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processor design Processor design is a subfield of computer engineering and electronics engineering (fabrication) that deals with creating a processor, a key component of computer hardware. The design process involves choosing an instruction set and a certain exe ...
, microcode (μcode) is a technique that interposes a layer of
computer organization In computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as µarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be impl ...
between the
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
(CPU) hardware and the programmer-visible
instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ' ...
of a computer. Microcode is a layer of hardware-level instructions that implement higher-level
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
instructions or internal
finite-state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
sequencing in many
digital processing Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digits. An example is ...
elements. Microcode is used in general-purpose
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
s, although in current desktop CPUs, it is only a fallback path for cases that the faster
hardwired control unit The control unit (CU) is a component of a computer's central processing unit (CPU) that directs the operation of the processor. A CU typically uses a binary decoder to convert coded instructions into timing and control signals that direct the ope ...
cannot handle. Microcode typically resides in special high-speed memory and translates machine instructions,
state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
data, or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
so that instructions can be designed and altered more freely. It also facilitates the building of complex multi-step instructions, while reducing the complexity of computer circuits. Writing microcode is often called ''microprogramming'' and the microcode in a particular processor implementation is sometimes called a ''microprogram''. More extensive microcoding allows small and simple microarchitectures to
emulate Emulate, Inc. (Emulate) is a biotechnology company that commercialized Organs-on-Chips technology—a human cell-based technology that recreates organ-level function to model organs in healthy and diseased states. The technology has applications ...
more powerful architectures with wider
word length In computing, a word is the natural unit of data used by a particular processor design. A word is a fixed-sized datum handled as a unit by the instruction set or the hardware of the processor. The number of bits or digits in a word (the ''word si ...
, more
execution unit In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...
s and so on, which is a relatively simple way to achieve software compatibility between different products in a processor family. Some hardware vendors, especially IBM, use the term ''microcode'' as a synonym for ''
firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
''. In that way, all code within a device is termed ''microcode'' regardless of it being microcode or
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
; for example,
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
s are said to have their microcode updated, though they typically contain both microcode and firmware.


Overview

The lowest layer in a computer's software stack is traditionally raw
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
instructions for the processor. In microcoded processors, fetching and decoding those instructions, and executing them, may be done by microcode. To avoid confusion, each microprogram-related element is differentiated by the ''micro'' prefix: microinstruction, microassembler, microprogrammer, microarchitecture, etc. Complex digital processors may also employ more than one (possibly microcode-based)
control unit The control unit (CU) is a component of a computer's central processing unit (CPU) that directs the operation of the processor. A CU typically uses a binary decoder to convert coded instructions into timing and control signals that direct the ope ...
in order to delegate sub-tasks that must be performed essentially asynchronously in parallel. For example, the
VAX 9000 The VAX 9000 is a discontinued family of Minicomputers developed and manufactured by Digital Equipment Corporation (DEC) using custom ECL-based processors implementing the VAX instruction set architecture (ISA). Equipped with optional vector proc ...
has a IBox unit to fetch and decode instructions, which it hands to a microcoded EBox unit to be executed, and the
VAX 8800 The VAX 8000 is a discontinued family of superminicomputers developed and manufactured by Digital Equipment Corporation (DEC) using processors implementing the VAX instruction set architecture (ISA). The 8000 series was introduced in October 19 ...
has both a microcoded IBox and a microcoded EBox. A high-level programmer, or even an
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
programmer, does not normally see or change microcode. Unlike machine code, which often retains some
backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially i ...
among different processors in a family, microcode only runs on the exact
electronic circuit An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...
ry for which it is designed, as it constitutes an inherent part of the particular processor design itself.


Design

Engineers normally write the microcode during the design phase of a processor, storing it in a
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM) or programmable logic array (PLA) structure, or in a combination of both. However, machines also exist that have some or all microcode stored in
static random-access memory Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The term ''static'' differen ...
(SRAM) or
flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...
. This is traditionally denoted as ''writeable
control store A control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. A control store implementation whose contents are unalterable is known as a Read Only Memory (ROM) or Read Only S ...
'' in the context of computers, which can be either read-only or read-write memory. In the latter case, the CPU initialization process loads microcode into the control store from another storage medium, with the possibility of altering the microcode to correct bugs in the instruction set, or to implement new machine instructions.


Microprograms

Microprograms consist of series of microinstructions, which control the CPU at a very fundamental level of hardware circuitry. For example, a single typical ''horizontal'' microinstruction might specify the following operations: * Connect register 1 to the ''A'' side of the ALU * Connect register 7 to the ''B'' side of the ALU * Set the ALU to perform
two's-complement Two's complement is a mathematical operation to reversibly convert a positive binary number into a negative binary number with equivalent (but negative) value, using the binary digit with the greatest place value (the leftmost bit in big- endian ...
addition * Set the ALU's carry input to zero * Store the result value in register 8 * Update the condition codes from the ALU status flags (''negative'', ''zero'', ''overflow'', and ''carry'') * Microjump to a given micro PC address for the next microinstruction To simultaneously control all processor's features in one cycle, the microinstruction is often wider than 50 bits; e.g., 128 bits on a 360/85 with an emulator feature. Microprograms are carefully designed and optimized for the fastest possible execution, as a slow microprogram would result in a slow machine instruction and degraded performance for related application programs that use such instructions.


