AMD Am186 Series, Instruction Set

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Instruction Set 4-162
MUL MUL
Flag Settings After Instruction
Examples
This example multiplies a 16-bit unsigned number in CX by a 16-bit unsigned number in
AX. If the product is small enough to fit in only the low word of the destination, this example
stores only the low word of the destination in memory.
Tips
Use SHL instead of MUL to multiply unsigned numbers by powers of 2. When multiplying
an unsigned number by a power of 2, it is faster to use SHL than MUL.
Much of the time, you can ignore the high half of the result because the product is small
enough to fit in only the low half of the destination. If it is, MUL clears CF and OF to 0;
otherwise, MUL sets CF and OF to 1.
If the result will fit in a register that is the size of the multiplicand, and you either want to
multiply an unsigned number by an immediate number or you don’t want the result to
overwrite AL or AX, use the second and third forms of IMUL instead of MUL. Although
designed for multiplying integers, these forms of IMUL calculate the same result as MUL
while letting you specify more than one operand.
Related Instructions
If you want to See
Convert an 8-bit unsigned binary product to its unpacked decimal equivalent AAM
Multiply two integers IMUL
Multiply an unsigned number by a power of 2 SHL
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
OF and CF = 0 if upper half of result is 0
OF and CF = 1 otherwise
– – – ? ? res ? res ? res
WPRODUCTH DW ?
WPRODUCTL DW ?
; 16-bit unsigned multiplication: DX::AX = CX * AX
MOV CX,32
MOV AX,300
MUL CX ; DX::AX = 00002580h = 9600
; store low word of product
MOV WPRODUCTL,AX
; if product fits in only low half of destination, then jump
JNC CONTINUE ; ignore high half
; store high word of product
MOV WPRODUCTH,DX
CONTINUE:
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