The zero page or base page is the block of memory at the very beginning of a computer'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 s ...

; that is, the

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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 microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System (latter dubbed ''68xx'') that also included serial and paral ...

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 te ...

processor families treat the first 256

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s of memory specially, whereas many other processors do not. Unlike more modern hardware, in the 1970s computer RAM 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 ...

and comparatively narrow

8-bit In computer architecture, 8-bit integers or other data units are those that are 8 bits wide (1 octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses ...

data bus, 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 technology 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 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

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, 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 i ...

, which facilitated its use for temporary storage of data and compensated for the relative shortage of CPU

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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 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

and comparatively narrow

data bus, accessing zero page locations could be faster than accessing other locations. Since zero page locations could be addressed by a single

, the instructions accessing them could be smaller and faster loading. For example, the

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 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 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, Linux and Microsoft Windows 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 leng ...

s. Such pointer values may legitimately indicate uninitialized values or sentinel nodes, 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

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Interrupt vectors

Some computer architectures still reserve the beginning of address space for other purposes, though; for instance, Intel x86 systems reserve the first 256 double-words of address space for the interrupt vector table (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 se ...

. 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. Initially ...

, 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 4-bit processor, memory was divided into (256 byte) pages and special precautions had to be taken when the

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crossed page boundaries, as some machine instructions exhibited different behaviour if located in the last few instructions of a page, so that only few instructions were recommended to jump between pages.

Size

Computers with few CPU registers

Null pointers

Interrupt vectors

CP/M

Page addressing

See also

References

Further reading