Justification

Microcode was originally developed as a simpler method of developing the control logic for a computer. Initially, CPU
instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ' ...
s were
hardwired Hardwire or hardwired may refer to: *Electrical wiring *Hardwired control unit, a part of a computer's central processing unit *In computer programming, a kludge to temporarily or quickly fix a problem *Wired communication In arts and entertainme ...
. Each step needed to fetch, decode, and execute the machine instructions (including any operand address calculations, reads, and writes) was controlled directly by
combinational logic In automata theory, combinational logic (also referred to as time-independent logic or combinatorial logic) is a type of digital logic which is implemented by Boolean circuits, where the output is a pure function of the present input only. This i ...
and rather minimal
sequential In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called th ...
state machine circuitry. While such hard-wired processors were very efficient, the need for powerful instruction sets with multi-step addressing and complex operations (''see below'') made them difficult to design and debug; highly encoded and varied-length instructions can contribute to this as well, especially when very irregular encodings are used. Microcode simplified the job by allowing much of the processor's behaviour and programming model to be defined via microprogram routines rather than by dedicated circuitry. Even late in the design process, microcode could easily be changed, whereas hard-wired CPU designs were very cumbersome to change. Thus, this greatly facilitated CPU design. From the 1940s to the late 1970s, a large portion of programming was done in
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
; higher-level instructions mean greater programmer productivity, so an important advantage of microcode was the relative ease by which powerful machine instructions can be defined. The ultimate extension of this are "Directly Executable High Level Language" designs, in which each statement of a high-level language such as
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
is entirely and directly executed by microcode, without compilation. The
IBM Future Systems project The Future Systems project (FS) was a research and development project undertaken in IBM in the early 1970s, aiming to develop a revolutionary line of computer products, including new software models which would simplify software development by ex ...
and
Data General Data General Corporation was one of the first minicomputer firms of the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation (DEC). Their first product, 1969's Data General Nova, was a 16-bit minicomputer ...
Fountainhead Processor are examples of this. During the 1970s, CPU speeds grew more quickly than memory speeds and numerous techniques such as memory block transfer, memory pre-fetch and
multi-level cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
s were used to alleviate this. High-level machine instructions, made possible by microcode, helped further, as fewer more complex machine instructions require less memory bandwidth. For example, an operation on a character string can be done as a single machine instruction, thus avoiding multiple instruction fetches. Architectures with instruction sets implemented by complex microprograms included the IBM
System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...
and
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
VAX VAX (an acronym for Virtual Address eXtension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The V ...
. The approach of increasingly complex microcode-implemented instruction sets was later called
complex instruction set computer A complex instruction set computer (CISC ) is a computer architecture in which single instructions can execute several low-level operations (such as a load from memory, an arithmetic operation, and a memory store) or are capable of multi-step o ...
(CISC). An alternate approach, used in many
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
s, is to use one or more programmable logic array (PLA) or
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM) (instead of combinational logic) mainly for instruction decoding, and let a simple state machine (without much, or any, microcode) do most of the sequencing. The
MOS Technology 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small t ...
is an example of a microprocessor using a PLA for instruction decode and sequencing. The PLA is visible in photomicrographs of the chip, and its operation can be seen in the
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
-level simulation. Microprogramming is still used in modern CPU designs. In some cases, after the microcode is debugged in simulation, logic functions are substituted for the control store. Logic functions are often faster and less expensive than the equivalent microprogram memory.


Benefits

A processor's microprograms operate on a more primitive, totally different, and much more hardware-oriented architecture than the assembly instructions visible to normal programmers. In coordination with the hardware, the microcode implements the programmer-visible architecture. The underlying hardware need not have a fixed relationship to the visible architecture. This makes it easier to implement a given instruction set architecture on a wide variety of underlying hardware micro-architectures. The IBM System/360 has a 32-bit architecture with 16 general-purpose registers, but most of the System/360 implementations use hardware that implements a much simpler underlying microarchitecture; for example, the System/360 Model 30 has 8-bit data paths to the arithmetic logic unit (ALU) and main memory and implemented the general-purpose registers in a special unit of higher-speed
core memory Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
, and the System/360 Model 40 has 8-bit data paths to the ALU and 16-bit data paths to main memory and also implemented the general-purpose registers in a special unit of higher-speed core memory. The Model 50 has full 32-bit data paths and implements the general-purpose registers in a special unit of higher-speed core memory. The Model 65 through the Model 195 have larger data paths and implement the general-purpose registers in faster transistor circuits. In this way, microprogramming enabled IBM to design many System/360 models with substantially different hardware and spanning a wide range of cost and performance, while making them all architecturally compatible. This dramatically reduces the number of unique system software programs that must be written for each model. A similar approach was used by Digital Equipment Corporation (DEC) in their VAX family of computers. As a result, different VAX processors use different microarchitectures, yet the programmer-visible architecture does not change. Microprogramming also reduces the cost of field changes to correct defects ( bugs) in the processor; a bug can often be fixed by replacing a portion of the microprogram rather than by changes being made to
hardware logic Electronic hardware consists of interconnected electronic components which perform analog circuit, analog or Digital electronics, logic operations on received and locally stored information to produce as output or store resulting new information ...
and wiring.


