GBDK 2020 Docs  4.1.1
API Documentation for GBDK 2020
GBDK Toolchain


GBDK 2020 uses the SDCC compiler along with some custom tools to build Game Boy ROMs.

  • All tools are located under bin/
  • The typical order of tools called is as follows (when using lcc these steps are usually performed automatically).
    1. Compile and assemble source files (.c, .s, .asm) with sdcc and sdasgb
    2. Optional: perform auto banking with bankpack on the object files
    3. Link the object files into .ihx file with sdldgb
    4. Validate the .ihx file with ihxcheck
    5. Convert the .ihx file to a ROM file (.gb, .gbc) with makebin

To see individual arguments and options for a tool, run that tool from the command line with either no arguments or with -h.

Data Types

For data types and special C keywords, see asm/sm83/types.h and asm/types.h.

Also see the SDCC manual (scroll down a little on the linked page):

Changing Important Addresses

It is possible to change some of the important addresses used by the toolchain at link time using the -Wl-g XXX=YYY and =Wl-b XXX=YYY flags (where XXX is the name of the data, and YYY is the new address).

lcc will include the following linker defaults for sdldgb if they are not defined by the user.

  • _shadow_OAM
    • Location of sprite ram (requires 0xA0 bytes).
    • Default -Wl-g _shadow_OAM=0xC000
  • .STACK
    • Initial stack address
    • Default -Wl-g .STACK=0xE000
  • .refresh_OAM
    • Address to which the routine for refreshing OAM will be copied (must be in HIRAM). Default
    • Default -Wl-g .refresh_OAM=0xFF80
  • _DATA
    • Start of RAM section (starts after Shadow OAM)
    • Default -Wl-b _DATA=0xc0A0
  • _CODE
    • Start of ROM section
    • Default -Wl-b _CODE=0x0200

Compiling programs

The lcc program is the front end compiler driver for the actual compiler, assembler and linker. It works out what you want to do based on command line options and the extensions of the files you give it, computes the order in which the various programs must be called and then executes them in order. Some examples are:

  • Compile the C source 'source.c', assemble and link it producing the Gameboy image ''
    lcc -o source.c
  • Assemble the file 'source.s' and link it producing the Gameboy image ''
    lcc -o source.s
  • Compile the C program 'source1.c' and assemble it producing the object file 'object1.o' for later linking.
    lcc -c -o object1.o source1.c
  • Assemble the file 'source2.s' producing the object file 'object2.o' for later linking
    lcc -c -o object2.o source2.s
  • Link the two object files 'object1.o' and 'object2.o' and produce the Gameboy image ''
    lcc -o object1.o object2.o
  • Do all sorts of clever stuff by compiling then assembling source1.c, assembling source2.s and then linking them together to produce
    lcc -o source1.c source2.s

Arguments to the assembler, linker, etc can be passed via lcc using -Wp..., -Wf..., -Wa... and -Wl... to pass options to the pre-processor, compiler, assembler and linker respectively. Some common options are:

  • To generate an assembler listing file.
  • To generate a linker map file.
  • To bind var to address 'addr' at link time.

For example, to compile the example in the memory section and to generate a listing and map file you would use the following. Note the leading underscore that C adds to symbol names.

lcc -Wa-l -Wl-m -Wl-g_snd_stat=0xff26 -o hardware.c


Using Makefiles

Please see the sample projects included with GBDK-2020 for a couple different examples of how to use Makefiles.

You may also want to read a tutorial on Makefiles. For example:

Linker Files and ROM Auto Banking

When bankpack is called through lcc it will now always use linkerfile output (-lkout=) for passing files to the linker (all input object files and linkerfiles will get get consolidated to a single linkerfile).


  • lkin=<filename> : Adds a input linkerfile (can specify multiple ones)
  • -lkout=<filename> : Enables linkerfile output and sets name (only one can be specified). ALL loaded object files, both from the command line and any loaded from linkerfiles will have their names written to this single output.

LCC + Bankpack:

  • lcc passes all input linkerfiles (from -Wl-f<name>) to bankpack (-lkin=)
  • Linkerfile output is always used when lcc calls bankpack (-lkout=)
  • A temporary file name is used for bankpack linkerfile output.
  • lcc clears out the linker object file and linkerfile lists, then uses the single linkerfile generated by bankpack

Also see the linkerfile example project.

Build Tools


lcc is the compiler driver (front end) for the GBDK/sdcc toolchain.

For detailed settings see lcc-settings

It can be used to invoke all the tools needed for building a rom. If preferred, the individual tools can be called directly.

  • the -v flag can be used to show the exact steps lcc executes for a build
  • lcc can compile, link and generate a binary in a single pass: lcc -o somesource.c
  • lcc now has a -debug flag that will turn on the following recommended flags for debugging
    • --debug for sdcc (lcc equiv: -Wf-debug)
    • -y enables .cdb output for sdldgb (lcc equiv: -Wl-y)
    • -j enables .noi output for sdldgb (lcc equiv: -Wl-j)


SDCC C Source compiler.

For detailed settings see sdcc-settings

  • Arguments can be passed to it through lcc using -Wf-<argument> and -Wp-<argument> (pre-processor)


SDCC Assembler for the Game Boy.

