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Update apple2_hires.md

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@ -206,39 +206,40 @@ The same is in fact true the other two A-zones. For the first one, addresses end
Now imagine you want scroll only the last 32 lines of the screen, then the MSB of the baseline will be either `#$22` or `#$23` to which you add 4 for each of the next seven lines while the LSB flips between `#$50` and `#$D0`. And you have many options to unroll the loops if you want to speed things up a little bit further
For instance this code would copy bytes 1-39 of each line of the 32 last lines of the screen to bytes 0-38 effectively scrolling that part of the screen one byte to the left.
(note: this is just one way, I'm not saying it's better than any other)
LDA #$22 ; base address MSB
CLC ; we're going to ADC !
LDX #38 ; 39 times per line
loop ; 4 lines at a time
.ldy1 LDY $2251,X ; copy byte X on 1st line
.sty1 STY $2250,X ; to previous byte
.ldy2 LDY $22D1,X ; same for 2nd line
.sty2 STY $22D0,X
.ldy3 LDY $2351,X ; 3rd line
.sty3 STY $2350,X
.ldy4 LDY $23D1,X ; 4th line
.sty4 STY $23D0,X
DEX
BPL loop ; stop when X goes back to #$FF
ADC #4 ; the next 4 lines base addr
CMP #$40 ; have we done them all ?
BCS .rts
STA ldy1+2 ; self modifying code
STA sty1+2 ; MSB for first 2 lines
STA ldy2+2
STA sty2+2
ADC #1 ; add one more for the
STA ldy3+2 ; MSB of the last 2 lines
STA sty3+2
STA ldy4+2
STA sty4+2
.rts RTS
```
LDA #$22 ; base address MSB
.loopx SEC ; set carry
STA .ldy1+2 ; self modifying code
STA .sty1+2 ; MSB for first 2 lines
STA .ldy2+2
STA .sty2+2
ADC #0 ; 0+carry = 1 !
STA .ldy3+2 ; MSB of the last 2 lines
STA .sty3+2 ; add one more for the
STA .ldy4+2
STA .sty4+2
TAY ; save in Y for now
LDX #0 ; init byte counter
.loop ; 4 lines at a time
.ldy1 LDA $2251,X ; copy byte X on 1st line
.sty1 STA $2250,X ; to previous byte
.ldy2 LDA $22D1,X ; same for 2nd line
.sty2 STA $22D0,X
.ldy3 LDA $2351,X ; 3rd line
.sty3 STA $2350,X
.ldy4 LDA $23D1,X ; 4th line
.sty4 STA $23D0,X
INX ; next byte
CPX #$27 ; last byte ?
BCC .loop ; not yet
TYA ; carry is set ! get back MSB
ADC #2 ; 2+carry = 3 !
CMP #$40 ; have we done them all ?
BCC .loopx ; not yet
```