History

In 1947, the design of the
MIT Whirlwind Whirlwind I was a Cold War-era vacuum tube computer developed by the MIT Servomechanisms Laboratory for the U.S. Navy. Operational in 1951, it was among the first digital electronic computers that operated in real-time for output, and the first ...
introduced the concept of a control store as a way to simplify computer design and move beyond ''
ad hoc Ad hoc is a Latin phrase meaning literally 'to this'. In English, it typically signifies a solution for a specific purpose, problem, or task rather than a generalized solution adaptable to collateral instances. (Compare with ''a priori''.) Com ...
'' methods. The control store is a
diode matrix A diode matrix is a two-dimensional grid of wires: each "intersection" wherein one-row crosses over another has either a diode connecting them, or the wires are isolated from each other. It is one of the most popular techniques for implementing a ...
: a two-dimensional lattice, where one dimension accepts "control time pulses" from the CPU's internal clock, and the other connects to control signals on gates and other circuits. A "pulse distributor" takes the pulses generated by the
CPU clock In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the ...
and breaks them up into eight separate time pulses, each of which activates a different row of the lattice. When the row is activated, it activates the control signals connected to it. Described another way, the signals transmitted by the control store are being played much like a
player piano A player piano (also known as a pianola) is a self-playing piano containing a pneumatic or electro-mechanical mechanism, that operates the piano action via programmed music recorded on perforated paper or metallic rolls, with more modern i ...
roll. That is, they are controlled by a sequence of very wide words constructed of
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
s, and they are ''played'' sequentially. In a control store, however, the ''song'' is short and repeated continuously. In 1951,
Maurice Wilkes Sir Maurice Vincent Wilkes (26 June 1913 – 29 November 2010) was a British computer scientist who designed and helped build the Electronic Delay Storage Automatic Calculator (EDSAC), one of the earliest stored program computers, and who inv ...
enhanced this concept by adding ''conditional execution'', a concept akin to a conditional in computer software. His initial implementation consisted of a pair of matrices: the first one generated signals in the manner of the Whirlwind control store, while the second matrix selected which row of signals (the microprogram instruction word, so to speak) to invoke on the next cycle. Conditionals were implemented by providing a way that a single line in the control store could choose from alternatives in the second matrix. This made the control signals conditional on the detected internal signal. Wilkes coined the term ''microprogramming'' to describe this feature and distinguish it from a simple control store.


