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Bit slicing is a technique for constructing a 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 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). Each of these component modules processes one bit field or "slice" of an operand. The grouped processing components would then have the capability to process the chosen full word-length of a given software design. Bit slicing more or less died out due to the advent of the
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 ...
. Recently it has been used in arithmetic logic units (ALUs) for
quantum computer Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Thoug ...
s and as a software technique, e.g. for
cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adve ...
in x86 CPUs.


Operational details

Bit-slice processors (BSPs) usually include 1-, 2-, 4-, 8- or
16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...
arithmetic logic unit (ALU) and control lines (including carry or overflow signals that are internal to the processor in non-bitsliced
CPU 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 ...
designs). For example, two 4-bit ALU chips could be arranged side by side, with control lines between them, to form an 8-bit ALU (result need not be power of two, e.g. three 1-bit units can make a 3-bit ALU, thus 3-bit (or ''n''-bit) CPU, while 3-bit, or any CPU with higher odd number of bits, hasn't been manufactured and sold in volume). Four 4-bit ALU chips could be used to build a 16-bit ALU. It would take eight chips to build a 32-bit word ALU. The designer could add as many slices as required to manipulate longer word lengths. A microsequencer or
control ROM Control may refer to: Basic meanings Economics and business * Control (management), an element of management * Control, an element of management accounting * Comptroller (or controller), a senior financial officer in an organization * Controll ...
would be used to execute logic to provide data and control signals to regulate function of the component ALUs. Known bit-slice microprocessors: * 2-bit slice: ** Intel 3000 family (1974, now discontinued), e.g. Intel 3002 with Intel 3001, second-sourced by Signetics and
Intersil Intersil is an American semiconductor company headquartered in Milpitas, California. As of February 24, 2017, Intersil is a subsidiary of Renesas. The previous Intersil was formed in August 1999 through the acquisition of the semiconductor bus ...
** Signetics
8X02 8X or 8-X may refer to: *8x, or eight times in multiplication *8X, code name for Enhanced Imaging System *8X, abbreviation for Octuple scull *Precorrin-8X methylmutase *South African Class 8X 2-8-0 locomotive *8X Bayshore; see List of San Franc ...
family (1977, now discontinued) * 4-bit slice: ** National IMP family, consisting primarily of the IMP-00A/520 RALU (also known as MM5750) and various masked ROM microcode and control chips (CROMs, also known as MM5751) *** National GPC/P / IMP-4 (1973), second-sourced by Rockwell *** National IMP-8, an 8-bit processor based on the IMP chipset, using two RALU chips and one CROM chip *** National
IMP-16 The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor, released in 1973. It consisted of five PMOS integrated circuits: four identical RALU chips, short for register and ALU, providing the data path, and one CR ...
, a 16-bit processor based on the IMP chipset, e.g. four RALU chips with one each IMP16A/521D and IMP16A/522D CROM chips (additional optional CROM chips could provide instruction set additionis) ** AMD Am2900 family (1975), e.g. AM2901, AM2901A, AM2903 ** Monolithic Memories 5700/6700 family (1974) e.g. MMI 5701 / MMI 6701, second-sourced by
ITT Semiconductors ITT Inc., formerly ITT Corporation, is an American worldwide manufacturing company based in Stamford, Connecticut. The company produces specialty components for the aerospace, transportation, energy and industrial markets. ITT's three businesse ...
**
Texas Instruments SBP0400 The Texas Instruments SBP0400 (SBP = silicon bipolar), also known as SBC 0400 and X0400, is a microprogrammable 4-bit slice processor that was introduced in 1976 (delivery began in December 1975). It was one of the first LSI processors and was ...
(1975) and SBP0401, cascadable up to 16 bits **
Texas Instruments SN74181 The 74181 is a 4-bit slice arithmetic logic unit (ALU), implemented as a 7400 series TTL integrated circuit. The first complete ALU on a single chip, it was used as the arithmetic/logic core in the CPUs of many historically significant minicomp ...
(1970) ** Texas Instruments SN74S281 with SN74S282 ** Texas Instruments SN74S481 with SN74S482 (1976) ** Fairchild 33705 **
Fairchild 9400 Fairchild may refer to: Organizations * Fairchild Aerial Surveys, operated in cooperation with a subsidiary of Fairey Aviation Company * Fairchild Camera and Instrument * List of Sherman Fairchild companies, "Fairchild" companies * Fairchild Fa ...
(MACROLOGIC), 4700 **
Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
M10800 family (1979), e.g. MC10800 **
Raytheon Raytheon Technologies Corporation is an American multinational aerospace and defense conglomerate headquartered in Arlington, Virginia. It is one of the largest aerospace and defense manufacturers in the world by revenue and market capitaliz ...
RP-16, a 16-bit processor consisting of seven integrated circuits, using four RALU chips and three CROM chips. * 8-bit slice: ** Four-Phase Systems AL1 (1969, considered to be the first microprocessor used in a commercial product, now discontinued) ** Texas Instruments SN54AS888 / SN74AS888 **
Fairchild 100K Fairchild may refer to: Organizations * Fairchild Aerial Surveys, operated in cooperation with a subsidiary of Fairey Aviation Company * Fairchild Camera and Instrument * List of Sherman Fairchild companies, "Fairchild" companies * Fairchild Fash ...
** ZMD (1978/1981), cascadable up to 32 bit * 16-bit slice: ** AMD Am29100 family **
Synopsys Synopsys is an American electronic design automation (EDA) company that focuses on silicon design and verification, silicon intellectual property and software security and quality. Products include tools for logic synthesis and physical desig ...
49C402 ** ZFT Robotron/ ZFTM Dresden (1979/1982), unreleased


