Intel XENIX 286 Programmers Guide (86) - Tenox.tc

as: A sse m bier
sub
subb
test
testb
wait
xchg
xchgb
xlat
xor
xorb
Byte Instructions
subtract
subtract byte
test
test byte
wait while TEST pin
exchange
exchange byte
translate
exclusive OR
exclusive OR byte
*
*
*
*
*
XENIX Programming
The XENIX assembler extends the definition of several instruction mne monics to include
an explicit byte "b" suffix. This suffix forces the operands in the instruction to be
treated as bytes when they would otherwise be treated as words. There are the
following byte instructions (not including byte string instructions):
adcb imulb rclb shlb
ad db incb rcrb shrb
an db movb rolb subb
cmpb mulb rorb testb
decb negb salb xchgb
divb notb sarb xorb
idivb orb sbbb
The byte instructions are especially useful when operating on memory operands defined
with the .byte directive. Since as does not assign an explicit type to symbols created
with the .byte or .word directives, it cannot detect the size of the associated item when
given in an instruction. For example, if test_byte and test_ word have been defined as
test byte:
test - word :
then the statements
.byte 1
.word 1
negb test byte
neg test - word
are required to operate on these values correctly. If neg were applied to test_byte, part
of test_word would be destroyed in the operation.
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