User's Manual
THE
80286
INSTRUCTION SET
AAS-ASCII
Adjust
AL
After Subtraction
Opcode Instruction Clocks
Description
3F
AAS 3 ASCII adjust AL after subtraction
FLAGS MODIFIED
Auxiliary carry,
carry
FLAGS UNDEFINED
Overflow, sign, zero, parity
OPERATION
AASshould
be
executed only
after
a subtraction instruction which left
the
byte result in
the
AL
register.
The
lower nibbles
of
the operands to
the
SUB
instruction should have been in the range 0
through 9 (BCD digits). In this case,
the
AAS
instruction will adjust
AL
to contain
the
correct decimal
digit result.
If
the
subtraction produced a decimal carry, the
AH
register
is
decremented,
and
the
carry
and
auxiliary carry flags
are
set to
1.
If
there was
no
decimal carry, the carry
and
auxiliary
carry
flags
are
set to 0,
and
AH
is
unchanged. In any case,
AL
is
left with its top nibble set to
O.
To convert.AL
to an
ASCII
result, you
can
follow
the
AAS instruction with
OR
AL,30H.
The
precise definition
of
AAS
is
as follows:
if
the
lower four bits
of
AL
are
greater than 9,
or
if
the
auxiliary
carry
flag
is
1,
then decrement
AL
by 6,
AH
by I, and set
the
carry
and
auxiliary carry flags.
Otherwise, reset the carry and auxiliary carry flags.
In
any case, conclude the AAS operation by setting
the
upper
four bits
of
AL
to zero.
PROTECTED MODE EXCEPTIONS
None
REAL ADDRESS MODE EXCEPTIONS
None
B-18