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The same carry bit is also generally used to indicate borrows in subtraction, though the bit's meaning is inverted due to the effects of two's complement arithmetic. Normally, a carry bit value of "1" signifies that an addition overflowed the ALU , and must be accounted for when adding data words of lengths greater than that of the CPU.
"Subtraction" is an English word derived from the Latin verb subtrahere, which in turn is a compound of sub "from under" and trahere "to pull". Thus, to subtract is to draw from below, or to take away. [7] Using the gerundive suffix-nd results in "subtrahend", "thing to be subtracted".
American schools teach a method of subtraction using borrowing. [11] A subtraction problem such as is solved by borrowing a 10 from the tens place to add to the ones place in order to facilitate the subtraction. Subtracting 9 from 6 involves borrowing a 10 from the tens place, making the problem into +. This is indicated by crossing out the 8 ...
The first uses the bit as a borrow flag, setting it if a<b when computing a−b, and a borrow must be performed. If a≥b, the bit is cleared. A subtract with borrow (SBB) instruction will compute a−b−C = a−(b+C), while a subtract without borrow (SUB) acts as if the borrow bit were clear.
Subtraction works in much the same way: 0 − 0 → 0 0 − 1 → 1, borrow 1 1 − 0 → 1 1 − 1 → 0. Subtracting a "1" digit from a "0" digit produces the digit "1", while 1 will have to be subtracted from the next column. This is known as borrowing. The principle is the same as for carrying.
The full subtractor is a combinational circuit which is used to perform subtraction of three input bits: the minuend , subtrahend , and borrow in . The full subtractor generates two output bits: the difference D {\displaystyle D} and borrow out B out {\displaystyle B_{\text{out}}} .
Subtract with borrow: B is subtracted from A (or vice versa) with borrow (carry-in) and the difference appears at Y and carry-out (borrow out). Two's complement: A (or B) is subtracted from zero and the difference appears at Y. Increment: A (or B) is increased by one and the resulting value appears at Y.
8-bit program status word PSW. This includes a carry flag (borrow bit on subtract), auxiliary carry flag and zero flag, but not a negative flag or overflow flag. The former is unnecessary, as it is possible to branch on the most significant bit of the accumulator directly, but the lack of the latter makes signed comparisons awkward.
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