Q.Divide 85 by 10

All 8051 arithmetic is carried out through the accumulator A, and memory-to-memory arithmetic is never allowed. Four operations are studied, each writing worked ALP programs in the board exam.

Addition uses ADD A,source, computing $A = A + source$; the source may be a register, a constant or a memory byte. A carry out of bit 7 sets the carry flag CY, and a carry from bit 3 to bit 4 sets the auxiliary carry AC. For example, $1FH + 5EH = 7DH$.

Subtraction is done only as subtract with borrow, SUBB A,source, giving $A = A - source - CY$. For a plain subtraction the carry is cleared first with CLR C. Internally the chip forms the 2's complement of the subtrahend (1's complement then add 1), adds it to A and inverts the carry; afterwards $CY = 0$ means the result is positive and $CY = 1$ means it is negative. …