[supreme] Help with C# implementation of ASM - SBC.

[Cups 97]

So I'm implementing a gameboy CPU in C# to make an emulator. And I need someone good in ASM to check an implementation I make which I have a doubt on.

 

The OPCODE 0x9F translates to instruction SBC (source here http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html) so substract with carry.

 

I just need someone to check my implementation, like @mesvak cause apparently he knows ASM.

 

Right now I have this made :

 

/// 
       /// Substract n + carry flag from A.
       /// 
       /// n
       private static void sbc(byte r1)
       {
           // TODO Double check this implementation
           uint substractResult = (uint)Gameboy.Cpu.Registers.A - r1 - Convert.ToUInt32(Gameboy.Cpu.Registers.F.C);

           // Set zero flag if result is 0.
           Gameboy.Cpu.Registers.F.Z = ((byte)substractResult) == 0;

           // Set half-carry flag if no borrow from bit 4. IM UNSURE ABOUT THIS. This should mean if result is 0b0001_0000, we have a carry, but is it really how to implement it?
           Gameboy.Cpu.Registers.F.H = ((Gameboy.Cpu.Registers.A ^ r1 ^ substractResult) & 0x10) != 0;

           // Set the carry flag if no borrow at all 0b0001_0000_0000 (so more than 8 bits)
           Gameboy.Cpu.Registers.F.C = ((substractResult & 0x100) != 0);
           
           // The substract flag is set as we performed a sub op.
           Gameboy.Cpu.Registers.F.N = true;
       }

[mesvak 362]

i never tried it with c# tbh but tried it with java like 2 years ago and about 3 months ago there was a person who created a similar shit

which is this guy https://github.com/trekawek/coffee-gb/tree/master/src/main/java/eu/rekawek/coffeegb/cpu

 

But its written in java i can translate it to c# if u didnt get it

 

just give a quick brief shit so basically for writing an emulator you needa consider 1- cpu ( timing and .....) 2- memory

 

and dont forget you needa write diff script for each of them seemingly in this script that tbh i m a bit confused about what u written there cuase you cannot bind them without importing them

 

APprently this is the case you working on

}

 

public OpcodeBuilder proceedIf(String condition) {

ops.add(new Op() {

@Override

public boolean proceed(Registers registers) {

switch (condition) {

case "NZ":

return !registers.getFlags().isZ();

 

case "Z":

return registers.getFlags().isZ();

 

case "NC":

return !registers.getFlags().isC();

 

case "C":

return registers.getFlags().isC();

}

 

 

And as a reminder you needa mention the the value of bit u going to use and what is z is it below z<8 bits or ......

[Cups 97]

No, I understand everything about the emulator but I just need someone to check the implementation of the half carry flag