Memory Address - Unit of Address Resolution

Unit of Address Resolution

See also: Word (computer architecture) and Binary prefix#Main memory

Most modern computers are byte-addressable, with each address identifying a single eight bit byte of storage; data too large to be stored in a single byte may reside in multiple bytes occupying a sequence of consecutive addresses. There exist word-addressable computers, where minimal addressable storage unit is exactly the processor's word. For example, the Data General Nova minicomputer, and the Texas Instruments TMS9900 and National Semiconductor IMP-16 microcomputers used 16 bit words, and there were many 36-bit mainframe computers (e.g., PDP-10) which used 18-bit word addressing, not byte addressing, giving an address space of 218 36-bit words, approximately 1 megabyte of storage. The efficiency of addressing of memory depends on the bit size of the bus used for addresses – the more bits used, the more addresses are available to the computer. For example, a 8-bit-byte-addressable machine with a 20-bit address bus (e.g. Intel 8086) can address 220 (1,048,576) memory locations, or one MiB of memory, while a 32-bit bus (e.g. Intel 80386) addresses 232 (4,294,967,296) locations, or a 4 GiB address space. In contrast, a 36-bit word-addressable machine with an 18-bit address bus addresses only 218 (262,144) 36-bit locations (9,437,184 bits), equivalent to 1,179,648 8-bit bytes, or 1152 KB, or 1.125 MiB—slightly more than the 8086.

Some older computers (decimal computers), were decimal digit-addressable. For example, each address in the IBM 1620's magnetic-core memory identified a single six bit binary-coded decimal digit, consisting of a parity bit, flag bit and four numerical bits. The 1620 used 5-digit decimal addresses, so in theory the highest possible address was 99,999. In practice, the CPU supported 20,000 memory locations, and up to two optional external memory units could be added, each supporting 20,000 addresses, for a total of 60,000 (00000–59999).