nesdev.org

And when subtracting, the carry is the ones' complement (i.e. the inverse) of bit 8 from the result of the previous subtraction. It makes sense, considering that a subtraction is just an addition that uses the number's ones' complement. that helps, I think I have it. It also makes it more clear why...

I'm just now getting back to this due to the holidays (happy belated holidays everyone!). I think part of my issue is it's been so long since I've worked directly with binary and two's complement that I no longer *think* in them. I understand what they are, their pros and cons, etc, but I'm slow. I ...

Hey guys, I've got SBC working correctly, but I don't understand why. I'm working off of the following document: http://teaching.idallen.com/dat2343/10f/notes/040_overflow.txt Here's pseudocodehere's my implementation var a = cpu.A; var b = mem.read(addr); int16 diff = a - b - !cpu.C cpu.A = (uint8)...

aha, makes sense, thanks

I'm curious about the implementation of GetOverflow. In particular, you have the 3 overflow flags, could you explain why you chose to model it that way?

Your post is fairly confusing, you're taking about PHP but then show stack.pull(), etc. I think this page explains it better: http://wiki.nesdev.com/w/index.php/CPU_status_flag_behavior You're right, it is confusing (and wrong). I've edited it for corrections and to make it more clear. My brain was...

http://wiki.nesdev.com/w/index.php/Status_flags On the 6502, bit 5 of the copy is always set, and bit 4 is set BRK or PHP, clear if an interrupt. When restoring the flags from memory, bits 4 and 5 are ignored, since again, there are only 6 status flags, and thus the extra 2 bits of the byte must be...

that's the plan

I'm learning as I'm going, this allows me to concentrate my efforts on specific areas and ignore the rest until I'm ready to take a closer look.

For now I'm just throwing an error when an illegal opcode gets reached, but I would eventually like to implement them. I've started going through nestest.nes (along with the log) to implement the official opcodes. When I get to the point that I'm ready to implement the other opcodes, are there any t...

According to this: http://www.6502.org/tutorials/6502opcodes.html#NOP NOP is 0xEA However, when looking at the matrix here: http://www.oxyron.de/html/opcodes02.html I'm seeing these listed as NOP also (listed below the question). My question is: what's the difference? is 0xEA the "official"...

Also, if my post seems disjointed it's because I'm tired

I appreciate the resources you've mentioned here, I'll be coming back and looking further into them when I'm a little more coherent.

There are two subjects being repeatedly intermixed here, and I've been having trouble figuring out what exactly is truly needed/wanted by the OP, while having to make blind assumptions about experience level. Thus I'll cover both: It's probably safe to assume I'm a dumbass :) This will be a little ...

And if you have a tool you think is better suited for my purposes, by all means, I just grabbed dasm because someone else stated that's what they've been using (successfully). I'm not too invested in the specific tool I'm using, I just want to be able to produce binaries myself from 6502 assembly.

I want to be able to start testing my CPU, and also be able to understand some of the other test suites out there. A lot of the information out there explains things I already understand such as addressing modes, etc, but don't really go into detail about 6502 asm dev. Also, with tidbits that I ment...

Found something useful: http://www.nintendoage.com/forum/messag ... eadid=4440

I can't believe how long it took me to find that.

I'm still looking for more resources though if you guys have anything to add to it.