Mario Sprites Problem

Mario Sprites Problem
by Anes on 2004-12-25 (#713)

i have problems emulating super mario bros.
Mario sprites itself goes to position x = 0 after scrolling 128 pixels, and after that mario's sprites are animated but i cant move it.

Help please

by Memblers on 2004-12-25 (#718)

Sounds like it could be a CPU bug. Do other games work ok?

by Disch on 2004-12-25 (#720)

Sounds almost like a V flag problem (bad wrapping at 128 could be because the V flag is being set wrong on ADC/SBC)

Double check your ADC/SBC. For ADC... the V flag should be set when:

Positive + Positive = Negative
or
Negative + Negative = Positive

In all other instances of ADC, the V flag should be cleared.

Likewise for SBC... V flag is set when:

Positive - Negative = Negative
or
Negative - Positive = Positive

by tepples on 2004-12-25 (#721)

You can make SBC always work by implementing SBC in terms of ADC: load the operand from memory as normal, invert it (^ 0xff), and then do your ADC code. Reducing complexity of your emulator's 6502 core is a good thing.

Nestress
by Anes on 2004-12-25 (#724)

I have tested my cpu with nestress rom and it throw me all "ok"
do i have to trust in that?

Thanks

by Hyde on 2004-12-25 (#725)

Do not trust it; I'm pretty sure there is something wrong with your ADC and SBC code (V flag).

adc implementation
by Anes on 2004-12-25 (#726)

sure, thanks i will try

by Zepper on 2004-12-26 (#735)

tepples wrote:
You can make SBC always work by implementing SBC in terms of ADC: load the operand from memory as normal, invert it (^ 0xff), and then do your ADC code. Reducing complexity of your emulator's 6502 core is a good thing.

Really? That's pretty good...
Is there any other similar case?

by Anes on 2004-12-26 (#738)

Fx3 wrote:
Really? That's pretty good...
Is there any other similar case?

Fx3 if you read carefully Mos 6502 manual, it says that the processor is always really adding. It inverts the bytes of the source, add carry and then add.

by Zepper on 2004-12-27 (#755)

Anes wrote:
Fx3 wrote:
Really? That's pretty good...
Is there any other similar case?

Fx3 if you read carefully Mos 6502 manual, it says that the processor is always really adding. It inverts the bytes of the source, add carry and then add.

Actually, it inverts the BITS of the data (byte)...