plan.txt

OpenFamicom

links:
http://yizhang82.me/nes-emu-overview
http://yizhang82.me/nes-emu-main-loop
http://wiki.nesdev.com/w/index.php/2A03 <- cpu
http://wiki.nesdev.com/w/index.php/Mapper
http://wiki.nesdev.com/w/index.php/Emulator_tests
http://www.emulator101.com/reference/6502-reference.html

HW info:

processor: 6502 (Ricoh 2A03) 1.79 MHZ
http://www.emulator101.com/6502-emulator.html
3 8 bit general purpose registers: A, X, and Y
8 bit register P (status)
8 bit stack pointer (fixed at RAM address $100, so can address $100-$1ff)
16 bit program counter
148 total instructions
little endian architecture

"PAL versions of the NES (sold in Europe, Asia and Australia) used the Ricoh 2A07 or RP2A07 processor, which was identical to the 2A03 except for a different clock divider and the 50 Hz vertical refresh rate used in the PAL television standard, with the exception of the 16 hard-coded sampling rates for the Delta modulation-encoded sample playback unit. The changes were made in order for sampled sounds to maintain the original pitch of the RP2A03 with the new clock divider."

PPU (Picture Processing Unit)
Renders 256x240 screen composed of 8x8 tiles for background, up to 64 8x8 or 8x16 sprites for moving objects. It supports pixel-level scrolling (which is a big deal back in that day).

APU (Audio Processing Unit)
Supporting 2 pulse channel, 1 triangle channel, 1 noise channel, and 1 DMC (delta modulation) channel.
http://wiki.nesdev.com/w/index.php/APU

Note on cartridges:
Cartridge boards (and mappers) - there are many different kinds of game catridge boards. They come with game data as ROMsm, sometimes their own battery-backed RAM, or some cases, their own audio processing unit. Most importantly, they also come with special hardware, refered to as mappers, that dynamically maps ROM/RAM into CPU and PPU memory space, bypassing the limitation of 16-bit address space. Some game catridge come with more than 256KB of CHR ROM and swap/map portion of it on demand.

Timing:
Every game runs at 60fps

Plan of attack:
1. Begin by emulating 6502
    - write tests for each instruction at every stage
2. Add rom support for test roms
3. Implement PPU w/o scrolling
4. Timing - "Make sure your game loop process as the exact cycle as the real hardware by doing some math over elapsed tick count and CPU MHZ"
5. Add scrolling
    - http://wiki.nesdev.com/w/index.php/PPU_scrolling
6. APU
7. Mappers

Architecture:
Data structures
    - Famicom
        - CPU
            - structure of registers
        - PPU
        - APU
    - Instructions/opcodes
        - must have an associated executed function

We need threads for CPU, PPU, and APU and they must sync with each other for timing.