I returned to one of my old projects - homemade computer with 6502 and TMS9918 video controller.
Computer itself works with EhBasic, now I am going to write some code to add the TMS9918 support. I already done that with my other project utilizing 8080 CPU, so it is tested design. I also have 8080 code to base on. Unfortunately I came across some problems.
I was able to communicate with TMS9918 and initialize its registers. At this point I have garbage on screen, because VRAM wasn't initialized yet. I need to transfer character definitions stored in EPROM to VRAM.
In my 8080 computer I used following procedure for that:
Code:
;RAM ADDRESS IN BC, VRAM ADDRES IN DE, DATA LENGTH IN HL
VDPWVRAM:
MOV A, E
OUT VDP_MODE
MOV A, D
ORI 40H
OUT VDP_MODE
VDPWVRAML:
LDAX B
OUT VDP_DATA
INX B
DCX H
MOV A, H
ORA L
JNZ VDPWVRAML
RET
I tried to translate it to the 6502 assembly, using following code:
Code:
;RAM ADDRESS IN BLKIND, VRAM PASSED BY STACK (LSB FIRST), DATA LENGTH BLKLEN
VDPWVRAM
PLA
STA VDP_MODE
PLA
ORA #$40
STA VDP_MODE
LDY #$00
VDPWVRAML
LDA (BLKIND), Y
STA VDP_DATA
INC BLKIND
BNE VDPWVRAML1
INC BLKIND+1
VDPWVRAML1
DEC BLKLEN
BNE VDPWVRAML
LDA BLKLEN+1
BEQ VDPWVRAMRET
DEC BLKLEN+1
BNE VDPWVRAML
VDPWVRAMRET:
RTS
Which is executed as follows:
Code:
;INITiALIZE VRAM
LDA #<CHARS
STA BLKIND
LDA #>CHARS
STA BLKIND+1
;0000H + (32*8)
LDA #<(32*8)
PHA
LDA #>(32*8)
PHA
LDA #<(CHARS_END-CHARS)
STA BLKLEN
LDA #>(CHARS_END-CHARS)
STA BLKLEN+1
JSR VDPWVRAM
Unfortunately code freezes when I try to call VDPWVRAM procedure. What Am I doing wrong?
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