13 replies to this topic

[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]

CPUWIZ said:

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]

vdub_bobby said:

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

Yes.

[Cybergoth]

Hi there!

vdub_bobby said:

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]

Cybergoth said:

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!

CPUWIZ said:

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]

Cybergoth said:

Hi there!

CPUWIZ said:

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]

CPUWIZ said:

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]

Thomas Jentzsch said:

CPUWIZ said:

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!

CPUWIZ said:

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]

Cybergoth said:

Hi there!

CPUWIZ said:

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