Frequently Asked Questions (FAQ)

General

• How can sound effects be made?
  • The simplest way is to use the Game Boy sound hardware directly. See the Sound Example for a way to test out sounds on the hardware.
  • Further discussion on using the Sound Example rom can be found in the ZGB wiki. Note that some example code there is ZGB specific and not part of the base GBDK API: https://github.com/Zal0/ZGB/wiki/Sounds

Graphics and Resources

• How do I use a tile map when it's tiles don't start at index zero?
  • The two main options are:
    • Use set_bkg_based_tiles(), set_bkg_based_submap(), set_win_based_tiles(), set_win_based_submap() and provide a tile origin offset.
    • Use utility_png2asset with -tile_origin to create a map with the tile index offsets built in.

ROM Header Settings

• How do I set the ROM's title?
  • Use the makebin -yn flag. For example with lcc -Wm-yn"MYTITLE" or with makebin directly -yn "MYTITLE". The maximum length is up to 15 characters, but may be shorter.
  • See "0134-0143 - Title" in Pandocs for more details.
    

• How do I set SGB, Color only and Color compatibility in the ROM header?
  • Use the following makebin flags. Prefix them with -Wm if using lcc.
    • -yc : GameBoy Color compatible
    • -yC : GameBoy Color only
    • -ys : Super GameBoy compatible
      
• How do I set the ROM MBC type, and what MBC values are available to use with the -yt makebin flag?
  • See setting_mbc_and_rom_ram_banks
    

Errors / Compiling / Toolchain

• What does z80instructionSize() failed to parse line node, assuming 999 bytes mean?
  • This is a known issue with SDCC Peephole Optimizer parsing and can be ignored. A bug report has been filed for it.
    

• What do these kinds of warnings / errors mean? WARNING: possibly wrote twice at addr 4000 (93->3E) Warning: Write from one bank spans into the next. 7ff7 -> 8016 (bank 1 -> 2)

  • You may have a overflow in one of your ROM banks. If there is more data allocated to a bank than it can hold it then will spill over into the next bank. The warnings are generated by ihxcheck during conversion of an .ihx file into a ROM file.

    See the section ROM/RAM Banking and MBCs for more details about how banks work and what their size is. You may want to use a tool such as romusage to calculate the amount of free and used space.
    

• What does error: size of the buffer is too small mean?

  • Your program is using more banks than you have configured in the toolchain. Either the MBC type was not set, or the number of banks or MBC type should be changed to provide more banks.

    See the section setting_mbc_and_rom_ram_banks for more details.
    

• Why is the compiler so slow, or why did it suddenly get much slower?
  • This may happen if you have large initialized arrays declared without the const keyword. It's important to use the const keyword for read-only data. See const_gbtd_gbmb and const_array_data
    
• What flags should be enabled for debugging?
  • You can use the lcc debug flag
    
• Is it possible to generate a debug symbol file (.sym) compatible with the bgb emulator?
  • Yes, turn on .noi output (LCC argument: -Wl-j or -debug and then use -Wm-yS with LCC (or -yS with makebin directly).
    
• How do I move the start of the DATA section and the Shadow OAM location?
  • The default locations are: _shadow_OAM=0xC000 and 240 bytes after it _DATA=0xC0A0
  • So, for example, if you wanted to move them both to start 256(0x100) bytes later, use these command line arguments for LCC:
    • To change the Shadow OAM address: -Wl-g_shadow_OAM=0xC100
    • To change the DATA address (again, 240 bytes after the Shadow OAM): -Wl-b_DATA=0xc1a0

API / Utilities

• Is there a list of all functions in the API?
  • Functions
  • Variables
    
• Can I use the float type to do floating point math?
  • There is no support for 'float' in GBDK-2020.
  • Instead consider some form of fixed point math (including the fixed type included in GBDK)
    
• Why are 8 bit numbers not printing correctly with printf()?
  • To correctly pass chars/uint8s for printing, they must be explicitly re-cast as such when calling the function. See docs_chars_varargs for more details.
    
• How can maps larger than 32x32 tiles be scrolled? & Why is the map wrapping around to the left side when setting a map wider than 32 tiles with set_bkg_data()?
  • The hardware Background map is 32 x 32 tiles. The screen viewport that can be scrolled around that map is 20 x 18 tiles. In order to scroll around within a much larger map, new tiles must be loaded at the edges of the screen viewport in the direction that it is being scrolled. set_bkg_submap can be used to load those rows and columns of tiles from the desired sub-region of the large map.
  • See the "Large Map" example program and set_bkg_submap()
  • Writes that exceed coordinate 31 of the Background tile map on the x or y axis will wrap around to the Left and Top edges.
    
• When using gbt_player with music in banks, how can the current bank be restored after calling gbt_update()? (since it changes the currently active bank without restoring it).
  • See restoring the current bank
    
• How can CGB palettes and other sprite properties be used with metasprites?
  • See Metasprites and sprite properties
    
• Weird things are happening to my sprite colors when I use png2asset and metasprites. What's going on and how does it work?
  • See utility_png2asset for details of how the conversion process works.