Examples

* The
EMIDEC 1100 The EMIDEC 1100 computer (became the ICT 1101 in 1962) was produced by the Computing Services Division of EMI Laboratories in the UK under the leadership of Godfrey Hounsfield in 1958, (first delivered in 1959) after one year's development. I ...
reputedly uses a hard-wired control store consisting of wires threaded through ferrite cores, known as "the laces". * Most models of the IBM System/360 series are microprogrammed: ** The Model 25 is unique among System/360 models in using the top 16 K bytes of core storage to hold the control storage for the microprogram. The 2025 uses a 16-bit microarchitecture with seven control words (or microinstructions). After system maintenance or when changing operating mode, the microcode is loaded from the card reader, tape, or other device. The IBM 1410 emulation for this model is loaded this way. ** The Model 30 uses an 8-bit microarchitecture with only a few hardware registers; everything that the programmer saw is emulated by the microprogram. The microcode for this model is also held on special punched cards, which are stored inside the machine in a dedicated reader per card, called "CROS" units (Capacitor Read-Only Storage). Another CROS unit is added for machines ordered with 1401/1440/1460 emulation and for machines ordered with 1620 emulation. ** The Model 40 uses 56-bit control words. The 2040 box implements both the System/360 main processor and the multiplex channel (the I/O processor). This model uses ''TROS'' dedicated readers similar to ''CROS'' units, but with an inductive pickup (Transformer Read-only Store). ** The Model 50 has two internal datapaths which operated in parallel: a 32-bit datapath used for arithmetic operations, and an 8-bit data path used in some logical operations. The control store uses 90-bit microinstructions. ** The Model 85 has separate instruction fetch (I-unit) and execution (E-unit) to provide high performance. The I-unit is hardware controlled. The E-unit is microprogrammed; the control words are 108 bits wide on a basic 360/85 and wider if an emulator feature is installed. * The
NCR 315 The NCR 315 Data Processing System, released in January 1962 by NCR, is a second-generation computer. All printed circuit boards use resistor–transistor logic (RTL) to create the various logic elements. It uses 12-bit ''slab'' memory structur ...
is microprogrammed with hand wired ferrite cores (a
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...
) pulsed by a sequencer with conditional execution. Wires routed through the cores are enabled for various data and logic elements in the processor. * The Digital Equipment Corporation
PDP-11 The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
processors, with the exception of the PDP-11/20, are microprogrammed. * Most
Data General Eclipse The Data General Eclipse line of computers by Data General were 16-bit minicomputers released in early 1974 and sold until 1988. The Eclipse was based on many of the same concepts as the Data General Nova, but included support for virtual memory ...
minicomputers are microprogrammed. The task of writing microcode for the Eclipse MV/8000 is detailed in the Pulitzer Prize-winning book titled ''
The Soul of a New Machine ''The Soul of a New Machine'' is a non-fiction book written by Tracy Kidder and published in 1981. It chronicles the experiences of a computer engineering team racing to design a next-generation computer at a blistering pace under tremendous p ...
''. * Many systems from Burroughs are microprogrammed: :* The B700 "microprocessor" execute application-level opcodes using sequences of 16-bit microinstructions stored in main memory; each of these is either a register-load operation or mapped to a single 56-bit "nanocode" instruction stored in read-only memory. This allows comparatively simple hardware to act either as a mainframe peripheral controller or to be packaged as a standalone computer. :* The
B1700 The Burroughs B1000 Series was a series of mainframe computers, built by the Burroughs Corporation, and originally introduced in the 1970s with continued software development until 1987. The series consisted of three major generations which were t ...
is implemented with radically different hardware including bit-addressable main memory but has a similar multi-layer organisation. The operating system preloads the interpreter for whatever language is required. These interpreters present different virtual machines for
COBOL COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural and, since 2002, object-oriented language. COBOL is primarily us ...
, Fortran, etc. *
Microdata Microdata can mean: * Microdata (statistics), a statistical term for individual response data in surveys and censuses * Microdata (HTML), a specification for semantic markup in HTML * Microdata Corporation Microdata Corporation was an American ...
produced computers in which the microcode is accessible to the user; this allows the creation of custom assembler level instructions. Microdata's
Reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, r ...
operating system design makes extensive use of this capability. * The
Xerox Alto The Xerox Alto is a computer designed from its inception to support an operating system based on a graphical user interface (GUI), later using the desktop metaphor. The first machines were introduced on 1 March 1973, a decade before mass-market ...
workstation used a microcoded design but, unlike many computers, the microcode engine is not hidden from the programmer in a layered design. Applications take advantage of this to accelerate performance. * The
IBM System/38 The System/38 is a discontinued minicomputer and midrange computer manufactured and sold by IBM. The system was announced in 1978. The System/38 has 48-bit addressing, which was unique for the time, and a novel integrated database system. It w ...
is described as having both horizontal and vertical microcode. In practice, the processor implements an instruction set architecture named the ''Internal Microprogrammed Interface'' (IMPI) using a horizontal microcode format. The so-called vertical microcode layer implements the System/38's hardware-independent Machine Interface instruction set in terms of IMPI instructions. Prior to the instruction of the
IBM RS64 The IBM RS64 is a family of microprocessors used in IBM's RS/6000 and AS/400 servers in the late 1990s. These microprocessors implement the "Amazon", or "PowerPC-AS", instruction set architecture (ISA). Amazon is a superset of the PowerPC instruc ...
processor line, early
IBM AS/400 The IBM AS/400 (Application System/400) is a family of midrange computers from IBM announced in June 1988 and released in August 1988. It was the successor to the System/36 and System/38 platforms, and ran the OS/400 operating system. Lower-cost ...
systems used the same architecture. * The
Nintendo 64 The (N64) is a home video game console developed by Nintendo. The successor to the Super Nintendo Entertainment System, it was released on June 23, 1996, in Japan, on September 29, 1996, in North America, and on March 1, 1997, in Europe and Au ...
's
Reality Coprocessor This article describes the processor, memory, and other components of the 1996 Nintendo 64 home video game console. Components * CPU: 64-bit NEC VR4300 ( MIPS R4300i) with 24 KB L1 cache, running at 93.75 MHz. ** Performance: 12 ...
(RCP), which serves as the console's
graphics processing unit A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobi ...
and audio processor, utilizes microcode; it is possible to implement new effects or tweak the processor to achieve the desired output. Some notable examples of custom RCP microcode include the high-resolution graphics, particle engines, and unlimited draw distances found in
Factor 5 Factor 5 GmbH is an Independent business, independent software and video game developer. The company was co-founded by five former Rainbow Arts employees in 1987 in Cologne, Germany, which served as the inspiration behind the studio's name. In ...
's ''
Indiana Jones and the Infernal Machine ''Indiana Jones and the Infernal Machine'' is an action-adventure video game by LucasArts released in 1999. The first 3D installment in the series, its gameplay focuses on solving puzzles, fighting enemies, and completing various platforming sec ...
'', '' Star Wars: Rogue Squadron'', and '' Star Wars: Battle for Naboo''; and the
full motion video Full-motion video (FMV) is a video game narration technique that relies upon pre-recorded video files (rather than sprites, vectors, or 3D models) to display action in the game. While many games feature FMVs as a way to present information duri ...
playback found in
Angel Studios Angel Studios is an American video streaming service, media company, and film distribution studio. The studio uses equity crowdfunding to finance its original productions by offering individual investors the opportunity to purchase shares in ...
' ''
Resident Evil 2 ''Resident Evil 2'' is a 1998 survival horror video game developed and published by Capcom for the PlayStation. The player controls Leon S. Kennedy and Claire Redfield, who must escape Raccoon City after its citizens are transformed into zom ...
''. * The VU0 and VU1 vector units in the
Sony , commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
PlayStation 2 The PlayStation 2 (PS2) is a home video game console developed and marketed by Sony Computer Entertainment. It was first released in Japan on 4 March 2000, in North America on 26 October 2000, in Europe on 24 November 2000, and in Australia on 3 ...
are microprogrammable; in fact, VU1 is only accessible via microcode for the first several generations of the SDK. * The MicroCore Lab
MCL86


