>Reply to your message of 5/16/97 9:56 AM
>> I have just written a little test program that does SC writing with code
>> executed in the VCS RAM and it works. So that does not semm to be the
>> explanation.
That wasn't necessary, Cheetah does just that, as a matter of fact, Cheetah
also switches banks using code in the RAM area too, take a look at the
source code, it is on my webpage.
>> What is strange is, that in all my programs I have to use a NOP in the
>> SC writing routine like
>>
>> LDX #$1d
>> CMP $F000,X
>> NOP
>> CMP $FFF8
>>
>> But Bob's Etch program, for example, runs perfectly on my system, although
>> it doesn't have the NOPs.
Well, here is what is happening, if you use an indexed address mode as in the
example above, you don't need a NOP, even though my 6502 opcode chart says
this takes the same number of cycles as an unindexed address mode as such:
lda $F01D
nop
sta $FFF8
the above example NEEDS the nop to work correctly.
>Could you post his code (or just the part that does the write)
O.K. here it is, pretty ugly...
plot
lda INPT4
bmi plot4
lda xpos
bmi plot4
cmp #$60
bpl plot4
tay
plot1
ror
ror
ror
ror
and #$f
clc
adc #$fa
sta tmpadrhi
tya
and #$0f
tax
and #$08
beq plot2
lda #$80
plot2
sta tmpadrlo
lda pixel,x
ldy ypos
bmi plot4
cpy #$78
bpl plot4
ldx mode
cpx #$1
beq plot2a
cpx #$5
beq clear
ora (tmpadrlo),y
jmp plot2b
clear
lda #$0
beq plot2b
plot2a
eor (tmpadrlo),y
plot2b
tax
ldy enable
lda $f000,y
sta $fff8
ldy ypos
lda $f000,x
lda (tmpadrlo),y
ldy $80
lda $f000,y
sta $fff8
plot3
rts
plot4
sta WSYNC
rts
>Can the read that sets the bank configuration be anywhere in the cart's
>memory or does it have to be within the 1000 to 10ff range?
Yes, it can be anywhere, but if you switch out the bank your code is in,
then I don't know exactly what happens. I used the RAM to store my BS
routine for Cheetah, which saved me a lot of headaches.
Later,
Bob
Wanna write a game for the Atari 2600?
Check out http://www.novia.net/~rcolbert
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