Help with BCD

[vdub_bobby]

Can someone please tell me why this doesn't work?

	sed  ;set decimal flag

lda #0

clc

adc #99  ;set timer to 99 for now.

sta Timer

cld  ;clear decimal flag. 

 

I want Timer to hold #%10011001 ($99)

 

but this puts #%01100011 (99) in Timer instead. What am I doing wrong?

 

Do I need to use hex?

[CPUWIZ]

Why don't you just do this?

 

lda #$99

sta Timer

 

[vdub_bobby]

Why don't you just do this?

 

lda #$99

sta Timer

 

 

Well, I could. But I would like to know how BCD works

 

Because I also want to put in a score at some point, and if I want to add 99 to the score, do I need to use hex?

[Thomas Jentzsch]

Yup, you need hex, so you missed the '$' before 99.

[CPUWIZ]

if I want to add 99 to the score, do I need to use hex?

 

Yes.

[Cybergoth]

Hi there!

 

Because I also want to put in a score at some point, and if I want to add 99 to the score, do I need to use hex?

 

Yup. It's easier to read.

The correct decimal value for adding 99 in bcd would be 153

 

For a 6 digit bcd score, this might come in handy at times:

 

AddScore

SED

CLC

ADC bcdScore+2

STA bcdScore+2

LDA bcdScore+1

ADC #$00

STA bcdScore+1

LDA bcdScore

ADC #$00

STA bcdScore

CLD

RTS

 

Greetings,

Manuel

[vdub_bobby]

Thanks, folks

[CPUWIZ]

For a 6 digit bcd score, this might come in handy at times:

 

AddScore

   SED

   CLC

   ADC bcdScore+2

   STA bcdScore+2

   LDA bcdScore+1

   ADC #$00

   STA bcdScore+1

   LDA bcdScore

   ADC #$00

   STA bcdScore

   CLD

   RTS

 

Greetings,

Manuel

 

Unless, of course, you are trying to add 200 to your score.

[Cybergoth]

Hi there!

 

Unless, of course, you are trying to add 200 to your score.  

 

So you're getting an idea why nothing in Star Fire or Seawolf scores more than 99 points?

 

Greetings,

Manuel

[CPUWIZ]

Hi there!

 

Unless, of course, you are trying to add 200 to your score.  

 

So you're getting an idea why nothing in Star Fire or Seawolf scores more than 99 points?

 

Greetings,

Manuel

 

Yep, those pesky extra bytes for a second routine.

[Thomas Jentzsch]

Yep, those pesky extra bytes for a second routine.  

Well, something like this does the Job too.

Add8:

 ldy #0

Add16: 

 sed

 clc

 adc score

 sta score

 tya

 adc score+1

 sta score+1

 cld

 rts

More sophisticated variants are of course possible.

[CPUWIZ]

Yep, those pesky extra bytes for a second routine.  

Well, something like this does the Job too.

Add8:

 ldy #0

Add16: 

 sed

 clc

 adc score

 sta score

 tya

 adc score+1

 sta score+1

 cld

 rts

More sophisticated variants are of course possible.

 

Yep, that works, still extra bytes though.

 

Interesting to see that you and Manuel have different endian styles. I do it your way myself.

[Cybergoth]

Hi there!

 

Interesting to see that you and Manuel have different endian styles. I do it your way myself.

 

I prefer to have my bits in the right order

 

Greetings,

Manuel

[+Andrew Davie]

Hi there!

 

Interesting to see that you and Manuel have different endian styles. I do it your way myself.

 

I prefer to have my bits in the right order

 

Greetings,

Manuel

 

Just to be absolutely clear about this, Manuel's way is wrong. The 6502 is a little-endian machine, and the lowest byte in, for example, a pointer to a memory address, is always the lowest byte of the memory address. Store your low byte in address, the next in address+1, etc.

 

One day, Manuel's screwy way of doing things is going to bite him in the bum. Also, I refuse to assist or debug code deliberately written in this fashion (big-endian), as a matter of principle.

 

To all those out there learning, don't do it Manuel's way. It is wrong for this machine/processor.

 

Cheers

A