8051 Program - 2s complement dptr

; Description:

; Subtract 'DPTR' From 0, Setting Carry If DPTR Underflows

;

; Entry Requirements:

; DPTR Has Value To Perform 2's Complement On

;

; On Exit:

; DPTR = 0 - DPTR

;

; Affected:

; PSW.CY

;

; Stack:

; 1 Bytes, Not Including Space Used By Called Routines

;

; Comments:

; None

UTIL_DPTR2C:
	push acc	; Make Sure Acc Gets Saved
	clr c	; Clear For SUBB
	clr a	; Clear For Subtract
	subb a,dpl	; Subtract
	mov dpl,a	; Move Back To DPL
	clr a	; Clear For Subtract
	subb a,dph	; If Carry Set, This Will Decrement
	mov dph,a	; Move Back To DPH
	pop acc	; Recover Original 'A'
	ret	; Return To Caller

Source: Assorted Utilities, John C. Wren 11/23/96

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List of topics: 8051

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