Over the years, in order to better understand how CPUs work, I’ve made two emulators for the 8-bit processor MOS 6502. This processor was widely used in the 1980s and 1990s, being employed in famous machines such as the Nintendo Entertainment System.
These two projects emulate a simplified version of the 6502: one ‘clock’ for the emulators corresponds to an entire instruction, while a real 6502 would require many (and a different amount of) clocks to execute an instruction.
The first of my 6502 emulators, lib6502-emulator, was written in C and is built as a library. The application using the library is responsible for handling the memory itself (RAM, ROM, disk, hardware registers…) while the library focuses on emulating the 6502 operations.
The library’s interface is designed to be very simple. To “wire” the
system memory to the 6502, the developer must set the functions
cpu_read_byte and cpu_write_byte. Each of the 6502’s interrupts
(RESET, IRQ and NMI) can be triggered by the corresponding function
(cpu_reset, cpu_irq, cpu_nmi). Finally, to tell the library to
simulate one instruction, cpu_clock can be called.
My second 6502 emulator, 6502-gba, was written in ARM assembly for the Game Boy Advance console. This project was made for a class in university in which computer architecture and assembly are taught.
The emulator is compiled into a Game Boy Advance ROM and can run programs put in the GBA cartridge’s save memory. It has access to the console’s hardware (display, buttons…) with many intended limitations, making it a sort of Fantasy Console based on the 6502 processor.
I found most of the knowledge I needed to make the emulators in
this website.
View on GitHub:
lib6502-emulator,
6502-gba