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Sasq
Registered: Apr 2004
Posts: 156 |
Problems overloading LOAD vector
I want to replace kernal LOAD to jump into the mmc64 bios. I have two problems;
- The mmc64 doesnt have any RAM like AR/RR so I need to use 6 bytes in main ram to switch in the mmc rom. I've been looking for a reasonably safe place and I' currently thinking about using the upper 6 bytes of the file number tables at 259. Can anyone think of safer places?
- My RR cart replaces LOAD per default as expected, but when I try to change it is instantly restored to the RR vector again. Why is this and how do you avoid it?
Also I wonder if there are any examples on using parts of the original kernal routines to do stuff like print error messages, print LOADING or generate basic listings for LOAD "$".
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iAN CooG
Registered: May 2002
Posts: 3197 |
Here are some code snippets I'm using sometimes.
The 1st send "I" and print status msg.
Is taken and adapted from some IDE64 examples
*= $c000
;callable from basic
jmp initdrv
jmp printstatus
initdrv
jsr openchan
lda $ba
jsr $ed0c ;listen
lda $b9
jsr $edb9 ;second
lda #$49 ;"I"
jsr $eddd ;send
jsr $edfe ;unlisten
jmp status
printstatus
;only print error/status
jsr openchan
status
lda $ba
jsr $ed09 ;talk
lda $b9
jsr $edc7 ;tksa
jsr $ee13 ;read
jsr $ffd2 ;print
bit $90
bvc *-8
jsr $edef ;untalk
jsr $f642 ;close
jsr $ffc3 ;close
rts
openchan
lda #0
sta $90
lda #8 ;drive 8
sta $ba
lda #$6f ;channel
sta $b9
lda #0
jsr $ffbd ;setname
jmp $f3d5 ;open
This one prints directory, wait for a key, exits.
Disassembled from a packer and commented by me.
*= $0334
JSR $E544 ;clear screen
LDA #$01
LDX #<dirname
LDY #>dirname
JSR $FFBD ; set filename "$"
LDA #$08
STA $BA ; device #8
LDA #$60
STA $B9 ; secondary chn
JSR $F3D5 ; open for serial bus devices
JSR $F219 ; set input device
LDY #$04
labl1 JSR $EE13 ; input byte on serial bus
DEY
BNE labl1 ; get rid of Y bytes
LDA $C6 ; key pressed?
ORA $90 ; or EOF?
BNE labl2 ; if yes exit
JSR $EE13 ; now get in AX the dimension
TAX ; of the file
JSR $EE13
JSR $BDCD ; print number from AX
labl3 JSR $EE13 ; now the filename
JSR $E716 ; put a character to screen
BNE labl3 ; while not 0 encountered
JSR $AAD7 ; put a CR , end line
LDY #$02 ; set 2 bytes to skip
BNE labl1 ; repeat
labl2 JSR $F642 ; close serial bus device
JSR $F6F3 ; restore I/O devices to default
labl4 JSR $F142 ; w8 a key
BEQ labl4
RTS
dirname .byte "$"
Hope it helps. |
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TNT
Account closed
Registered: Oct 2004
Posts: 189 |
Plenty of good messages at $f0bd, printing routine at $f12b
To print "loading" ldy #$49;jsr $f12b
If you update $93 for load/verify you can print appropriate message with jsr $f5d2
jsr $f5af is "searching" + filename
ldx #01..1e; jsr $a43a for some more errors
What code you use to jump into MMC64 Flash? inc $df11;jmp?
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Sasq
Registered: Apr 2004
Posts: 156 |
Quote:What code you use to jump into MMC64 Flash? inc $df11;jmp?
This is wat I use (by saving carry at destination I can preserve $01 too). It assumes no program maps char rom while loading (which I think is pretty safe).
dec $df11
lsr $01
jmp $8xxx
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TNT
Account closed
Registered: Oct 2004
Posts: 189 |
That's eight bytes and causes side effects (tape control). I would try splitting code into three parts and use all three file tables.
*=$25e
dec $df11
bpl $267
*=$267
ldx $01
ldy #$37
bne $272
*=$272
sty $01
jmp $8xxx
That way you can have four open files without overwriting switching code.
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Sasq
Registered: Apr 2004
Posts: 156 |
I use the rest of the space to LDA different values and some BIT trickery to be able to overload 6 vectors, which takes all 3 * 8 bytes. I doubt a typical game has more than two open files though, and the tape motor thing is unfortunate but not that big a deal IMO since I wont be using tape.
There might be other problems, I wont know until I try it properly (havent had time thse last couple of days).
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