User's Manual
BASIC INSTRUCTION SET
Example:
SUB
WORDOPRND,
AX.
Replaces
the
contents
of
the
destination
operand
WORDOPRND
with the result obtained by subtracting the contents of AX from the
contents of the memory word labeled
WORDOPRND.
SBB
(Subtract Integers with Borrow) subtracts the source operand from the destination operand,
subtracts 1
if
CF
is
set, and returns the result to the destination operand. The operands may be signed
or unsigned bytes or words.
SBB may be used to subtract numbers longer than
16
bits. This instruction
affects
OF, SF, ZF, AF, PF, and CF. The carry flag
is
set if a borrow
is
required.
Example:
SBB BL, 32. Subtracts
32
from the contents of BL and then decrements the result
of
this
subtraction by one if
CF
is
set.
If
CF
is
cleared, SBB performs the same operation as SUB.
DEC
(Decrement) subtracts 1 from the destination operand.
DEC
updates AF, OF, PF, SF, and ZF,
but it does not affect CF.
Use SUB with an immediate value of 1 to perform a decrement
that
affects
carry.
Example:
DEC
BX. Subtracts 1 from the contents of BX and places the result back in
BX.
3.3.3
Multiplication Instructions
MUL (Unsigned Integer Multiply) performs an unsigned multiplication of the source operand and the
accumulator.
If
the source
is
a byte, the processor multiplies it by the contents of AL and returns the
double-length result to AH and AL.
If
the source operand
is
a word, the processor multiplies
it
by the contents of AX and returns the
double-length result to DX and AX.
MUL
sets
CF
and OF to indicate that the upper half of the result
is
nonzero; otherwise, they are cleared. This instruction leaves SF, ZF, AF, and
PF
undefined.
Example:
MUL
BX. Replaces the contents of DX and AX with the product of BX and
AX.
The
low-
order
16
bits of the result replace the contents of AX; the high-order word goes to DX. The
processor sets
CF
and OF if the unsigned result
is
greater than
16
bits.
IMUL
(Signed Integer Multiply) performs a signed multiplication operation.
IMUL
uses AX and
DX
in the same way as the
MUL
instruction, except when used in the immediate form.
The immediate form of
IMUL
aiiows the specificaiion of a destination register other than the combi-
nation of
DX
and AX. In this case, the result cannot exceed
16
bits without causing an overflow.
If
the immediate operand
is
a byte, the processor automatically extends it to
16
bits before performing
the multiplication.
The immediate form of
IMUL
may also be used with unsigned operands because the
low
16
bits of a
signed or unsigned multiplication of two 16-bit values will always be the same.
IMUL
clears
CF
and
OF
to indicaie ihai iht: upper half of thc rcsult b the
gig!'!
of the
low,,"!
h~Jf
This
instruction leaves SF, ZF, AF, and
PF
undefined.
Example:
IMUL
BL. Replaces the contents
of
AX
with the product of BL and AL. The processor
sets
CF
and
OF
if the result
is
more than 8 bits long.
Example:
IMUL
BX, SI,
5.
Replaces the contents of BX with the product of the contents of
SI
and
an immediate value of
5.
The processor sets
CF
and
OF
if the signed result
is
longer than
16
bits.
3-8