[A83] Re: variable horizontal line routine?

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[A83] Re: variable horizontal line routine?


Just some general ideas for optimizing it a bit:

horizline:
  ;42 bytes
  ;c = x, l = y
  ld h,0
  add hl,hl      ; * 2
  add hl,hl      ; * 4
  ld d,h
  ld e,l
  add hl,hl      ; * 8
  add hl,de      ; * 12
  ld a,c
  rra
  rra
  rra
  and %00011111  ; x / 8
  ld e,a
  ld a,12
  sub e          ;get number of bytes left in row
  push af        ;save it
  add hl,de      ;add x component
  ld de,graphbuf ;whatever that address is ;-)
  add hl,de
  ld a,c
  cpl
  and %00000111
  inc a            ;shift 1-8 times depending on x value
  ld b,a
  xor a
getbitmask:
  sl1 a            ;undocumented opcode ;-)
  djnz getbitmask
  pop bc           ;number of bytes left in row
copybytes:
  ld (hl),a
  inc hl
  ld a,$ff         ;write $ff to all the other bytes
  djnz copybytes
  ret


In a message dated Fri, 1 Jun 2001  8:19:47 AM Eastern Daylight Time, Olle Hedman <oh@hem.passagen.se> writes:

<< 
At 12:32 2001-06-01, you wrote:

>ok, this should work, and should be fast enough, if you want you could

I'm sorry, but that is a _really_ crappy horiz-line routine.
It is about as slow as you can make a horiz-lineroutine. It is even quite 
possible that useing the rom linedrawingroutine is much faster.

what you should do is find the byte in screenram where the line should start,
with the formula x/8 + y*12  (12 is the width of the buffer in bytes)
then find wich pixel it starts on, and then fill this part of a byte, and 
then do bytewrites directly to the mem.

something along these lines: (Sorry, havn't even touched an 83 in 2 years, 
and almost not used any z80 so it could be a bit of here and there, but 
approx this: (its heavily based on my pixelplotting routine I posted in 
this list about 2* years ago or so)

input: a=x e=y

         ld      d,0             ; Do y*12
         sla     e
         sla     e
         ld      hl,0
         add     hl,de
         add     hl,de
         add     hl,de

         ld      d,0             ; Do x/8
         ld      e,a
         srl     e
         srl     e
         srl     e
         add     hl,de
          ld     b,e             ; save bytexpos in b too.

         ld      de,graphbuf  <-- or whatever. the adress of the graphbuf 
anyway.
         add     hl,de           ; Add address to graphbuf

         ld      de,0007h        ; Get the remainder of x/8
         and     e
          ld     e,a
          ld     IX,pixtable     ; Look up in table where line should start
          add    IX,de
          ld     a,(IX+0)        ; and load this

         ld     (HL),a

         ld     a,12   <--- maybe this should be 11 or something.
         sub    b
         ld     b,a

loop:
         inc    HL
         ld     (HL),0ffh  <--- you can do this can't you?
         djnz   loop

          ret

pixtable
         .db     11111111b
         .db     01111111b
         .db     00111111b
         .db     00011111b
         .db     00001111b
         .db     00000111b
         .db     00000011b
         .db     00000001b


------

Ok. this should work, untested now though. if it doesn't work it should 
with just a minor bugfixing. (I can help you bugfix if you want)
And this can at least claim to be fast. Maybe you can do it even faster, 
but I don't think it is possible to do it much faster.

///Olle



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