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Jeff calc 84
Power User (Posts: 325)

05 Sep 2026 06:04:45 am

Here's the code for that horizontal line drawer I've been working on:

Code:

xpos .equ appbackupscreen


ypos .equ appbackupscreen+1


len .equ appbackupscreen+2


   call ClrScreen


   ld ix,xpos


   ld (ix),0


   ld (ix+1),0


   ld (ix+2),8


   call HorizLine


   bcall(_GrBufCpy)


   ret


;start of horizontal line routine.


;Inputs:


;x location: xpos


;y location: ypos


;Length: len


;Outputs:


;A line, leftmost pixel on (xpos,ypos), length equal to len.


;Need to call _GrBufCpy to actually display the line.


;Destroys:





HorizLine:


   ld hl,(ypos)


   ld h,0


   add hl,hl


   add hl,hl


   ld d,h


   ld e,l


   add hl,hl


   add hl,de      ;HL=ypos*12


   ld de,(xpos)


   ld a,e         ;A=xpos


   and %00000111  ;A=xpos%8, the bit we need


   srl e


   srl e


   srl e          ;E=xpos//8


   ld d,0


   add hl,de      ;HL=ypos*12+xpos//8, the byte we need


   ld de,(len-1)  ;Gets (len) into D


   add a,d        ;Check to see if can be drawn in 1 byte


   cp 9


   jr nc,HNotInAByte ;a>8


   jr c,HInAByte  ;a<=8


HInAByte:


   sub d          ;Get back the original A


   ld b,d


   ld e,1


HLoop1:


   sla e


   djnz HLoop1


   push af


   ld a,e


   sub 1


   ld e,a


   pop af         ;E-=1


   ;After this operation, E has the lower D bits as 1


   cpl


   add a,9


   sub d


   or a


   jr z,HAIsZero


   ld b,a         ;B=8-A-D


HLoop2:


   sla e


   djnz HLoop2


HAIsZero:


   ld a,e


   ld de,PlotSScreen


   add hl,de


   ld (hl),a


   ret


HNotInAByte:


   ret


   


ClrScreen:


   ;Clears the screen.


sp_save .equ AppBackupScreen


   ld (sp_save),sp


   di


   ld sp,PlotSScreen+768


   ld b,64


   ld hl,0





ClrLoop:


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   push hl


   djnz ClrLoop





   ld sp,(sp_save)


   ei


   bcall(_GrBufCpy)


   ret

Currently it'd draw anything that can be drawn in 1 byte of writes.

ti_kid
Advanced Member (Posts: 239)

05 Sep 2026 06:42:38 am

About your clear routine, why not just loop over each byte and set it to 0. Something like this

Code:

set_zero:


    ld hl, 768


    ld de, plotsscreen


loop:


    xor a


    ld (de), a


    dec hl ;unfortunately inc and dec dont change flags if used on 16 bit registers


    ld a, h


    or l ; these 2 lines check if hl is 0


    jr loop


; end

Have you learned about ldir yet?

ldir copies over BC amount of bytes from (hl) to (de) incrementing hl and de aswell.

What would happen if perhaps you set the first byte of plotsscreen to 0 and copied over 767 bytes from plotsscreen to plotsscreen+1 ? Not going to provide sample code to see if you can do it yourself.

Jeff calc 84
Power User (Posts: 325)

05 Sep 2026 09:43:17 am

Tbh the only reason why is because SP is really fast. Yes, it's bad for readability, but it's, well, cool. 😉

Jeff calc 84
Power User (Posts: 325)

06 Sep 2026 02:16:15 am

And now the routine is done! And I've already refactored, and optimized, the code. Routine is only 154 bytes large. 🙂

Code:

xpos .equ appbackupscreen


ypos .equ appbackupscreen+1


len .equ appbackupscreen+2





   ld bc,767


   ld hl,PlotSScreen


   ld de,PlotSScreen+1


   ld (hl),0


   ldir


   ld ix,xpos


   ld (ix),2


   ld (ix+1),0


   ld (ix+2),5


   call HorizLine


   ld (ix),5


   ld (ix+1),2


   ld (ix+2),10


   call HorizLine


   ld (ix),7


   ld (ix+1),4


   ld (ix+2),10


   call HorizLine


   bcall(_GrBufCpy)


   ret


;start of horizontal line routine.


;Inputs:


;x location: xpos


;y location: ypos


;Length: len


;Outputs:


;A line, leftmost pixel on (xpos,ypos), length equal to len.


;Need to call _GrBufCpy to actually display the line.


;Destroys: all, including (appbackupscreen) to (appbackupscreen+3)





HorizLine:


   ld hl,(ypos)


   ld h,0


   add hl,hl


   add hl,hl


   ld d,h


   ld e,l


   add hl,hl


   add hl,de      ;HL=ypos*12


   ld de,(xpos)


   ld a,e         ;A=xpos


   and %00000111  ;A=xpos%8, the bit we need


   srl e


   srl e


   srl e          ;E=xpos//8


   ld d,0


   add hl,de      ;HL=ypos*12+xpos//8, the byte we need


   ld de,PlotSScreen


   add hl,de


   ld de,(len-1)  ;Gets (len) into D


   add a,d        ;Check to see if can be drawn in 1 byte


   ;cp 17


   ;jr nc,HNotInTwoBytes ;a>16


   cp 8


   jr nc,HNotInAByte ;a>7


   ;If a<=7...


HInAByte:


   sub d          ;Get back the original A


   ld b,d


   ld e,1


HLoop1:


   sla e


   djnz HLoop1


   push af


   ld a,e


   sub 1          ;E-=1


   ld e,a


   pop af


   ;After this operation, E has the lower D bits as 1


   cpl


   add a,9


   sub d


   or a


   jr z,HAIsZero


   ld b,a         ;B=8-A-D


HLoop2:


   sla e


   djnz HLoop2


HAIsZero:


   ld (hl),e


   ret


HNotInAByte:


   ld (len-1),a


   sub d          ;Get back original A


   or a


   jr z,HIf1      ;If A=0


   cpl


   add a,9        ;A=8-A


   ld b,a


   cpl


   add a,d


   ld d,a         ;D=D-A-1


   ld a,1


HLoop3:


   sla a


   djnz HLoop3


   sub 1


   ;After this, the lower B bits of A are set to one.


   ld (hl),a


HIf1:


   ld a,(len-1)   ;Get (len-1) into A


   cp 17


   jr nc,HIf1Cond1


   push de


   ld de,1


   add hl,de      ;HL+=1


   pop de         ;Get D back


   ld a,d


   jr HIf1End


HIf1Cond1:


   ld a,d


   srl a


   srl a


   srl a


   push de


   ld de,1


   ;Loop: just write $FF to (hl) until d is less than 8.


HLoop6:


   add hl,de


   ld (hl),$FF


   sub 1


   jr nz,HLoop6


   add hl,de


   pop de


   ld a,d


   and %00000111


   ld d,a


HIf1End:


   or a


   ld a,$80       ;Flags are unchanged


   jr z,HDIsZero


   ld b,d


HLoop4:


   sra a


   djnz HLoop4


   ;The upper B bits of A are set to one.


HDIsZero:


   ld (hl),a


   ret

I used an LDIR there to save space.
The code's still very buggy, darn. ☹️

Now onto sprites!

Jeff calc 84
Power User (Posts: 325)

03 Oct 2026 12:53:55 am

Psst: Nibbles game is in the works!
https://ourl.ca/22897 for more details.
Planned features:
12*16 screen. (4*4 pixels to one block)
Yeah, that's about it...

EDIT: After rethinking things, perhaps a guessing game could be easier to code.

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