an

are examples of highly encoded "vertical" microsequencer implementations of the Intel 8086/8088, 8051, and MOS 6502. * Th
Digital Scientific Corp.
Meta 4 Series 16 computer system was a user-microprogammable system first available in 1970. The microcode had a primarily vertical style with 32-bit microinstructions. The instructions were stored on replaceable program boards with a grid of bit positions. One (1) bits were represented by small metal squares that were sensed by amplifiers, zero (0) bits by the absence of the squares. The system could be configured with up to 4K 16-bit words of microstore. One of Digital Scientific's products was an emulator for the
IBM 1130 The IBM 1130 Computing System, introduced in 1965, was IBM's least expensive computer at that time. A binary 16-bit machine, it was marketed to price-sensitive, computing-intensive technical markets, like education and engineering, succeeding th ...
. * The
MCP-1600 The MCP-1600 is a multi-chip 16-bit microprocessor introduced by Western Digital in 1975 and produced through the early 1980s. Used in the Pascal MicroEngine, the WD16 processor in the Alpha Microsystems AM-100, and the DEC LSI-11 microcompute ...
is a
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
made by
Western Digital Western Digital Corporation (WDC, commonly known as Western Digital or WD) is an American computer drive manufacturer and data storage company, headquartered in San Jose, California. It designs, manufactures and sells data technology produc ...
in the late 1970s through the early 1980s used to implement three different computer architectures in microcode: the
Pascal MicroEngine Pascal MicroEngine is a series of microcomputer products manufactured by Western Digital from 1979 through the mid-1980s, designed specifically to run the UCSD p-System efficiently. Compared to other microcomputers, which use a machine language ...
, the WD16, and the DEC
LSI-11 The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
, a cost-reduced PDP-11. * Earlier
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
processors are fully microcoded; starting with the
Intel 80486 The Intel 486, officially named i486 and also known as 80486, is a microprocessor. It is a higher-performance follow-up to the Intel 386. The i486 was introduced in 1989. It represents the fourth generation of binary compatible CPUs following the ...
, less complicated instructions are implemented directly in hardware. x86 processors implemented
patchable microcode A control store is the part of a Central processing unit, CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. A control store implementation whose contents are unalterable is known as a Read-only memor ...
(patch by
BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
or
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
) since Intel P6 microarchitecture and AMD K7 microarchitecture. * Some
video card A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer moni ...
s,
wireless network interface controller A wireless network interface controller (WNIC) is a network interface controller which connects to a wireless network, such as Wi-Fi or Bluetooth, rather than a wired network, such as a Token Ring or Ethernet. A WNIC, just like other NICs, wor ...
s implemented patchable microcode (patch by operating system).


Implementation

Each microinstruction in a microprogram provides the bits that control the functional elements that internally compose a CPU. The advantage over a hard-wired CPU is that internal CPU control becomes a specialized form of a computer program. Microcode thus transforms a complex electronic design challenge (the control of a CPU) into a less complex programming challenge. To take advantage of this, a CPU is divided into several parts: * An
I-unit The instruction unit (I-unit or IU), also called, e.g., instruction fetch unit (IFU), instruction issue unit (IIU), instruction sequencing unit (ISU), in a central processing unit (CPU) is responsible for organizing program instructions to be fetche ...
may decode instructions in hardware and determine the microcode address for processing the instruction in parallel with the
E-unit EMD E-units were a line of passenger train streamliner diesel locomotives built by the General Motors Electro-Motive Division (EMD) and its predecessor the Electro-Motive Corporation (EMC). Final assembly for all E-units was in La Grange, Illino ...
. * A
microsequencer In computer architecture and engineering, a sequencer or microsequencer generates the addresses used to step through the microprogram of a control store. It is used as a part of the control unit of a CPU or as a stand-alone generator for address ...
picks the next word of the control store. A sequencer is mostly a counter, but usually also has some way to jump to a different part of the control store depending on some data, usually data from the
instruction register In computing, the instruction register (IR) or current instruction register (CIR) is the part of a CPU's control unit that holds the instruction currently being executed or decoded. In simple processors, each instruction to be executed is loaded ...
and always some part of the control store. The simplest sequencer is just a register loaded from a few bits of the control store. * A
register Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
set is a fast memory containing the data of the central processing unit. It may include the
program counter The program counter (PC), commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), the instruction counter, or just part of the instruction sequencer, is ...
and
stack pointer In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or mach ...
, and may also include other registers that are not easily accessible to the application programmer. Often the register set is a triple-ported
register file A register file is an array of processor registers in a central processing unit (CPU). Register banking is the method of using a single name to access multiple different physical registers depending on the operating mode. Modern integrated circuit- ...
; that is, two registers can be read, and a third written at the same time. * An arithmetic and logic unit performs calculations, usually addition, logical negation, a right shift, and logical AND. It often performs other functions, as well. There may also be a
memory address register In a computer, the memory address register (MAR) is the Central processing unit, CPU Hardware register, register that either stores the memory address from which data will be fetched to the CPU registers, or the address to which data will be sent ...
and a
memory data register A memory buffer register (MBR) or memory data register (MDR) is the register in a computer's CPU that stores the data being transferred to and from the immediate access storage. It contains a copy of the value in the memory location specified by th ...
, used to access the main
computer storage Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a compute ...
. Together, these elements form an "
execution unit In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...
". Most modern
CPUs A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
have several execution units. Even simple computers usually have one unit to read and write memory, and another to execute user code. These elements could often be brought together as a single chip. This chip comes in a fixed width that would form a "slice" through the execution unit. These are known as "
bit slice Bit slicing is a technique for constructing a Processor (computing), processor from modules of processors of smaller bit width, for the purpose of increasing the word length; in theory to make an arbitrary ''n''-bit central processing unit ( ...
" chips. The
AMD Am2900 Am2900 is a family of integrated circuits (ICs) created in 1975 by Advanced Micro Devices (AMD). They were constructed with bipolar devices, in a bit-slice topology, and were designed to be used as modular components each representing a different ...
family is one of the best known examples of bit slice elements. The parts of the execution units and the whole execution units are interconnected by a bundle of wires called a
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
. Programmers develop microprograms, using basic software tools. A microassembler allows a programmer to define the table of bits symbolically. Because of its close relationship to the underlying architecture, "microcode has several properties that make it difficult to generate using a compiler." A
simulator A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of models; the model represents the key characteristics or behaviors of the selected system or process, whereas the s ...
program is intended to execute the bits in the same way as the electronics, and allows much more freedom to debug the microprogram. After the microprogram is finalized, and extensively tested, it is sometimes used as the input to a computer program that constructs logic to produce the same data. This program is similar to those used to optimize a programmable logic array. Even without fully optimal logic, heuristically optimized logic can vastly reduce the number of transistors from the number needed for a
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM) control store. This reduces the cost to produce, and the electricity used by, a CPU. Microcode can be characterized as ''horizontal'' or ''vertical'', referring primarily to whether each microinstruction controls CPU elements with little or no decoding (horizontal microcode) or requires extensive decoding by combinatorial logic before doing so (vertical microcode). Consequently, each horizontal microinstruction is wider (contains more bits) and occupies more storage space than a vertical microinstruction.


