AMD Am186 Series, Набор инструкций

Страница: 105 / 320

Instruction Set 4-65
IMUL IMUL
Flag Settings After Instruction
Examples
This example uses the single-operand form of IMUL to multiply an 8-bit integer in memory
by an integer in AL.
/* (continued) */
if (operands() == 2)
/* substitute ”product” for multiplicand */
multiplicand = product;
if (operands() >= 2)
{
temp = multiplicand * multiplier;
if (size(temp) == size( product))
/* product will fit */
{
/* store result */
product = temp;
/* clear overflow and carry flags */
OF = CF = 0;
}
else
/* product won’t fit */
{
/* store only lower half of result */
product = 0x00FF & temp;
/* set overflow and carry flags */
OF = CF = 1;
}
}
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Processor Status
Flags Register
reserved
OF DF IF TF SF ZF AF PF CF
? = undefined; – = unchanged
For the single-operand form:
CF and OF = 1 if the product is large enough to
require the full destination.
CF and OF = 0 if the product is small enough to fit
in the
low half of the destination.
– – – ? ? res ? res ? res
For the two- and three-operand forms:
CF and OF = 1 if the product is too large to fit in the
destination.
CF and OF = 0 if the product is small enough to fit
in the destination.
BMULTIPLICAND DB -10 ; F6h
; 8-bit integer multiplication: AX = BMULTIPLICAND * AL
MOV AL,7 ; AL = 07h = 7
IMUL BMULTIPLICAND ; AX = FFBAh = -70