Zero page

The zero page is the series of memory addresses 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 bytes 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 registers. The PDP-8 had only one register, so zero page addressing was essential.

Unlike more modern hardware, in the 1970s computer RAM used to be as fast 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. 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.

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