The zero page or base page is the block of memory at the very beginning of a
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 generic sets of operations known as C ...
's
address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.
For software programs to save and retrieve st ...
; that is, the
page
Page most commonly refers to:
* Page (paper), one side of a leaf of paper, as in a book
Page, PAGE, pages, or paging may also refer to:
Roles
* Page (assistance occupation), a professional occupation
* Page (servant), traditionally a young m ...
whose starting address is zero. The size of a page depends on the context, and the significance of zero page memory versus higher addressed memory is highly dependent on machine architecture. For example, the
Motorola 6800
The 6800 ("''sixty-eight hundred''") is an 8-bit computing, 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the Motorola 6800 family, M6800 Microcomputer System (latter dubbed ''68xx' ...
and
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 ...
processor families treat the first 256
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
s of
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...
specially,
whereas many other processors do not.
Unlike more modern hardware, in the 1970s computer
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
was as fast as or faster than the CPU. Thus it made sense to have few registers and use the main memory as an extended pool of extra registers. In machines with a relatively wide
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 ...
address bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
and comparatively narrow
8-bit
In computer architecture, 8-bit Integer (computer science), integers or other Data (computing), data units are those that are 8 bits wide (1 octet (computing), octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) arc ...
data bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
, accessing zero page locations could be faster than accessing other locations.
Zero page addressing now has mostly historical significance, since the developments in
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
technology
Technology is the application of knowledge to reach practical goals in a specifiable and reproducible way. The word ''technology'' may also mean the product of such an endeavor. The use of technology is widely prevalent in medicine, science, ...
have made adding more registers to a CPU less expensive and CPU operations much faster than RAM accesses.
Size
The actual size of the zero page in bytes is determined by 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 ...
design and in older designs, is often equal to the largest value that can be referenced by the processor's indexing registers. For example, the aforementioned 8-bit processors have 8-bit index registers and a page size of 256 bytes. Therefore, their zero page extends from address 0 to address 255.
Computers with few CPU registers
In early computers, including the
PDP-8
The PDP-8 is a 12-bit computing, 12-bit minicomputer that was produced by Digital Equipment Corporation, Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's ...
, the zero page had a special fast
addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions in ...
, which facilitated its use for temporary
storage of data and compensated for the relative shortage of
CPU 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 ...
s. The PDP-8 had only one register, so zero page addressing was essential. In the original
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 ...
KA-10 models, the registers it has are simply the first 16 words,
36-bits long, of main memory. Those locations can be accessed as both registers and memory locations.
Unlike more modern hardware, 1970s computer
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
was as fast as or faster than the CPU. Thus, it made sense to have few registers and use the main memory as an extended pool of extra registers. In machines with a relatively wide
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 ...
address bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
and comparatively narrow
8-bit
In computer architecture, 8-bit Integer (computer science), integers or other Data (computing), data units are those that are 8 bits wide (1 octet (computing), octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) arc ...
data bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
, accessing zero page locations could be faster than accessing other locations. Since zero page locations could be addressed by a single
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
, the instructions accessing them could be smaller and faster loading.
For example, 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 ...
has only one general purpose register (the accumulator). To offset this weakness and gain a performance advantage, it was designed to make special use of the zero page, providing special instructions that are smaller, thus requiring fewer memory fetch cycles and executing faster. Many
instructions
Instruction or instructions may refer to:
Computing
* Instruction, one operation of a processor within a computer architecture instruction set
* Computer program, a collection of instructions
Music
* Instruction (band), a 2002 rock band from Ne ...
are coded differently for zero page and non-zero page addresses, this is called ''zero-page addressing'' in 6502 terminology (it was called ''direct addressing'' in 6800 terminology):
LDA $00 ; zero page
LDA $0000 ; non-zero page
The above two instructions both accomplish the same thing: they load the value of memory location $00 into the .A register (accumulator). However, the first instruction is only two bytes long and requires three clock cycles to complete. The second instruction is three bytes in length and requires four clock cycles to execute. This difference in execution time could become significant in repetitive code.