Horizontal microcode

"Horizontal microcode has several discrete micro-operations that are combined in a single microinstruction for simultaneous operation." Horizontal microcode is typically contained in a fairly wide control store; it is not uncommon for each word to be 108 bits or more. On each tick of a sequencer clock a microcode word is read, decoded, and used to control the functional elements that make up the CPU. In a typical implementation a horizontal microprogram word comprises fairly tightly defined groups of bits. For example, one simple arrangement might be: For this type of micromachine to implement a JUMP instruction with the address following the opcode, the microcode might require two clock ticks. The engineer designing it would write microassembler source code looking something like this: For each tick it is common to find that only some portions of the CPU are used, with the remaining groups of bits in the microinstruction being no-ops. With careful design of hardware and microcode, this property can be exploited to parallelise operations that use different areas of the CPU; for example, in the case above, the ALU is not required during the first tick, so it could potentially be used to complete an earlier arithmetic instruction.


Vertical microcode

In vertical microcode, each microinstruction is significantly encoded, that is, the bit fields generally pass through intermediate combinatory logic that, in turn, generates the control and sequencing signals for internal CPU elements (ALU, registers, etc.). This is in contrast with horizontal microcode, in which the bit fields either directly produce the control and sequencing signals or are only minimally encoded. Consequently, vertical microcode requires smaller instruction lengths and less storage, but requires more time to decode, resulting in a slower CPU clock. Some vertical microcode is just the assembly language of a simple conventional computer that is emulating a more complex computer. Some processors, such as
DEC Alpha Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set computers ...
processors and the CMOS microprocessors on later IBM mainframes
System/390 The IBM System/390 is a discontinued mainframe product family implementing the ESA/390, the fifth generation of the System/360 instruction set architecture. The first computers to use the ESA/390 were the Enterprise System/9000 (ES/9000 ...
and
z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture-b ...
, use machine code, running in a special mode that gives it access to special instructions, special registers, and other hardware resources unavailable to regular machine code, to implement some instructions and other functions, such as page table walks on Alpha processors. This is called
PALcode PALcode (Privileged Architecture Library code) is the name used by DEC in the Alpha instruction set architecture (ISA) for a set of functions in the System Reference Manual (SRM) or AlphaBIOS firmware, providing a hardware abstraction layer for ...
on Alpha processors and millicode on IBM mainframe processors. Another form of vertical microcode has two fields: The ''field select'' selects which part of the CPU will be controlled by this word of the control store. The ''field value'' controls that part of the CPU. With this type of microcode, a designer explicitly chooses to make a slower CPU to save money by reducing the unused bits in the control store; however, the reduced complexity may increase the CPU's clock frequency, which lessens the effect of an increased number of cycles per instruction. As transistors grew cheaper, horizontal microcode came to dominate the design of CPUs using microcode, with vertical microcode being used less often. When both vertical and horizontal microcode are used, the horizontal microcode may be referred to as ''nanocode'' or ''picocode''.