For detailed settings see sdasgb-settings

  • Arguments can be passed to it through lcc using -Wa-<argument>


Automatic Bank packer.

For detailed settings see bankpack-settings

When enabled, automatically assigns banks for object files where bank has been set to 255, see rom_autobanking. Unless an alternative output is specified the given object files are updated with the new bank numbers.

  • Can be enabled by using the -autobank argument with lcc.
  • Must be called after compiling/assembling and before linking.
  • Arguments can be passed to it through lcc using -Wb-<argument>


The SDCC linker for the gameboy.

For detailed settings see sdldgb-settings

Links object files (.o) into a .ihx file which can be processed by makebin

  • Arguments can be passed to it through lcc using -Wl-<argument>


IHX file validator.

For detailed settings see ihxcheck-settings

Checks .ihx files produced by sdldgb for correctness.

  • It will warn if there are multiple writes to the same ROM address. This may indicate mistakes in the code or ROM bank overflows
  • Arguments can be passed to it through lcc using -Wi-<argument>


IHX to ROM converter.

Converts .ihx files produced by sdldgb into ROM files (.gb, .gbc). Also used for setting some ROM header data.

  • Arguments can be passed to it through lcc using -Wm-<argument>

GBDK Utilities


Compression utility.

For detailed settings see gbcompress-settings

Compresses (and decompresses) binary file data with the gbcompress algorithm (also used in GBTD/GBMB). Decompression support is available in GBDK, see gb_decompress().

Can also compress (and decompress) using block style RLE encoding with the --alg=rle flag. Decompression support is available in GBDK, see rle_decompress().


Tool for converting PNGs into GBDK format MetaSprites and Tile Maps.

  • Convert single or multiple frames of graphics into metasprite structured data for use with the ...metasprite...() functions.
  • When -map is used, converts images into Tile Maps and matching Tile Sets
  • Supports Game Boy 2bpp, GBC 4bpp, SGB 4bpp, and SMS/GG 4bpp

For detailed settings see png2asset-settings
For working with sprite properties (including cgb palettes), see metasprite_and_sprite_properties
For API support see move_metasprite() and related functions in metasprites.h

Working with png2asset

  • The origin (pivot) for the metasprite is not required to be in the upper left-hand corner as with regular hardware sprites. See -px and -py.
  • The conversion process supports using both SPRITES_8x8 (-spr8x8) and SPRITES_8x16 mode (-spr8x16). If 8x16 mode is used then the height of the metasprite must be a multiple of 16.


The following abbreviations are used in this section:

  • Original Game Boy and Game Boy Pocket style hardware: DMG
  • Game Boy Color: CGB

Conversion Process

png2asset accepts any png as input, although that does not mean any image will be valid. The program will follow the next steps:

  • The image will be subdivided into tiles of 8x8 or 8x16.
  • For each tile a palette will be generated.
  • If there are more than 4 colors in the palette it will throw an error.
  • The palette will be sorted from darkest to lightest. If there is a transparent color that will be the first one (this will create a palette that will also work with DMG devices).
  • If there are more than 8 palettes the program will throw an error.

With all this, the program will generate a new indexed image (with palette), where each 4 colors define a palette and all colors within a tile can only have colors from one of these palettes

It is also posible to pass a indexed 8-bit png with the palette properly sorted out, using -keep_palette_order

  • Palettes will be extracted from the image palette in groups of 4 colors.
  • Each tile can only have colors from one of these palettes per tile.
  • The maximum number of colors is 32.

Using this image a tileset will be created

  • Duplicated tiles will be removed.
  • Tiles will be matched without mirror, using vertical mirror, horizontal mirror or both (use -noflip to turn off matching mirrored tiles).
  • The palette won't be taken into account for matching, only the pixel color order, meaning there will be a match between tiles using different palettes but looking identical on grayscale.


Passing -map the png can be converted to a map that can be used in both the background and the window. In this case, png2asset will generate:

  • The palettes
  • The tileset
  • The map
  • The color info
    • By default, an array of palette index for each tile. This is not the way the hardware works but it takes less space and will create maps compatibles with both DMG and CGB devices.
    • Passing -use_map_attributes will create an array of map attributes. It will also add mirroring info for each tile and because of that maps created with this won't be compatible with DMG.
      • Use -noflip to make background maps which are compatible with DMG devices.

Meta sprites

By default the png will be converted to metasprites. The image will be subdivided into meta sprites of -sw x -sh. In this case png2asset will generate:

  • The metasprites, containing an array of:
    • tile index
    • y offset
    • x offset
    • flags, containing the mirror info, the palettes for both DMG and GBC and the sprite priority
  • The metasprites array

Super Game Boy Borders (SGB)

Screen border assets for the Super Game Boy can be generated using png2asset.

The following flags should be used to perform the conversion:

  • <input_border_file.png> -map -bpp 4 -max_palettes 4 -pack_mode sgb -use_map_attributes -c <output_border_data.c>
  • Where <input_border_file.png> is the image of the SGB border (256x224) and <output_border_data.c> is the name of the source file to write the assets out to.

See the sgb_border example project for more details.


Converts a binary .rom file to .msxdos com format, including splitting the banks up into separate files.