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Subroutine Subbtraction for 8051 Microcontroller

;====================================================================
; subroutine SUB16
; 16-Bit Signed (2's Complement) Subtraction
;
; input: r1, r0 = X
; r3, r2 = Y
;
; output: r1, r0 = signed difference D = X - Y
; Carry C is set if the result (D) is out of range.
;
; alters: acc, C, OV
;====================================================================

SUB16:     anl PSW, #0E7H ; Register Bank 0
           mov a, r0 ; load X low byte into acc
           clr C ; clear carry flag
           subb a, r2 ; subract Y low byte
           mov r0, a ; put result in Z low byte
           mov a, r1 ; load X high into accumulator
           subb a, r3 ; subtract Y high with borrow
           mov r1, a ; save result in Z high byte
           mov C, OV
           ret

;====================================================================
; subroutine SUB32
; 32-Bit Signed (2's Complement) subtraction
;
; input: r3, r2, r1, r0 = X
; r7, r6, r5, r4 = Y
;
; output: r3, r2, r1, r0 = signed difference D = X - Y
; Carry C is set if the result (D) is out of range.
;
; alters: acc, C, OV
;====================================================================

SUB32:     anl PSW, #0E7H ; Register Bank 0
           mov a, r0 ; load X low byte into acc
           clr C ; clear carry flag
           subb a, r4 ; subract Y low byte
           mov r0, a ; put result in Z low byte
           mov a, r1 ; repeat with other bytes...
           subb a, r5
           mov r1, a
           mov a, r2
           subb a, r6
           mov r2, a
           mov a, r3
           subb a, r7
           mov r3, a
           mov C, OV ; set C if external borrow
           ret

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