GameBoy emulator targeting Lua 5.1

How to run with LÖVE:

make
love . tetris.gb

Controls:

Arrows: WASD
A/B: J/K
Start/Select: Return/Space

How to build for Nspire:

<git clone of Ndless>/ndless/src/tools/LuaBin/luabin tetris.gb - rom | sed 's/&quot;/"/g' > rom.lua
make gbc4lua.tns

Minimum OS version is technically 3.1, but until 3.6 (?) the OS just crashes immediately on open.

Not implemented

• GBC: Currently it only provides DMG emulation
• Line based rendering: The entire screen is rendered at the beginning of vblank, so raster effects will not work.
• LCD modes: Interrupts and mode status are only partially implemented
• Audio: The Nspire doesn't support audio output
• Timer registers: Implementing those properly would add noticable overhead and it appears like they're not necessary for most games without audio.
• Mapper support: Only MBC1 and MBC5 (without rumble) are implemented
• BG+Window and OBJ enable bits: Everything is always drawn to not rely on line based toggling
• Cycle accurate memory access: Instruction cycles are tracked, but all accesses triggered by an instruction appear to happen within the same cycle. As the timer is not implemented this should not be noticable.
• Cycle accurate interrupts: The CPU only checks for interrupts every line.
• CPU flags for BCD: The N and H flags are not set by instructions other than pop af. This will lead to weird score calculations and more.

TODO

• Input handling in main_nspire.lua
• Save/load sram contents to variables
• Optimize! For now most parts are not optimized, especially tile/object drawing and blitting to the screen. The code could benefit from loop unrolling and inlining as well.

Writing fast Lua code

The Lua language has various traps which slow code down and need to be avoided to get code that's just slow instead of abysmally slow:

• Use locals and function parameters if possible. Access to global variables does a table lookup, access to upvalues is a GETUPVALUE for each read and SETUPVALUE for each write.
• Perform obvious simplifications like constant folding and avoiding redundant reads/writes. There is no optimization performed by the interpreter.
• Avoid table lookups. Cache them if possible, e.g. outside a loop or even outside of a function.
• Unroll loops
• Avoid accessing tables at [0] if not needed. The array part is tbl[1]-tbl[#tbl], using tbl[0] does a hash lookup.