Decimal computers are computers which can represent numbers and addresses in decimal as well as providing instructions to operate on those numbers directly in decimal, without conversion to a pure binary representation. Examples of such representations are binary-coded decimal or BCD, Excess-3, two-out-of-five code, ASCII, and EBCDIC.
Many early computers used decimal arithmetic, some examples being the ENIAC, IBM 702, IBM 705, IBM 650, IBM 1401, IBM 1620, IBM NORC, IBM 7070, IBM 7080, Electrodata 200, Burroughs B2500/3xxx/4xxx, UNIVAC I, UNIVAC II and UNIVAC III. The Burroughs machines listed above used only decimal arithmetic, while other Burroughs machines offered both binary and decimal operations. The IBM 1401 used a combination of decimal and binary arithmetic. Some also had a variable wordlength, which enabled operations on numbers with a large number of digits. The Burroughs 2500-4900 supported up to 99 digit floating point precision.
Later, several microprocessors, such as 6502 and Z80, offered limited BCD arithmetic support in the form of special modes, flags and/or decimal adjust instructions. The Intel 80x86 family of microprocessors provide instructions to convert one-byte BCD numbers to binary. These operations were not extended to wider formats and hence are now slower than using 32-bit or wider BCD 'tricks' to compute in BCD (see ).
Decimal arithmetic is now becoming more common; for instance, three decimal floating-point types with two encodings have been added to the new IEEE 754 standard, with 7, 16, and 34-digit decimal significands. The IBM Power6 processor, the IBM System z9, and the IBM System z10 have implemented these types using the Densely Packed Decimal scheme for encoding the digits of the significand, although a binary encoding is still used for the exponent., the first and third in hardware and the second in microcode.
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