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much less useful for scientific and engineering users. Or consider 32-bit x86 processors. Their 32-bit linear addresses can address 4 billion different items. Using word addressing, a 32-bit processor could address 4 Gigawords; or 16 Gigabytes using the modern 8-bit byte. If the 386 and its successors had used word addressing, scientists, engineers, and gamers could all have run programs that were 4x larger on 32-bit machines. However, word processing, rendering HTML, and all other text applications would have run more slowly.
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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
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The advantage of word addressing is that more memory can be addressed in the same number of bits. The IBM 7094 has 15-bit addresses, so could address 32,768 words of 36 bits. The machines were often built with a full complement of addressable memory. Addressing 32,768 bytes of 6 bits would have been
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in the new one, and then store back the amended word. At least six machine instructions. Usually, these are relegated to a subroutine, so every store or fetch of a single character involves the overhead of calling a subroutine and returning. With byte addressing, that can be achieved in one
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instructions incremented the byte pointer and then loaded or stored the next byte. These instructions could operate on arbitrary-width bit fields. Programs took advantage of this flexibility: those not needing lowercase letters used the limited character set of
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and can handle 32-bit (4-byte) data in single instructions. However, data in memory may be of various lengths. Instruction sets that support byte addressing supports accessing data in units that are narrower than the word length. An eight-bit processor like the
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When computers were so costly that they were only or mainly used for science and engineering, word addressing was the obvious mode. As it became cost-effective to use computers for handling text, hardware designers moved to byte addressing.
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bytes before or after the byte to which it currently pointed required a sequence of multiple instructions. The KL10 PDP-10 model extended the
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instruction: store this character code at that byte address. Text programs are easier to write, they are smaller, and run faster.
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Neither of these machines originally had direct machine support for random access to bytes; adjusting a byte pointer to point
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To illustrate why byte addressing is useful, consider the
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DECsystem-10/DECSYSTEM-20 Processor
Reference Manual
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On the PDP-6/10, special instructions operated on a
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