Types of Memory Addresses
There are several types of memory addresses. In other words, a computer, and even one program may have several different memory address spaces.
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a physical address, a code, which the CPU (or other device) can use to access it. Generally only system software, i.e. the BIOS, operating systems, and some specialized utility programs (e.g., memory testers), address physical memory using machine code operands or processor registers, instructing the CPU to direct a hardware device, called the memory controller, to use the memory bus or system bus, or separate control, address and data busses, to execute the program's commands. The memory controllers' bus consists of a number of parallel lines, each represented by a binary digit (bit). The width of the bus, and thus the number of addressable storage units, and the number of bits in each unit, varies among computers.
A computer program uses memory addresses to execute machine code, store and retrieve data. Most application programs do not have a knowledge about physical addresses. Rather, they address logical addresses, or virtual addresses, using computer's memory management unit and operating system memory mapping; see below.
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