;
; fast textmode putpixel
; (made for c64 but can be used anywhere)
;
;
; it's real fast but takes 50 bytes of zeropage
; for 25 columns screen
; -> good for demos ;-)
;
;
; how does it work:
; on zeropage there are 2byte columns' pointers
; and there is only Y index reg used to determine
; row of pixel
; just like STA ([column*2+zeropageptr]),y
;
; Written from scratch by Rafal Szyja , Poland
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

zeropage= 100 ; zeropage pointer for generating tables

_x= 251   ; x coordinate of
_y= 252   ; y

temp= 99   ; temporary storage
   

   *= $1000
   
   JSR gentab   ; this is called only once for this routine
         ; by altering gentab you can for example make table
         ; for different screen posiotion and organization
         ; than c64's poweron default 40x25 @ 0400

; this is only testpiece - pixels are to be set somewhere else
; and then jump to setpix
   LDA #0
   STA _x
   LDA #10
   STA _y

; real start of routine

setpix   
   LDA _y
   ASL
   CLC   ; y coor will be always < 128 so you can gain 2 cycles by deleting this line
   ADC #100
   sta dest+1   ; program automodification = faster operation
   
   ldy _x
   lda #255   ; just to test routine , there is actual pixelval
dest   
   STA ($00),y   
   rts
   
;---------------------------------------------------------)
   
gentab   ; generates tables with columns' values
   LDA #$00   ; LSB of screen
   LDX #$04   ; MSB of screen
   
   LDY #0
   
loop   
   STA zeropage,y
   INY
   sta temp
   txa
   STa zeropage,y
   lda temp
   INY
   CLC
   ADC #40      ; 40 rows
   BCC _a
   INX
_a   
   CPY #50
   BEQ stop
   jmp loop

stop    
   RTS

   *= $1000
   
_x= 251   ; x
_y= 252   ; y
   
dyl = 253   ; destination lo
dyh = 254   ;         hi
   
   LDA #2
   STA _y
   LDA #2
   sta _x
   
   LDA #0
   STA dyh
   LDA _y
   ASL ;A
   ROL dyh
   ASL ;a
   ROL dyh
   CLC
   ADC _y
   BCC x1
   INC dyh
x1
   ASL ;A
   ROL dyh
   ASL ;A
   ROL dyh
   ASL ;A
   ROL dyh
   STA dyl
   
   CLC
   LDA dyh
   ADC #$04
   STA dyh
   
   
   LDY _x   
   LDA #160
   STA (dyl),y
   JMP *