![]() ![]() It obviously makes things simpler to have a whole number of bytes per data item. These developments essentially forced computers to have larger address spaces, and ended the practice of having an address in each instruction. At the same time, magnetic core memory allowed building much larger memories than vacuum tubes, electrostatic storage or delay lines. A byte-addressed machine allows you to access individual characters easily, but demands a larger address field. ![]() Doing that in a word-addressed machine is cumbersome, at best. Addresses are 12 bits.Īs the range of uses for computers expanded, handling text data became more and more important. The IBM 704/709 is an example it had a maximum of 4096 words of 36 bits, with six characters per word, each of 6 bits. The size of the address field in instructions determined the memory size. Instructions were normally a single word, and there was often a single address as part of each instruction. A number was typically stored in a machine word, and the desired numerical range determined the size of the word. The priority in the earliest computer designs was to process numbers as rapidly as possible. Overall, an 8-bit byte allowed a reasonably large character set, by the standards of the time, and also allowed two BCD digits per byte. Investment, better use of core storage, faster tapes, and more This efficiency implies, for a given hardware Numeric data in business records is more than twice as frequent asĪlphanumeric. Was coding efficiency, which arises from the fact that the use of The set to grow), and commensurability with a 32/64-bit floating-point Greater coding efficiency, spare bits in the alphabetic set (allowing The 4/8 approach, used in the IBM 650-7074 family and elsewhere, had Specification field structure, and of commensurability with a 48-bitįloating-point word and a 24-bit instruction field. The straight-6 approach, used in the IBM 702-70-7010įamilies, as well as in other manufacturers' systems, had theĪdvantages of familiar usage, existing I/O equipment, simple Might well cost more than the wasted bits in the character. Short-sighted, and (c) the engineering complexities of this approach Used, (b) limiting the alphabetic character to 6 bits seemed Individual characters, even in models where decimal arithmetic is not Versatility and power of manipulating character streams and addressing The 4/6 approach was rejected because (a) was desired it to have the Incorporated a binary recoding of decimal digit pairs, was also Module as the minimum addressable element. Three obvious alternatives were considered - 6 bits for all, with 2 bits wasted on numeric data 4 bits for digits, 8 for alphanumeric, with 2 bits wasted on alphanumeric and 4 bits for digits, 6 for alphanumeric, which would require adoption of a 12-bit In an issue that year of the IBM Technical Journal, an explanation of the choice was offered:Ĭharacter size, 6 vs 4/8: In character size, the fundamental problem is that decimal digits require 4 bits, the alphanumeric characters require 6 bits. That became popular with the introduction of the IBM 360 family of computers in 1964. Much of this grows out of the adoption of the 8-bit byte. ![]()
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