COMPO & PLATFORMS

This is for the 64 Byte Oldschool Intro compo of LoveByte22.

Target platforms are the Game Boy Color (CGB) and for v1.0.1 also Game Boy Advance (AGB).

WHAT RUNS WHERE

tinymirrorboy.384b.cgb should work with all emulators and cartridges, where this doesn’t work tinymirrorboy.16k.cgb or tinymirrorboy.32k.cgb should run.

It was tested on SameBoy (v0.14.3; CGB modus), Emulicious (only with original CGB bootrom; CGB modus), BGB (v1.5.9; CGB modus) and on CGB (CGB-D) with EVERDRIVE GB X3.

Only Emulicious can run the actual 64B ROM tinymirrorboy.64b.cgb.

v1.0.1

Tested on SameBoy (v0.14.3; AGB modus) and Emulicious (only with original AGB bootrom; CGB modus), but not on real hardware

v1.0.2

Tested on Emulicious (only with original AGB/CGB bootrom; CGB modus)

TRICKS USED

• Only half logo (24B;CGB doesn’t check more)
• Abuse logo to load custom 4x4 as upscaled 8x8 tiles into VRAM
• Use ROM mirroring to fill rest of header with data from logo
• DMG compatibility mode (has standard colors configurable via 1B and palette shifting is cheaper)

TRICKS NOT USED

• Running through the logo (only the second half has illegal instructions; instead I sacrificed 2B for a relative jump and cleaner/reliable register values)

CONCEPT

The Game Boy header is consider being 80B ($100-$14F as defined by gbdev pandocs) long. I already knew that the CGB bootrom only needs half the logo and the header checksum. I wanted to explore wether necessary ROM size can be further reduced by wrapping the ROM around, which would be done by ignoring the upper 13 bits of the address. I wanted to see if the logo sets the header to some values that makes the bootrom refuse to start it and if it’s feasable to hack the header checksum like I hacked the title checksum in my last intro.

MOTIVATION

This self built Game Boy 96B cartridge by insideGadgets: https://www.youtube.com/watch?v=Ss0evFqgB6M

Their approach gives them only 16B that can be used for code. With my apporach there would be 71 (or 69 if you don’t count the mandatory jr) Bytes free.

PITFALLS

The header checksum can’t be directly set and must be fixed with bytes $134-13F or $100-$10C.

All changable header bytes relevant for the header checksum are within the title.

$103 needs values <$80, this allows only following opcodes:

• nop
• stop
• jr
• ld (without ldh, ldhl, ld with (c), ld with 16b address, ld sp, hl)
• inc
• dec
• halt
• rlca
• rla
• daa
• scf
• add (only with 16b registers as second operand)
• rrca
• rra
• cpl
• ccf

PERSPECTIVE

What is considered the header could be from $100-$11B (28B): 2B jr + 1B header checksum + $00 (CGB Byte) + 24B logo.

Also hacking the title checksum is not possible in 64B.

Somebody could try to build this type $66 (a value from the logo) cartridge that ignores the upper 13 bits of the address.