Writable control store

A few computers were built using ''writable microcode''. In this design, rather than storing the microcode in ROM or hard-wired logic, the microcode is stored in a RAM called a ''writable control store'' or ''WCS''. Such a computer is sometimes called a ''writable instruction set computer'' (WISC). Many experimental prototype computers use writable control stores; there are also commercial machines that use writable microcode, such as the
Burroughs Small Systems The Burroughs B1000 Series was a series of mainframe computers, built by the Burroughs Corporation, and originally introduced in the 1970s with continued software development until 1987. The series consisted of three major generations which were t ...
, early
Xerox Xerox Holdings Corporation (; also known simply as Xerox) is an American corporation that sells print and electronic document, digital document products and services in more than 160 countries. Xerox is headquartered in Norwalk, Connecticut (ha ...
workstations, the DEC
VAX VAX (an acronym for Virtual Address eXtension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The V ...
8800 (''Nautilus'') family, the
Symbolics Symbolics was a computer manufacturer Symbolics, Inc., and a privately held company that acquired the assets of the former company and continues to sell and maintain the Open Genera Lisp system and the Macsyma computer algebra system.
L- and G-machines, a number of IBM System/360 and
System/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path f ...
implementations, some DEC
PDP-10 Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, especi ...
machines, and the
Data General Eclipse MV/8000 The Eclipse MV/8000 was the first in a family of 32-bit minicomputers produced by Data General during the 1980s. Codenamed ''Eagle'' during development, its architecture was a new 32-bit design backward compatible with the previous 16-bit Eclipse ...
. Many more machines offer user-programmable writable control stores as an option, including the
HP 2100 The HP 2100 is a series of 16-bit minicomputers that were produced by Hewlett-Packard (HP) from the mid-1960s to early 1990s. Tens of thousands of machines in the series were sold over its twenty-five year lifetime, making HP the fourth largest mi ...
, DEC PDP-11/60 and
Varian Data Machines Varian Data Machines was a division of Varian Associates which sold minicomputers. It entered the market in 1967 through acquisition of Decision Control Inc. (DCI) in Newport Beach, California. It met stiff competition and was bought by Sperry Co ...
V-70 series
minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...
s. The IBM System/370 includes a facility called ''Initial-Microprogram Load'' (''IML'' or ''IMPL'') that can be invoked from the console, as part of ''
power-on reset A power-on reset (PoR, POR) generator is a microcontroller or microprocessor peripheral that generates a reset signal when power is applied to the device. It ensures that the device starts operating in a known state. PoR generator In VLSI dev ...
'' (''POR'') or from another processor in a tightly coupled
multiprocessor Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There ar ...
complex. Some commercial machines, for example IBM 360/85, have both a read-only storage and a writable control store for microcode. WCS offers several advantages including the ease of patching the microprogram and, for certain hardware generations, faster access than ROMs can provide. User-programmable WCS allows the user to optimize the machine for specific purposes. Starting with the
Pentium Pro The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel and introduced on November 1, 1995. It introduced the P6 microarchitecture (sometimes termed i686) and was originally intended to replace the original P ...
in 1995, several
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
CPUs have writable Intel Microcode. This, for example, has allowed bugs in the
Intel Core 2 Intel Core 2 is the processor family encompassing a range of Intel's consumer 64-bit x86-64 single-, dual-, and quad-core microprocessors based on the Core microarchitecture. The single- and dual-core models are single-die, whereas the quad-cor ...
and Intel
Xeon Xeon ( ) is a brand of x86 microprocessors designed, manufactured, and marketed by Intel, targeted at the non-consumer workstation, server, and embedded system markets. It was introduced in June 1998. Xeon processors are based on the same arc ...
microcodes to be fixed by patching their microprograms, rather than requiring the entire chips to be replaced. A second prominent example is the set of microcode patches that Intel offered for some of their processor architectures of up to 10 years in age, in a bid to counter the security vulnerabilities discovered in their designs –
Spectre Spectre, specter or the spectre may refer to: Religion and spirituality * Vision (spirituality) * Apparitional experience * Ghost Arts and entertainment Film and television * ''Spectre'' (1977 film), a made-for-television film produced and writ ...
and
Meltdown Meltdown may refer to: Science and technology * Nuclear meltdown, a severe nuclear reactor accident * Meltdown (security vulnerability), affecting computer processors * Mutational meltdown, in population genetics Arts and entertainment Music * Me ...
– which went public at the start of 2018. A microcode update can be installed by Linux,
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
, Microsoft Windows, or the motherboard BIOS.


Comparison to VLIW and RISC

The design trend toward heavily microcoded processors with complex instructions began in the early 1960s and continued until roughly the mid-1980s. At that point the
RISC In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a complex instruction set comput ...
design philosophy started becoming more prominent. A CPU that uses microcode generally takes several clock cycles to execute a single instruction, one clock cycle for each step in the microprogram for that instruction. Some CISC processors include instructions that can take a very long time to execute. Such variations interfere with both
interrupt latency In computing, interrupt latency refers to the delay between the start of an Interrupt Request (IRQ) and the start of the respective Interrupt Service Routine (ISR). For many operating systems, devices are serviced as soon as the device's interrup ...
and, what is far more important in modern systems, pipelining. When designing a new processor, a
hardwired control The control unit (CU) is a component of a computer's central processing unit (CPU) that directs the operation of the processor. A CU typically uses a binary decoder to convert coded instructions into timing and control signals that direct the op ...
RISC has the following advantages over microcoded CISC: * Programming has largely moved away from assembly level, so it's no longer worthwhile to provide complex instructions for productivity reasons. * Simpler instruction sets allow direct execution by hardware, avoiding the performance penalty of microcoded execution. * Analysis shows complex instructions are rarely used, hence the machine resources devoted to them are largely wasted. * The machine resources devoted to rarely used complex instructions are better used for expediting performance of simpler, commonly used instructions. * Complex microcoded instructions may require many clock cycles that vary, and are difficult to
pipeline Pipeline may refer to: Electronics, computers and computing * Pipeline (computing), a chain of data-processing stages or a CPU optimization found on ** Instruction pipelining, a technique for implementing instruction-level parallelism within a s ...
for increased performance. There are counterpoints as well: * The complex instructions in heavily microcoded implementations may not take much extra machine resources, except for microcode space. For example, the same ALU is often used to calculate an effective address and to compute the result from the operands, e.g., the original
Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
,
8086 The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...
, and others. * The simpler non-RISC instructions (i.e., involving direct memory
operand In mathematics, an operand is the object of a mathematical operation, i.e., it is the object or quantity that is operated on. Example The following arithmetic expression shows an example of operators and operands: :3 + 6 = 9 In the above examp ...
s) are frequently used by modern compilers. Even immediate to stack (i.e., memory result) arithmetic operations are commonly employed. Although such memory operations, often with varying length encodings, are more difficult to pipeline, it is still fully feasible to do so - clearly exemplified by the
i486 The Intel 486, officially named i486 and also known as 80486, is a microprocessor. It is a higher-performance follow-up to the Intel 386. The i486 was introduced in 1989. It represents the fourth generation of binary compatible CPUs following the ...
,
AMD K5 The K5 is AMD's first x86 processor to be developed entirely in-house. Introduced in March 1996, its primary competition was Intel's Pentium microprocessor. The K5 was an ambitious design, closer to a Pentium Pro than a Pentium regarding techn ...
,
Cyrix 6x86 The Cyrix 6x86 is a line of sixth-generation, 32-bit x86 microprocessors designed and released by Cyrix in 1995. Cyrix, being a fabless company, had the chips manufactured by IBM and SGS-Thomson. The 6x86 was made as a direct competitor to In ...
,
Motorola 68040 The Motorola 68040 ("''sixty-eight-oh-forty''") is a 32-bit microprocessor in the Motorola 68000 series, released in 1990. It is the successor to the 68030 and is followed by the 68060, skipping the 68050. In keeping with general Motorola na ...
, etc. * Non-RISC instructions inherently perform more work per instruction (on average), and are also normally highly encoded, so they enable smaller overall size of the same program, and thus better use of limited cache memories. Many RISC and
VLIW Very long instruction word (VLIW) refers to instruction set architectures designed to exploit instruction level parallelism (ILP). Whereas conventional central processing units (CPU, processor) mostly allow programs to specify instructions to exe ...
processors are designed to execute every instruction (as long as it is in the cache) in a single cycle. This is very similar to the way CPUs with microcode execute one microinstruction per cycle. VLIW processors have instructions that behave similarly to very wide horizontal microcode, although typically without such fine-grained control over the hardware as provided by microcode. RISC instructions are sometimes similar to the narrow vertical microcode. Microcode has been popular in application-specific processors such as
network processors A network processor is an integrated circuit which has a feature set specifically targeted at the Computer networking, networking application domain. Network processors are typically software programmable devices and would have generic characteris ...
,
microcontroller A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable i ...
s,
digital signal processor A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on MOS integrated circuit chips. They are widely used in audio si ...
s, channel controllers,
disk controller {{unreferenced, date=May 2010 The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus conne ...
s,
network interface controller A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. Ear ...
s,
graphics processing unit A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobi ...
s, and in other hardware.


