Help needed - BCD

[Heaven/TQA]

Hi guys,

 

as some of you might know i am heavily working on a diablo-look a like on a800 (so...thats why i am posting my question here in 2600... ) and i am doing some BCD calculations/conversions esp. for the on-screen display. i got bin2bcd routines from www.6502.org but i am not sure if there work with signed numbers as well...

 

so...my question is... does the BCD mode of 6502 calculate with signed numbers as well? is there anything special i have to keep in mind? some of my weapons player can found should have malus instead of bonus and should alter the stats correctly...

 

http://www.6502.org/source/

 

so... how would be -5 be represented in BCD?

[Heaven/TQA]

issue solved... it seems that negative values can not work in bcd as $fe does not convert correctly to bcd...

 

here is the code

 

	org $4000
	
init cld
lda #-5

jsr writebcd_ascii

ende jmp ende

writebcd_ascii jsr bin2bcd
	lda bcd

writebcd_ascii_direct pha
	lsr @
	lsr @
	lsr @
	lsr @
	ldy #0
	jsr writenumber_ascii
	pla
	and #$0f
writenumber_ascii clc
	adc #48
	sta ascii+1,y
	iny
	rts



;first convert A into BCD
bin2bcd	tax
	lda #"+"
	sta ascii
	txa
	bpl INBCD8	;if already positive then procede as normal
;negative value so make it positve
	eor #$ff
	clc
	adc #1
	ldx #"-"
	stx ascii
INBCD8	SED	; Switch to decimal mode
	sta bin
	LDA #0	; Ensure the result is clear
	STA BCD+0
	STA BCD+1
	LDX #8	; The number of source bits
	
CNVBIT	ASL BIN	; Shift out one bit
	LDA BCD; And add into result
	ADC BCD
	STA BCD
	LDA BCD+1; propagating any carry
	ADC BCD+1
	STA BCD+1
	DEX	; And repeat for next bit
	BNE CNVBIT
	CLD	; Back to binary
	rts

	
	org $0600
bin		dta 0
bcd		dta 0,0

ascii	dta c"+00",0			;holds 2 digits in ascii and the sign 


run init

[+SpiceWare]

Use SBC #$05 to subtract 5 in decimal mode.

[Heaven/TQA]

yes... that would be a solution but i want to have a hex value, doesnt matter if positive or negative range into BCD and then into an ascii string "+00" or "-00" to print that correctly on screen...

[supercat]

so... how would be -5 be represented in BCD?

 

It would be $95. Note that the "N" flag is generally not useful in BCD unless the most negative value used is -$20 (also known as $80); note also that even with that caveat the ADC/SBC instructions will not set the N flag in consistent or useful fashion.