Gameboy Development Forum

a) is correct. title_screen_tile_data_vram0, or more generally the start of the section, is the only thing that's being aligned by the align argument to a section. Yes, currently, the only way to achieve this together with your other requirements (floating bank etc) is to use padding.

To make your life easier, you can use the assert keyword to check the alignment at link time. It will then abort the assembly process if the pieces go out of alignment.

SECTION FRAGMENT "TITLE_SCREEN ROMX", ROMX, ALIGN[ 4 ] ; 16 byte aligned
 
title_screen_tile_data_vram0::
INCBIN "test_output/title_screen_data.bin", 760, 2528
.end

ASSERT ASSERT ((@) % 16)==0

title_screen_tilemap::
INCBIN "test_output/title_screen_data.bin", 40, 360
.end
 
db 0, 0, 0, 0, 0, 0, 0, 0

ASSERT ASSERT ((@) % 16)==0
 
title_screen_tilemap_attr::
INCBIN "test_output/title_screen_data.bin", 400, 360
.end

Starting in newer versions of RGBDS (v0.5.0+) there's an align keyword that you can use anywhere in a section. It doesn't insert padding though. Instead, it tries to align the whole section around the place of the align statement. But you can use still use it in this case for a similar effect to the assert above. If the data is misaligned, the assembly fails.

SECTION FRAGMENT "TITLE_SCREEN ROMX", ROMX, ALIGN[ 4 ] ; 16 byte aligned
 
title_screen_tile_data_vram0::
INCBIN "test_output/title_screen_data.bin", 760, 2528
.end

ALIGN 4

title_screen_tilemap::
INCBIN "test_output/title_screen_data.bin", 40, 360
.end
 
db 0, 0, 0, 0, 0, 0, 0, 0

ALIGN 4
 
title_screen_tilemap_attr::
INCBIN "test_output/title_screen_data.bin", 400, 360
.end

However, I have to question the need for alignment for title_screen_tilemap and title_screen_tilemap_attr. I presume those are used to fill up one screen with 20x18 tilemap entries. Where does the 16 byte alignment come from in this case? I can't think of a way that (efficient) code would both use 16 byte alignment and copy 20 bytes at a time.

This makes sense for the tile data, but then you can simply put all your tiles in one place and they will be naturally aligned because each tile is 16 bytes big. (And you can use an assert as above to confirm that you didn't enter the size incorrectly.

Furthermore, even if you're doing bit math that requires alignment, it would be recommended to base the math on the destination address and/or length counter if possible. This way you can use ld A,[HL+] for the read which reduces the need for alignment of the source data.