nesdev.org

Posted: **Wed Mar 05, 2008 7:16 pm**


reset:
    sei        ; ignore IRQs
    cld        ; disable decimal mode
    ldx #$40
    stx $4017  ; disable APU frame IRQ
    ldx #$ff
    txs        ; Set up stack
    inx        ; now X = 0
    stx $2000  ; disable NMI
    stx $2001  ; disable rendering
    stx $4010  ; disable DMC IRQs

I was wondering how x became zero in this bit of code.

First x contains #$40

After the stx $4017 ; #$40 still in the x reg?

Then ldx #$ff ; x now contains 255?

Then txs ; transfering the contents (#$ff) of x reg to stack?

x still contains 255?

Then inx ; does x reset back to 0 (255 + 1 =0) here?

Thanks,
T

Posted: **Wed Mar 05, 2008 7:25 pm**

electro wrote: Then inx ; does x reset back to 0 (255 + 1 =0) here?

Yes. X is only 8 bits wide, so it gets clipped at 8-bits.

$FF + 1 = $100

but you need to truncate $100 to 8 bits, so you're left with $00

Posted: **Wed Mar 05, 2008 7:28 pm**

I see. Thanks.

T

Posted: **Thu Mar 06, 2008 7:53 pm**

triiiiiicky tricky tricky tricky.
btw i'm still taking in the last post in the controllers thead. that's why i haven't replied.

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Understanding a bit of code

Understanding a bit of code

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