Zero page
Encyclopedia
The zero page is the series of memory address
es at the absolute beginning of a computer
's address space
; that is, the page 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 MOS Technology 6502 processor treats the first 256 byte
s of memory specially, whereas many other processors do not.
The actual size of the zero page in bytes is determined by the microprocessor
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 6502
's index registers' size is 8 bits and the page size is 256 bytes. Therefore, its zero page extends from address 0 to address 255.
In early computers, including the PDP-8
, the zero page had a special fast addressing mode
, which facilitated its use for temporary storage of data
and compensated for the relative shortage of CPU
register
s. The PDP-8 had only one register, so zero page addressing was essential.
Possibly unimaginably by computer users after the 1980s, computer RAM used to be faster than or as fast as the CPU during the 1970s. Thus it made sense to have few registers and use the main memory as an extended pool of extra registers. Since each memory location within the zero page of a 16-bit
address bus
computer may be addressed by a single byte
, it was faster, in 8-bit
data bus machines, to access such a location rather than a non-zero page location.
For example, the MOS Technology 6502
has only one general purpose register (the accumulator). As a result, it used the zero page extensively. Many instruction
s are coded differently for zero page and non-zero page addresses:
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. Obviously, the difference in execution time could significantly improve performance in repetitive code.
Zero page addressing now has mostly historical significance, since the developments in integrated circuit
technology
have made adding more registers to a CPU less expensive and CPU operations much faster than RAM accesses. Some computer architectures still reserve the beginning of address space for other purposes, though; for instance, Intel x86 systems reserve the first 512 words of address space for the interrupt
table if they run in real mode
. A similar technique of using the zero page for hardware related vectors was employed in the ARM architecture, leading in some badly written programs to the infamous "ofla" behaviour.
In fact, quite contrary to the zero page's original preferential use, some modern operating systems such as Linux
and Microsoft Windows
actually make the zero page inaccessible to trap uses of NULL pointers. This is useful, as NULL pointers are the method used to represent the value of a reference that points to nothing. Since code operating on a reference will be written assuming it that the reference actually refers to some valid structure or value, catching the case where the reference points to nothing is of use since it is a sure sign of a bug.
, the zero page is used for communication between the running program and the operating system. See Zero page (CP/M)
for more information.
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
es at the absolute beginning of a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
's address space
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.- Overview :...
; that is, the page 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 MOS Technology 6502 processor treats the first 256 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s of memory specially, whereas many other processors do not.
The actual size of the zero page in bytes is determined by the microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
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 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
's index registers' size is 8 bits and the page size is 256 bytes. Therefore, its zero page extends from address 0 to address 255.
In early computers, including the PDP-8
PDP-8
The 12-bit PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date. It was the first widely sold computer in the DEC PDP series of...
, the zero page had a special fast addressing mode
Addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...
, which facilitated its use for temporary storage of data
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
and compensated for the relative shortage of CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
s. The PDP-8 had only one register, so zero page addressing was essential.
Possibly unimaginably by computer users after the 1980s, computer RAM used to be faster than or as fast as the CPU during the 1970s. Thus it made sense to have few registers and use the main memory as an extended pool of extra registers. Since each memory location within the zero page of a 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
address bus
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
computer may be addressed by a single byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
, it was faster, in 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
data bus machines, to access such a location rather than a non-zero page location.
For example, the MOS Technology 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
has only one general purpose register (the accumulator). As a result, it used the zero page extensively. Many instruction
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
s are coded differently for zero page and non-zero page addresses:
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. Obviously, the difference in execution time could significantly improve performance in repetitive code.
Zero page addressing now has mostly historical significance, since the developments in integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
have made adding more registers to a CPU less expensive and CPU operations much faster than RAM accesses. Some computer architectures still reserve the beginning of address space for other purposes, though; for instance, Intel x86 systems reserve the first 512 words of address space for the interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
table if they run in real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
. A similar technique of using the zero page for hardware related vectors was employed in the ARM architecture, leading in some badly written programs to the infamous "ofla" behaviour.
In fact, quite contrary to the zero page's original preferential use, some modern operating systems such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
and Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
actually make the zero page inaccessible to trap uses of NULL pointers. This is useful, as NULL pointers are the method used to represent the value of a reference that points to nothing. Since code operating on a reference will be written assuming it that the reference actually refers to some valid structure or value, catching the case where the reference points to nothing is of use since it is a sure sign of a bug.
CP/M
In 8-bit CP/MCP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
, the zero page is used for communication between the running program and the operating system. See Zero page (CP/M)
Zero page (CP/M)
The Zero Page is a data structure used in CP/M systems for programs to communicate with the operating system. In 8-bit CP/M versions it is located in the first 256 bytes of memory, hence its name....
for more information.