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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 (page-sized) structure, which is by default located exactly before offset 0 of the program's load segment, rather than in segment 0.
The address and value parameters may contain expressions, as long as the evaluated expressions correspond to valid memory addresses or values, respectively.A valid address in this context is an address within the computer's address space, while a valid value is (typically) an unsigned value between zero and the maximum unsigned number that the minimum addressable unit (memory cell) may hold.
In a computer using virtual memory, accessing the location corresponding to a memory address may involve many levels. In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. [1] These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned ...
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.
Note that this is more or less the same as base-plus-offset addressing mode, except that the offset in this case is large enough to address any memory location. Example 1: Within a subroutine, a programmer may define a string as a local constant or a static variable. The address of the string is stored in the literal address in the instruction.
Atari BASIC can call machine code subroutines stored in strings or POKEed into memory. The 256 byte area starting at address 1536 10 (600 16) is often used for this purpose. Machine code is invoked with the USR function. The first parameter is the address of the subroutine and the following values are parameters.
; MEMCPY --; Copy a block of memory from one location to another;; Entry parameters in registers; HL: 13-bit address of source data block; DE: 13-bit address of target data block; C: 8-bit count of bytes to copy. (1 to 256 bytes) ORG 2000Q;Code at 002000 octal MEMCPY LBM;Read source byte into B CALL XCHGI;Exchange HL<->DE and increment DE LMB ...