IBM System/7 Functional Characteristics - All about the IBM 1130 ...

11111 !III VIII I 1101 1,411111111,111111 IIII•111111.11.111111111111111,11 1111 III 1111111 1111111111111.11111111111111111 111111.1,Subtract Register (PSR)0 5 8 15Op codeModifier1 1 1 1 1 X X X 0 0 0 00 1 0 1 0III)F 8-F 0 2The contents of the register specified by R are subtracted algebraically from the contentsof the accumulator. The resulting difference replaces the contents of the accumulator. IfR specifies an index register (R01 to 111), the contents of the index register are notchanged.An R field of 000 specifies the accumulator. This means that the accumulator can besubtracted from itself, giving a simple method of setting the accumulator contents to a0 value.The carry and result indicators are set to reflect the resulting difference in theaccumulator. The carry indicator is set on by a borrow from the high-order bit in theaccumulator.The overflow indicator is set to reflect the result only if it has not been set by aprevious operation. If the overflow indicator was previously set on, it will not be changedby the result of a subtract register instruction. If an overflow occurs, the accumulatorwill contain the correct low-order 16 bits of the result and the carry indicator will beturned on if the result is negative.The sign of the result is dependent upon the signs and magnitudes of both operands.Possible combinations (where operand B is always numerically greater than operand A,regardless of signs) are:From Operand Subtract Operand Sign of Resultin Accumulator in Storage in Accumulator+B +A+B —A—B +A ——B —A+A +B+A —B—A +B—A —BThe value in the accumulator is positive if the leftmost bit is a 0; the value in theaccumulator is negative if the leftmost bit is a 1. Negative numbers are in twos-complementform.Example:0 5 8 15PSR instruction(R = 101 specifies index registerOp code R Modifier5 is subtracted from accumulator) 1 1 1 1 1 1 0 1 0 0 0 010 0 1 0III) I I iiiiiiiF D 0 2Contents of accumulator 0000 0000 1110 1111Contents of index register 5 —0000 0000 0011 0101Result in accumulator0000 0000 1011 1010(in decimal: 239-53 = 186)