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Oswald
Registered: Apr 2002
Posts: 5094 |
Misaligned Char reads ?!
I was just fooling and found this.
the code does this:
near but well before the left border edge :
lda $d012 ;8
clc ;10
adc #$02 ;12
and #7 ;14
ora #$18 ;16
sta $d011
and 8 cycles after that:
lda $d012 ;8
clc ;10
adc #$00 ;12
and #7 ;14
ora #$18 ;16
sta $d011 ;20
as I add #2 to d011 in the first write I thought this could be used to rotate the characters, because the misalignment is exactly 2 lines, but the misalignment stays 2 even if I write 3 to d011 (while trying 0-7 in the 2nd d011 write, well most cases results in fld), or change starting rasterline 1 line lower, etc.
anyone can explain this?
yeah.. only tested in emulator. 3.3 x64sc has same results as older vice and non sc. |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
What does it do on real hardware? |
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Oswald
Registered: Apr 2002
Posts: 5094 |
results are some on real hw. btw the code does run only every 2nd line as indicated by the raster coloring. |
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Flavioweb
Registered: Nov 2011
Posts: 463 |
Looks to me like a mix of last line repeat, fld and linecrunch... in this order.
Last char line is repeated then 1 line fld is inserted then vic restart draw "next line" with "old data" but crunching first line of the new char...
...if this make some sense... |
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Oswald
Registered: Apr 2002
Posts: 5094 |
"crunching first line of the new char."
thats still something that was never seen before. cant be fld because there would be empty lines between. the misalignment is also present in bitmap mode:
 |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
Can you post the prg?
There is also what looks like a 0xff glitch on the top chars STU which is interesting. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
+1 for prg. also source please, this looks like a good candidate for a regression test :) |
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Oswald
Registered: Apr 2002
Posts: 5094 |
as is, this was originally ment to test if repeating 2 lines from the top char row endlessly is possible. Long after achieving the goal I returned to it with some random hacking and stumbled upon the result above.
d018tab = $1e00
dd00tab = $1f00
ysize = 200
*= $1000
sei
lda #$7f
sta $dc0d
lda #$01
sta $d01a
lda $dc0d
lda $dd0d
lda #$1b
sta $d011
lda #$36
sta $d012
lda #$35
sta $01
lda #<irq1
sta $fffe
lda #>irq1
sta $ffff
ldx #$00
- txa
sta $db00,x
dex
bne -
.for ue=0,ue<25,ue=ue+1
lda #$28+ue
sta $0410+ue*40
.next
;jmp *
lda #3
sta $dd00
u
ldx #200
- txa
and #%00000001
sta d018tab,x
dex
bne -
ldx #$27
- txa
sta $0400,x
dex
bpl -
lda ($01,x)
bit $eaea
cli
jmp u
irq1
pha
txa
pha
tya
pha
inc $d019
inc $d020
lda $d012
- cmp $d012
beq -
ldx #12
dex
bne *-1
nop
nop
nop
;nop
ldx #$00
rloop ldy d018tab,x ;4
lda $d012 ;8
clc ;10
adc #$04 ;12
and #7 ;14
ora #$18 ;16
sta $d011 ;20
sty $d020 ;24
;bit $eaea ;28
;bit $eaea ;32
;bit $eaea ;36
;bit $eaea ;40
;bit $eaea ;44
;bit $ea ;47
;bit $ea ;50
;bit $eaea
;bit $ea
;bit $ea
;nop ;52
ldy d018tab,x ;4
lda $d012 ;8
clc ;10
adc #$00 ;12
and #7 ;14
ora #$18 ;16
sta $d011 ;20
sty $d020 ;24
;bit $eaea ;28
;bit $eaea ;32
nop
nop
bit $eaea ;36
bit $eaea ;40
bit $eaea ;44
bit $eaea ;48
bit $eaea ;52
bit $eaea ;56
bit $eaea
inx ;58
cpx #ysize/2 ;60
bne rloop ;63
dec $d020
lda #<irq2
sta $fffe
lda #>irq2
sta $ffff
lda #$ff
sta $d012
inc $d020
pla
tay
pla
tax
pla
rti
irq2
pha
txa
pha
tya
pha
inc $d019
inc $d020
lda #$1b
sta $d011
lda #$2
sta $d020
lda #<irq1
sta $fffe
lda #>irq1
sta $ffff
lda #3
sta $dd00
lda #20
sta 53272
lda #$33
sta $d012
pla
tay
pla
tax
pla
rti
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Scan
Registered: Dec 2015
Posts: 111 |
I don't have an explanation for what I'm looking at, but I only looked briefly and will examine closer using C64Debugger in the evening. But I see something off in the comments with the cycle counting.
ldy d018tab,x ;4
lda $d012 ;8
clc ;10
adc #$00 ;12
and #7 ;14
ora #$18 ;16
sta $d011 ;20 (introduce badline condition)
sty $d020 ;24
;bit $eaea ;28
;bit $eaea ;32
nop
nop
bit $eaea ;36
bit $eaea ;40
bit $eaea ;44
bit $eaea ;48
bit $eaea ;52
bit $eaea ;56 This part will introduce a badline, but the cycle counting in the comments assumes it's not a badline and that the VIC won't stall the CPU.
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Oswald
Registered: Apr 2002
Posts: 5094 |
dont rely on the comments, they are leftovers, garbage, random lines were insterted etc. |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
The lda ($01,x) could introduce introduce memory read, CIA or VIC ack, issues.
However probably not related to this effect.
I'm currently puzzled. |
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