gianlucag / mos6502 Goto Github PK

I've converted your code to Delphi but you're missing a license. Thank you

The README suggests the C64 used the 6502, but actually, it used the 6510, is this a 6502 emulation or 6510? It suggests you could make a C64 with this emulation, but if it is a 6502 emulator, it may or may not work.

The stack pointer hardware in the 6502 can only update the stack pointer after the stack access has happened. So, it decrements the stack pointer after pushing a value onto the stack, so that the stack pointer is ready for another push.

But when pulling, this means it has to pull some dummy data (just to update the stack pointer) before pulling the real data.

It looks as though gianlucag/mos6502 does not do this dummy read. While executing a plp instruction, which is a four-cycle instruction, gianlucag/mos6502 performs only two memory reads:

1. the opcode byte
2. the PSW from the stack

but the real chip will perform the following four memory reads:

1. the opcode byte
2. the next byte in the instruction stream (all instructions do this in cycle two, while the instruction is being decoded, just in case)
3. the byte immediately below where the PSW is about to be read from
4. the PSW from the stack

Exactly the same problem affects other instructions that read from the stack: rts, pla, and probably also rti.

This causes include loops when used as a part of larger projects

A real chip, and Mike Chambers' fake6502 as well, read the reset vector low byte first (i.e. $fffc) and then $fffd. But it seems like this emulator does it in reverse order.

I appreciate it's a tiny thing but it's showing as a difference in my test suite

What do we think? Do we care about such tiny differences?

Would it be technically possible to use this to access a graphical config menu thats present on a sidkick pico? I use your sidberry to play sids via the sidkick pico and want to tune some stuff thats in the config menu.

There is reference to the Run method, but it doesn't match the code.

The README says void Run(uint32_t n); but the code says void Run(int32_t cycles, uint64_t& cycleCount, CycleMethod cycleMethod = CYCLE_COUNT);.

The "status" member has uninitialized bits. When I noticed that a static instance of mos6502 would work as expected but one on the stack or heap would show subtle differences in program execution (static data=bss), it pointed me to the bug.

Otherwise, this is a very nice and efficient piece of code. Using it for my second project. I love it!