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;====================================================================
; 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