and again thanks dclxvi . probably there's some typos in some docs.

usually the operation is written some docs (the correct ones) as "A = A - M - (IF_CARRY() ? 0 : 1)".
then, if C is set the formula goes "A = A - M" and "A = A - M - 1" if it's not set, wich is the correct behavior.

...so it means that the instruction DO overwrite the original data, even in all the others addressing modes.

thanks a lot mate!

...that's what i'm wondering now while looking at the ROR behavior.

e.g.: ROR Accumulator:
byte src; // just an unsigned 8-bit number

src = (ACC >> 1); // store the rotation temporary
if(C_FLAG) src |= 0x80; // if C was set, then set the #7 bit of the rotation's result
C_FLAG = ACC & 0x1;
Z_FLAG ...

First, the formula is A = A - M - 1 + C
that's not way the i've found the formula. that "-1" doesn't appears in any doc i've checked.

it's strange that src is used in the temp calculation but the Memoria array is used in the V_FLAG calculation
my mistake. forgot to chanage that "Memoria[PC ...

yes i did Garth, actually that's the doc if was reffering to.

looks like that expression was whipped up from a Karnaugh map or something like that, who knows...

i came across this code to emulate the SBC instruction:

uint temp = (uint)(ACC - src - C_FLAG);
Z_FLAG = !(temp & 0xff);
N_FLAG = (temp & 0x80);
V_FLAG = ((ACC ^ temp) & (Memoria[PC] ^ temp) & 0x80)
C_FLAG = (temp < 0x100);
ACC = (byte)(temp & 0xff);

i tested the code it do work perfectly, but ...

sorry for revive old posts, but let me just paste my final code:

(C# Language)
private void Branch(bool Flag, bool Condition)
{
PC += 2; // PC points to the next instruction after the branch before the branch is taken
if (Flag == Condition)
{
PC--;
UInt16 Displacement = Memory[PC++]; // Post ...

ok thanks!

but it affects the value in the memory address. isn't it? i.e.:

assume $FE is #AA (170 in decimal)

ASL $FE

...after that $FE is #54 (84 in decimal)

that means that it may ovewrite memory address... thanks Lee!

thanks Ed! i suppose it is also valid for "ASL Zero Page" and all the others ASL's... i mean that the result of any kind of ASL is always stored into the accumulator. is that right?

just to be sure: in the instruction ASL A (ASL accumulator) the operation is:

1) A = ASL accumulator

...or just

2) ASL accumulator

...???

the result of the ASL operation is stored into the accumulator? or the ASL instruction is just used to set/reset flags?

ok, thank you all for your answers! specially kc5tja, i'll try to make just like you said above.

this is a project i've been working on since i'm 18 (now i'm 20) and i always got stucked in some point (because i've internet access in my house since 2 years ago and because the complexness of this ...

There are already NES emulators, some probably open source. Also, the rise on the NOAC chips (Nintendo On A Chip) are populart because the PPU patent has expired.

One place to look, the Playpower $10 computer which uses one. They have an open source emulator so you will wan to take a look there ...

hi everyone! i'm trying to write a nintendo nes emulator. i'd written almost the 50% of the cpu core, but i have some doubts with the video emulation.

where should i start writing the ppu emulator? any advice will be greatly apreciated!