Micro-operations

Modern CISC implementations, such as the
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
family, decode instructions into dynamically buffered
micro-operation In computer central processing units, micro-operations (also known as micro-ops or μops, historically also as micro-actions) are detailed low-level instructions used in some designs to implement complex machine instructions (sometimes termed m ...
s ("μops") with an instruction encoding similar to RISC or traditional microcode. A hardwired instruction decode unit directly emits μops for common x86 instructions, but falls back to a more traditional microcode ROM containing μops for more complex or rarely used instructions. For example, an x86 might look up μops from microcode to handle complex multistep operations such as loop or string instructions,
floating-point unit In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can b ...
transcendental function In mathematics, a transcendental function is an analytic function that does not satisfy a polynomial equation, in contrast to an algebraic function. In other words, a transcendental function "transcends" algebra in that it cannot be expressed alge ...
s or unusual values such as
denormal number In computer science, subnormal numbers are the subset of denormalized numbers (sometimes called denormals) that fill the underflow gap around zero in floating-point arithmetic. Any non-zero number with magnitude smaller than the smallest normal ...
s, and special-purpose instructions such as
CPUID In the x86 architecture, the CPUID instruction (identified by a CPUID opcode) is a processor supplementary instruction (its name derived from CPU IDentification) allowing software to discover details of the processor. It was introduced by Intel ...
.


See also

*
Address generation unit The address generation unit (AGU), sometimes also called address computation unit (ACU), is an execution unit inside central processing units (CPUs) that calculates addresses used by the CPU to access main memory. By having address calculations ...
(AGU) *
CPU design Processor design is a subfield of computer engineering and electronics engineering (fabrication) that deals with creating a processor, a key component of computer hardware. The design process involves choosing an instruction set and a certain ...
*
Finite-state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
(FSM) *
Firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
*
Floating-point unit In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can b ...
(FPU) * Pentium FDIV bug *
Instruction pipeline In computer engineering, instruction pipelining or ILP is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing inco ...
*
Microsequencer In computer architecture and engineering, a sequencer or microsequencer generates the addresses used to step through the microprogram of a control store. It is used as a part of the control unit of a CPU or as a stand-alone generator for address ...
* MikroSim * Millicode * Superscalar


Notes


References


Further reading

* * * * * *


External links


Writable Instruction Set Computer

Capacitor Read-only Store






(fixes the issues when running 32-bit virtual machines in PAE mode)
Notes on Intel Microcode Updates
March 2013, by Ben Hawkes, archived from the original on September 7, 2015

''
EE Times ''EE Times'' (''Electronic Engineering Times'') is an electronics industry magazine published in the United States since 1972. EE Times is currently owned by AspenCore, a division of Arrow Electronics since August 2016. Since its acquisition ...
'', 2002, by Alexander Wolfe, archived from the original on March 9, 2003
Opteron Exposed: Reverse Engineering AMD K8 Microcode Updates
July 26, 2004
WepSIM: An Online Interactive Educational Simulator Integrating Microdesign, Microprogramming, and Assembly Language Programming
July 26, 2022 {{Processor technologies Instruction processing Firmware Central processing unit BIOS