Logical & Arithmetic Operations

1, mul operation instruction

In the Assembly language, the multiplication operation is convenient to store the value, and the high and low bits of the value are generally distinguished in different registers

Multiplication Prestore:

The number to be multiplied the number of digits is the same (8bits/16bits)

In the case of eight bits, one is stored in AL, and the other is docked and placed in a memory unit or register;

In the case of sixteen bits, the number two is stored in AX, and the other is placed in a 16-bit register or memory unit

Data results:

If the multiplication operation is multiplied by 8-bit data, the result is placed in AX by default; if it is 16-bit data, the high-order result is placed in DX by default, and the position is in AX by default

mov ax , 100 mov bx , 1000 mul bx ---->ax = 4240H dx = 000FH

2, adc command

Use the flag register CF (carry flag) to carry out the addition operation with carry

①Low-order addition

②The carry value generated by the addition of the upper and lower bits and the upper and lower bits

adc ax , bx ; ( ax ) = ( ax ) + ( bx ) + CF

3, sbb instruction

Use the flag register CF (carry flag) to carry out the subtraction operation with borrowing

sbb ax , bx ; ( ax ) - ( bx ) - CF

For multi-digit subtraction calculations:

003E1000H - 00202000H

mov bx 003EH mov ax 1000H sub ax , 2000H sbb bx , 0020H

4. cmp command

Subtraction and comparison operations, logarithmic subtraction, do not save the result, record the cmp result by changing the register bit value of ZF, CF, SF, etc.

cmp ax , ax zf = 1,pf = 1,sf = 0,cf = 0, of = 0

The rewriting of a single register by cmp cannot get the correct result, and it needs to be combined with multiple bits of the flag register before making a judgment

OF and SF cmp command judgment: (cmp ah , bh)

①sf = 1,of = 0---->ahah>bh

③sf = 0,of = 0---->ahah>=bh

The actual result caused by the overflow is the opposite of the logical true result