Displaying any image on an MSX, loading from a ROM

This article is basically just a note that I can come back to in case I forget some details.

The tool on this page (Japanese) takes an image file and adds dithering and stuff: https://nazo.main.jp/prog/retropc/gcmsx.html

The output is something that can be BLOAD’ed straight to VRAM memory using BLOAD’s S parameter. Didn’t even know that option existed!

This means we can easily convert this to a ROM by adding a loader that pokes everything into VRAM. We just need to get rid of the BSAVE header at the start (or ignore it in the loader), which looks like this according to http://www.faq.msxnet.org/suffix.html#BIN:

byte 0  : ID byte #FE
byte 1+2: start-address
byte 3+4: end-address
byte 5+6: execution-address

So we just cut off 8 bytes at the beginning, e.g. by doing:

tail -c +8 msx_20231111232339933.SC2 > foo.bin

And the loader in assembly (z80asm-flavor) could look like this:

SetVdpWrite: macro high low ; from http://map.grauw.nl/articles/vdp_tut.php
	ld a,low
	out (0x99),a
	ld a,high+0x40
	out (0x99),a
endm

vpoke: macro value
; 	ld a,value ; not needed in this implementation
	out (0x98),a
; 	nop ; nops not needed in this implementation
; 	nop
; 	nop
endm

	org 0x4000
	db "AB" ; magic number
	dw entry_point
	db 00,00,00,00,00,00,00,00,00,00,00,00 ; ignored

entry_point:
	ld a,2
	call 0x005f
	SetVdpWrite 00 00
	ld hl,data
loop:
	ld a,(hl)
	vpoke a
	inc hl
	ld a,h
	cp end>>8
	jr nz,loop
	ld a,l
	cp end&0xff
	jr nz,loop
inf:
	jr inf
data:
	incbin "foo.bin" ; read data from file foo.bin
end:
	ds 0x8000-$ ; fill remainder of 16 KB chunk with 0s

Of course, this approach is quite wasteful; we need almost 16 KB of memory to display any image, even if it’s mostly empty.