SUBR loc16,AX
6-354
SUBR loc16,AX Reverse-Subtract Specified Location From AX
SYNTAX OPTIONS OPCODE OBJMODE RPT CYC
SUBR loc16,AX 1110 101A LLLL LLLL 1 − 1
Operands loc16 Addressing mode (see Chapter 5)
AX Accumulator high (AH) or accumulator low (AL) register
Description Subtract the 16−bit content of the location pointed to by the “loc16”
addressing mode from the specified AX register (AH or AL), and store the
result in location pointed to by ”loc16”:
[loc16] = AX − [loc16]
This instruction performs a read-modify-write operation.
Flags and
Modes
N After the subtraction, [loc16] is tested for a negative condition. If bit 15 of
[loc16] is 1, then the negative flag bit is set; otherwise it is cleared.
Z
After the subtraction, [loc16] is tested for a zero condition. The zero flag
bit is set if the operation generates [loc16] = 0, otherwise it is cleared
C
If the subtraction generates a borrow, C is cleared; otherwise C is set.
V
If an overflow occurs, V is set; otherwise V is not affected. Signed positive
overflow occurs if the result crosses the max positive value (0x7FFF) in
the positive direction. Signed negative overflow occurs if the result
crosses the max negative value (0x8000) in the negative direction.
Repeat This instruction is not repeatable. If this instruction follows the RPT
instruction, it resets the repeat counter (RPTC) and executes only once.
Example
; Subtract index register AR0 from VarA and store in AR0:
MOV AL,@VarA ; Enable
sign extensio ;
with a left shift of 3
; Load AL with contents of VarA
SUBR @AR0,AL ; AR0 = AL − AR0
; Subtract the contents of VarC from VarB and store in VarC:
MOV AH,@VarB ; Load AH with contents of VarB
SUBR @VarC,AH ; VarC = AH − VarC
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