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In computer architecture, word addressing means that addresses of memory on a computer uniquely identify words of memory. It is usually used in contrast with byte addressing, where addresses uniquely identify bytes. Almost all modern computer architectures use byte addressing, and word addressing is largely only of
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 a byte-addressable machine with storage-to-storage (SS) instructions, there are typically move instructions to copy one or multiple bytes from one arbitrary location to another. In a byte-oriented (byte-addressable) machine without SS instructions, moving a single byte from one arbitrary location to another is typically: LOAD the source byte
Very often, when referring to the word size of a modern computer, one is also describing the size of address space on that computer. For instance, a computer said to be "32-bit" also usually allows 32-bit memory addresses; a byte-addressable 32-bit computer can address 2 32 = 4,294,967,296 bytes of memory, or 4 gibibytes (GiB). This allows one ...
An architecture may use "big" or "little" endianness, or both, or be configurable to use either. Little-endian processors order bytes in memory with the least significant byte of a multi-byte value in the lowest-numbered memory location. Big-endian architectures instead arrange bytes with the most significant byte at the lowest-numbered address.
The smallest unit of addressable and writable memory is the 8-bit byte. Bytes can also be held in the lower half of registers R0 through R5. 16-bit words are stored little-endian with least significant bytes at the lower address. Words are always aligned to even memory addresses. Words can be held in registers R0 through R7.
A one-word sequence descriptor in memory, called a "byte pointer", held the current word address within the sequence, a bit position within a word, and the size of each byte. Instructions existed to load and store bytes via this descriptor, and to increment the descriptor to point at the next byte (bytes were not split across word boundaries).
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.