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In computer architecture, 256-bit integers, memory addresses, or other data units are those that are 256 bits (32 octets) wide. Also, 256-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers , address buses , or data buses of that size.
In 8-bit CP/M versions it is located in the first 256 bytes of memory, hence its name. The equivalent structure in DOS is the Program Segment Prefix (PSP), a 256-byte structure, which, however, is by default located at offset 0 in the program's load segment (rather than in segment 0) immediately preceding a loaded program.
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 and MOS Technology 6502 processor families treat the first 256 bytes of memory specially, [1] whereas many other processors do not.
For instance, many 8-bit processors, such as the MOS Technology 6502, supported 16-bit addresses— if not, they would have been limited to a mere 256 bytes of memory addressing. The 16-bit Intel 8088 and Intel 8086 supported 20-bit addressing via segmentation , allowing them to access 1 MiB rather than 64 KiB of memory.
On most modern computers, this is an eight bit string. Because the definition of a byte is related to the number of bits composing a character, some older computers have used a different bit length for their byte. [2] In many computer architectures, the byte is the smallest addressable unit, the atom of addressability, say. For example, even ...
Designers of these processors included a partial remedy known as "zero page" addressing. The initial 256 bytes of memory ($0000 – $00FF; a.k.a., page "0") could be accessed using a one-byte absolute or indexed memory address. This reduced instruction execution time by one clock cycle and instruction length by one byte.
A significant use of this feature is to allow small routines that can fit within the 256 bytes of a page to use zero-page addressing (now known as base page addressing) which makes the code smaller because addresses no longer have a second byte, which also makes the code run faster because the second byte does not have to be fetched from memory ...
(Because the AVR program counter counts 16-bit words, not bytes, a 12-bit offset is sufficient to address 2 13 bytes of ROM.) Additional memory addressing capabilities are present as required to access available resources: Models with >256 bytes of data address space (≥256 bytes of RAM) have a 16-bit stack pointer, with the high half in the ...