Q.Write the instructions to move value 34H into register A and value 3FH into register B, then add them together.

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.

Multiplication uses MUL AB (no comma): the unsigned bytes in A and B are multiplied to give a 16-bit product whose low byte stays in A and high byte goes to B, e.g. $35H \times 45H = 0E49H$.

Division uses DIV AB: the byte in A is divided by the byte in B, leaving the quotient in A and the remainder in B. If $B = 0$ the operation is invalid and the overflow flag OV is set; otherwise DIV clears CY and OV. Mastering these four operations, and how each affects the flags, is essential for 8051 assembly programming.