In Grauw's test program you could change ModifyR18 (at line 141 in VDPReg18.asm) as follows
ModifyR18: PROC
ld b,0
ld c,VDP_PORT_1
ld d,18 | VDP_REGISTER
ld e,0FH
Loop:
ld a,b
and e
reverse: equ $
cpl
di
out (c),a
in a,(0x98)
out (c),d
ei
djnz Loop
ret
ENDP
In Grauw's test program you could change ModifyR18 (at line 141 in VDPReg18.asm) as follows
ModifyR18: PROC
ld b,0
ld c,VDP_PORT_1
ld d,18 | VDP_REGISTER
ld e,0FH
Loop:
ld a,b
and e
reverse: equ $
cpl
di
out (c),a
in a,(0x98)
out (c),d
ei
djnz Loop
ret
ENDP
It seems that you cannot read the data port between two accesses to the command port without resetting its logic.
R18 doesn't change at all
We could maybe use R17....
I will try a different test using it
maybe one could try something like
in a,(0x98)
then following out(c), ... out(c), .... to change the register?
or the vdp could have enough time between the first out to fetch vram data and be free for the cmd engine, before the cpu could have finisced to set r18?
I would try to be as tight as possible
This is the fastest thing I came with
ModifyR18: PROC
di
ld a,18 + 128 ; no auto increment
out (0x99),a
ld a,17 + 128
ei
out (0x99),a
ld bc,0x0098
ld e,0FH
Loop:
ld a,b
and e
reverse: equ $
cpl
in f,(c) ; db EDh,70h ; read data port
out (0x9B),a ; set R#18
djnz Loop
ret
ENDP
I would try to be as tight as possible
This is the fastest thing I came with
ModifyR18: PROC
di
ld a,18 + 128 ; no auto increment
out (0x99),a
ld a,17 + 128
ei
out (0x99),a
ld bc,0x0098
ld e,0FH
Loop:
ld a,b
and e
reverse: equ $
cpl
in f,(c) ; db EDh,70h ; read data port
out (0x9B),a ; set R#18
djnz Loop
ret
ENDP
Max could you try this ?
yes, will compile and try shortly.
I assume in f,(c) is an attempt to read data into nowhere? It wouldn't compile. I tried to write in (HL),(C) and it also didn't work. So I just inserted DB EDh,70h there and it compiled. I wonder is there profer way to write it in assembler without using DB...
Here is a slightly different video of the same program. I didn't disable the internal LAN module before executing, so I think it slightly affected the result.