Historical necessity

Bit slicing, although not called that at the time, was also used in computers before large-scale integrated circuits (LSI, the predecessor to today's
VLSI Very large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining millions or billions of MOS transistors onto a single chip. VLSI began in the 1970s when MOS integrated circuit (Metal Oxide Semiconductor) ...
, or very-large-scale integration circuits). The first bit-sliced machine was
EDSAC 2 EDSAC 2 was an early computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform gene ...
, built at the University of Cambridge Mathematical Laboratory in 1956–1958. Prior to the mid-1970s and late 1980s there was some debate over how much bus width was necessary in a given computer system to make it function. Silicon chip technology and parts were much more expensive than today. Using multiple simpler, and thus less expensive, ALUs was seen as a way to increase computing power in a cost-effective manner. While
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calcula ...
microprocessors were being discussed at the time, few were in production. The UNIVAC 1100 series mainframes (one of the oldest series, originating in the 1950s) has a 36-bit architecture, and the 1100/60 introduced in 1979 used nine
Motorola MC10800 The Motorola MC10800 is a 4-bit bit-sliced processor designed by Motorola and introduced in 1979. It is implemented in ECL logic and is part of the M10800 family. A clone of the MC10800 was manufactured in the Soviet Union under the designation ...
4-bit ALU chips to implement the needed word width while using modern integrated circuits. At the time 16-bit processors were common but expensive, and 8-bit processors, such as the Z80, were widely used in the nascent home-computer market. Combining components to produce bit-slice products allowed engineers and students to create more powerful and complex computers at a more reasonable cost, using off-the-shelf components that could be custom-configured. The complexities of creating a new computer architecture were greatly reduced when the details of the ALU were already specified (and debugged). The main advantage was that bit slicing made it economically possible in smaller processors to use bipolar transistors, which switch much faster than NMOS or CMOS transistors. This allowed much higher clock rates, where speed was needed for example, for DSP functions or matrix transformation or, as in 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 G ...
, the combination of flexibility and speed, before discrete CPUs were able to deliver that.


Modern use


Software use on non-bit-slice hardware

In more recent times, the term bit slicing was reused by Matthew Kwan to refer to the technique of using a general-purpose CPU to implement multiple parallel simple
virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...
s using general logic instructions to perform single-instruction multiple-data (
SIMD Single instruction, multiple data (SIMD) is a type of parallel processing in Flynn's taxonomy. SIMD can be internal (part of the hardware design) and it can be directly accessible through an instruction set architecture (ISA), but it should ...
) operations. This technique is also known as SIMD within a register (SWAR). This was initially in reference to Eli Biham's 1997 article ''A Fast New DES Implementation in Software'', which achieved significant gains in performance of DES by using this method.


Bit-sliced quantum computers

To simplify the circuit structure and reduce the hardware cost of
quantum computer Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Thoug ...
s (proposed to run the MIPS32 instruction set) a 50 GHz superconducting "4-bit bit-slice arithmetic logic unit (ALU) for 32-bit rapid single-flux-quantum microprocessors was demonstrated".


See also

*
Bit-serial architecture In digital logic applications, bit-serial architectures send data one bit at a time, along a single wire, in contrast to bit-parallel word architectures, in which data values are sent all bits or a word at once along a group of wires. All d ...


References


External links

* a bitslicing primer presenting a pedagogical bitsliced implementation of the Tiny Encryption Algorithm (TEA), a
block cipher In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called ''blocks''. Block ciphers are specified cryptographic primitive, elementary components in the design of many cryptographic protocols and ...
{{Authority control Digital electronics Central processing unit University of Cambridge Computer Laboratory Bit-slice chips