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Guy Harris (talk | contribs) As the last sentence in the paragraph says, "With standardization on 8-bit bytes and byte addressability, stating memory sizes in bytes, kilobytes, and megabytes ... has become the norm". Some older byte-addressable machines may still have given memory sizes in words, but I don't know of anybody doing that *now* for *current* computers. |
Peter Flass (talk | contribs) →{{anchor|Variable word length}}Variable-word architectures: add timing for 1620 |
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Early machine designs included some that used what is often termed a ''variable word length''. In this type of organization, an operand has no fixed length. Depending on the machine and the instruction, the length might be denoted by a count field, by a delimiting character, or by an additional bit called, e.g., flag, or [[word mark (computer hardware)|word mark]]. Such machines often use [[binary-coded decimal]] in 4-bit digits, or in 6-bit characters, for numbers. This class of machines includes the [[IBM 702]], [[IBM 705]], [[IBM 7080]], [[IBM 7010]], [[UNIVAC 1050]], [[IBM 1401]], [[IBM 1620]], and [[RCA]] 301.
Most of these machines work on one unit of memory at a time and since each instruction or datum is several units long, each instruction takes several cycles just to access memory. These machines are often quite slow because of this. For example, instruction fetches on an [[IBM 1620 Model I]] take 8 cycles (160 μs) just to read the 12 digits of the instruction (the [[IBM 1620 Model II|Model II]] reduced this to 6 cycles, or 4 cycles if the instruction did not need both address fields). Instruction execution takes a variable number of cycles, depending on the size of the operands.
=== Word, bit and byte addressing ===
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