Almost none of the following is implemented yet. This README was mainly a roadmap of sorts. Behaviours outlined here will almost certainly be changed in some way or outright removed, especially non-standard 6502 behaviour.
The purpose of this project is to have a working 6502 in software.
Cycle accuracy is not a goal of this project. Instead, the focus will be on making sure the instructions operate properly and sequentially (ie, pipelining as seen on real hardware is ignored).
Special registers will be implemented to allow communication with the host system. Most notably the host's keyboard and console. Additional APIs may be defined later (networking? lol).
Similar to mappers on the NES, the emulator will have the option to use memory bank switching. This will probably only be one or two banking schemes that require little to no setup.
File system IO. Direct and/or via open/close file and map to work RAM?
Configurable work RAM? Enabled/Disabled, WorkRAM = open file buffer? filename would need to be written somewhere.
Fire an NMI at 60hz? This could allow synchronizing and timing things with the host system a bit easier. Should probably be a configuration option (either command line, register, or both).
- Lock/read (a-la NES controllers)
- Interrupt/status register based system
- A simple input buffer (FIFO style)
This method would require the a sizeable chunk of memory addresses assigned to just the keyboard (at least a full page), but it would probably be the simplest to program for on the ASM side.
This might not be it's own separate input method, but rather a modification of the Lock/Read method. A register would be made available to check for a new keyboard state (button pressed/released) and would not update the keyboard state until the next frame/NMI (clocked at a configurable rate? 60hz default?).
An input buffer method would need to properly handle modifier keys. Maybe signal that there are modifiers to read by setting the 7th bit and making flags for which modifiers available in another register.
Registers:
- [R] Input in buffer
- [R] Modifier for last read input from buffer
- [W] Drop buffer
Probably only two schemes: Full 32k banks or one fixed bank at $C000 and a
swapple bank of 16k starting at $8000.
Simple swapping mechanism: write the bank number to a register (eg. no shifting
like MMC1).
NES mapper support is fully implemented. Currently, only three mappers are written: FullRW, MMC1, and NROM.
Open file by writing full path to a page between $4000 and $5FFF. Once opened, fill work RAM with file's contents. If more than 8k, allow bank swapping? Write/flush file contents on register write. Close/drop file contents/changes on register write. Open/reload file from disk on register write.
Implement a file system API to list files and directories, create and delete files, change working directory, etc.
Registers:
- IO command/API
- Open file (WRAM)
- Reload file (WRAM). Used when other commands use WRAM for data.
- Write file (WRAM)
- Close file. Do not flush. A write would be required before this command to save data.
- chdir
- lsdir
- create/delete file
- Swap WRAM page (read/write more of file or avoid overwriting contents of WRAM with command output)
- Write-only filename bank (half a page?)
- All data returned from IO command will be written to WRAM (directory listings, command errors, etc).
- Command return code.