Some CPUs like the
Motorola 6809
The Motorola 6809 ("''sixty-eight-oh-nine''") is an 8-bit microprocessor with some 16-bit features. It was designed by Motorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible with the earlier Motorola 6800, the 6 ...
extend this concept by allowing fast accesses to whichever 256-byte page, the so called ''direct page'', is selected by a special 8-bit "Direct Page" (DP) register. This is called ''direct page addressing''.
Null pointers
Contrary to the zero page's original preferential use, some modern operating systems such as
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 ...
,
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
and
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
actually make the zero page inaccessible to trap uses of
null pointer
In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown lengt ...
s. Such pointer values may legitimately indicate uninitialized values or
sentinel node
In computer programming, a sentinel node is a specifically designated node used with linked lists and trees as a traversal path terminator. This type of node does not hold or reference any data managed by the data structure.
Benefits
Sentinel ...
s, but they do not point to valid objects.
Buggy code may try to access an object via a null pointer, and this can be trapped at the operating system level as a memory
access violation
Access may refer to:
Companies and organizations
* ACCESS (Australia), an Australian youth network
* Access (credit card), a former credit card in the United Kingdom
* Access Co., a Japanese software company
* Access Healthcare, an Indian BPO se ...
.
Interrupt vectors
Some computer architectures still reserve the beginning of address space for other purposes, though; for instance,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
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 ...
systems reserve the first 256 double-words of address space for the
interrupt vector table
An interrupt vector table (IVT) is a data structure that associates a list of interrupt handlers with a list of interrupt requests in a table of interrupt vectors. Each entry of the interrupt vector table, called an interrupt vector, is the addr ...
(IVT) if they run in
real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit seg ...
.
A similar technique of using the zero page for hardware related vectors was employed in the ARM architecture. In badly written programs this could lead to "ofla" behaviour, where a program tries to read information from an unintended memory area, and treats executable code as data or vice versa. This is especially problematic if the zero page area is used to store system jump vectors and the firmware is tricked into overwriting them.
CP/M
In 8-bit
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
, the zero page is used for communication between the running program and the operating system.
Page addressing
In some processor architectures, like that of the
Intel 4004
The Intel 4004 is a 4-bit central processing unit (CPU) released by Intel Corporation in 1971. Sold for US$60, it was the first commercially produced microprocessor, and the first in a long line of Intel CPUs.
The 4004 was the first signific ...
4-bit processor, memory was divided into (256 byte) pages and special precautions had to be taken when the
control flow
In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...
crossed
page boundaries, as some
machine instruction
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 ...
s exhibited different behaviour if located in the last few instructions of a page, so that only few instructions were recommended to jump between pages.
See also
*
Low memory
In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system ...
– the first 64 KB of memory (segment 0) in DOS
*
Page boundary relocation
Relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses. Prior to the advent of multiprocess systems, and still in many embedded ...
References
Further reading
*
*
* {{cite book , title=8086 Family Utilities - User's Guide for 8080/8085-Based Development Systems , chapter=1. Introduction: Segment Alignment , id=Order Number: 9800639-04 , edition=A620/5821 6K DD , version=Revision E , date=May 1982 , orig-year=1980, 1978 , publisher=
Intel Corporation
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 series ...
, location=Santa Clara, California, USA , page=1-6 , url=http://bitsavers.trailing-edge.com/pdf/intel/ISIS_II/9800639-04E_8086_Famility_Utilities_Users_Guide_May82.pdf , access-date=2020-02-29 , url-status=live , archive-url=https://web.archive.org/web/20200229032355/http://bitsavers.trailing-edge.com/pdf/intel/ISIS_II/9800639-04E_8086_Famility_Utilities_Users_Guide_May82.pdf , archive-date=2020-02-29
Memory management