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IBM 7090/94 Binary Representation

The 7090/94 was a binary computer (there were, at the time and before, also decimal (not to mention analog) machines) and was also considered at the time to be a parallel machine in that all of the bits in the registers and in any data word in memory were all read at the same time -- not serially from, for example, a mercury delay line that was commonly used a few years before for memory before the advent of ferrite core memory.

Each memory location contained 36-bits and the machine had a fixed-length word (some other machines of the era, especially serial machines, had variable length words). There was no byte addressing (it is, in fact, not clear if that term had yet been invented). With integers, each number would occupy a full 36-bit location even though the number might require fewer bits for representation. The left-most bit of the set was the sign bit so that 35-bits were available for representing the quantity (the machine was, therefore, a 1's-complement machine which was common for the time -- the machines of today are usually 2's-complement -- even Cray supercomputers which finally switched to 2's-complement not too very long ago (Press Here to see an interesting discourse on the 1's-complement Univac 1100 machines written by John Walker).

A '0' in the sign bit indicated a positive quantity and a '1' indicated a negative quantity which lead to the interesting fact that '-0' existed and was not equal to '+0' (which is why things like IF( IABS(K) .EQ. 0) ... are frequently seen in ancient FORTRAN decks).

Binary Examples
Binary Representation in MemoryDecimal Equivalent
000000000000000000000000000010101011+171
100000000001000000000000000000000111-16,777,223
0000000000000000000000000000000000000
100000000000000000000000000000000000-0

The Octal numbering convention was used with the 7090/94. With the FAP assembler, octal constants were denoted, in source code, with a leading O (the letter 'oh') -- O253 = +171 (decimal). In assembly listings, 36-bit octal constants were most frequently printed as the sign-bit ('+' or '-') followed by a 35-bit octal value (e.g., OCT -2777 yielded '-00000002777').

http://www.frobenius.com/binary.htm -- Last Revision: 23 August 2001
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