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Although on some early computers there were register addresses at the high end of the address range, e.g., IBM 650, [27] [a] IBM 7070, [28] [c] the trend has been to use only register address at the low end and to use only the first 8 or 16 words of memory (e.g. ICL 1900, DEC PDP-6/PDP-10). This meant that there was no need for a separate "add ...
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 ...
There are several memory banks which are one word wide, and one word wide bus. There is some logic in the memory that selects the correct bank to use when the memory gets accessed by the cache. Memory interleaving is a way to distribute individual addresses over memory modules. Its aim is to keep the most of modules busy as computations proceed.
Thus, word addressing allows a computer to address substantially more memory without increasing its address width and incurring the corresponding large increase in memory usage. However, this is valuable only within a relatively narrow range of working set sizes, and it can introduce substantial runtime overheads depending on the application.
In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. [3] A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
The root of the problem is that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range are available. [citation needed] Flat addressing is possible by applying multiple instructions, which however leads to slower programs. The memory model concept derives from the setup of the segment registers.
When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes data from MDR to the memory location whose address is stored in MAR. MAR, which is found inside the CPU, goes either to the RAM (random-access memory) or cache.
The largest possible address size, used to designate a location in memory, is typically a hardware word (here, "hardware word" means the full-sized natural word of the processor, as opposed to any other definition used). Documentation for older computers with fixed word size commonly states memory sizes in words rather than bytes or characters.