8051 Program - decrement dptr

; Description:

; Decrement DPTR. CY == 1 If Underflow

;

; Entry Requirements:

; DPTR Has Value To Decrement

;

; On Exit:

; DPTR = DPTR - 1, CY Set Accordingly

;

; Affected:

; PSW.CY, DPTR

;

; Stack:

; 1 Bytes, Not Including Space Used By Called Routines

;

; Comments:

; None

UTIL_DPTRDEC:
	push acc	; Save Acc
	clr c	; Clear For SUBB
	mov a,dpl	; Move Low Of DPTR To A
	subb a,#1	; Subtract 1
	mov dpl,a	; Store Back
	mov a,dph	; Get High Of DPTR
	subb a,#0	; Subtract CY If Set
	mov dph,a	; Move Back
	pop acc	; Recover Acc
	ret	; Return To Caller

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

; another approach
DEC_DPTR:
	XCH A,DPL	;Exchange A for DPL
	DEC A	; Decrement A (which is DPL)
	CJNE A,#0FFh,_dec_dptr2	; If A (DPL) is not #0FFh, continue normally
	DEC DPH	; If A=FFh, we need to decrement DPH
_dec_dptr2:	XCH A,DPL	; Exchange A for DPL (thus saving DPL and restoring A)
	RET

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List of topics: 8051

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