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An eight-bit processor like the Intel 8008 addresses eight bits, but as this is the full width of the accumulator and other registers, this could be considered either byte-addressable or word-addressable. 32-bit x86 processors, which address memory in 8-bit units but have 32-bit general-purpose registers and can operate on 32-bit items with a ...
In theory, modern byte-addressable 64-bit computers can address 2 64 bytes (16 exbibytes), but in practice the amount of memory is limited by the CPU, the memory controller, or the printed circuit board design (e.g., number of physical memory connectors or amount of soldered-on memory).
A memory address a is said to be n-byte aligned when a is a multiple of n (where n is a power of 2). In this context, a byte is the smallest unit of memory access, i.e. each memory address specifies a different byte. An n-byte aligned address would have a minimum of log 2 (n) least-significant zeros when expressed in binary.
If that memory is arranged in a byte-addressable flat address space using 8-bit bytes, then there are 65,536 (2 16) valid addresses, from 0 to 65,535, each denoting an independent 8 bits of memory. If instead it is arranged in a word-addressable flat address space using 32-bit words, then there are 16,384 (2 14 ) valid addresses, from 0 to ...
Each byte is identified and accessed in hardware and software by its memory address. If the total number of bytes in memory is n, then addresses are enumerated from 0 to n − 1. Computer programs often use data structures or fields that may consist of more data than can be stored in one byte.
A processor with 128-bit byte addressing could directly address up to 2 128 (over 3.40 × 10 38) bytes, which would greatly exceed the total data captured, created, or replicated on Earth as of 2018, which has been estimated to be around 33 zettabytes (over 2 74 bytes). [1] A 128-bit register can store 2 128 (over 3.40 × 10 38) different values.
An iconic example of virtual-to-physical address translation is virtual memory, where different pages of virtual address space map either to page file or to main memory physical address space. It is possible that several numerically different virtual addresses all refer to one physical address and hence to the same physical byte of RAM.
Hence, a microprocessor with 12-bit memory addresses can directly access 4096 words (4 kW) of word-addressable memory. IBM System/360 instruction formats use a 12-bit displacement field which, added to the contents of a base register, can address 4096 bytes of memory in a region that begins at the address in the base register.