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Decimal Add Subtract

IP.com Disclosure Number: IPCOM000074689D
Original Publication Date: 1971-May-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Daas, AC: AUTHOR

Abstract

One decimal order module of BCD arithmetic performs the addition A+B, subtraction A-8, or complementation -A, while providing in each case a prediction of the odd parity bit Rp of the result digit R. The carry-out C10 functions as a borrow during subtraction or complementing. The circuit operates in either parallel or serial arithmetic configurations.

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Decimal Add Subtract

One decimal order module of BCD arithmetic performs the addition A+B, subtraction A-8, or complementation -A, while providing in each case a prediction of the odd parity bit Rp of the result digit R. The carry-out C10 functions as a borrow during subtraction or complementing. The circuit operates in either parallel or serial arithmetic configurations.

In each mode of operation, a hexadecimal addition is performed. From the intermediate result C16, S8, S4, S2, S1 a decimal carry C10 is computed, and if this signal is up it actuates the correction circuit to add sic to the immediate sum. During addition, the V-circuits 10 and 12 and V-circuits 11 in the input and output leads have no influence, and R , A+B+C1 is calculated as usual.

Subtraction A-8 is performed by: R = A-B-C1 =15- [15-A)+B+C1]. The Y- circuits 10 in the input leads for A as well as 12 in the output leads for R provide a 15-complement, when the Subtract signal is up. If (15-A)+B+C1 > 15 then a C16 bit is generated. This indicates that A-B-C1 < 0, so the C16 serves as a borrow bit C10 in a straightforward manner, while AND-gate 13 is disabled by inverter 14. Since C16 removed a magnitude of 16 from the intermediate sum, a + 6 correction is performed after which only 10 stays removed, or, after final complementing, 10 has been added to the result in this decimal order, as is due to the borrow C10. Thus: 15- {15-A)+B+C1-16i] = 10-C1+(A-B). Negative results appear in complement form, indicated by C10 from highest order. fo...