Raster scaling 6502

scaler.bbc

.scale_rast   LDY #0
              STY rast_in_y
              STY rast_out_y
.scr1         LDX #256-4
.scr2         LDA (rast_in), Y
              PHA
              AND #&07
              STA scratch - 4, X
              PLA
              AND #&70
              LSR A
              ORA scratch - 4, X
              STA scratch - 4, X
              INX
              BNE scr2
              STY rast_in_y

              LDY rast_out_y

\ Byte 0
              LDA scratch + 0
              ASL A
              ASL A
              LSR scratch + 1
              ROR A
              LSR scratch + 1
              ROR A
              STA (rast_out), Y
              INY

\ Byte 1
              LDA scratch + 2
              LSR A
              LSR A
              LSR A
              LSR A
              ORA scratch + 1
              STA (rast_out), Y
              INY

\ Byte 2
              LDA scratch + 2
              ASL A
              ASL A
              ASL A
              ROL scratch + 3
              ASL A
              ROL scratch + 3
              LDA scratch + 3
              STA (rast_out), Y
              INY

              STY rast_out_y
              LDY rast_in_y
              CPY #256 DIV 8 * 3
              BCC scr1

\ Update pointers
              CLC
              LDA rast_in + 0
              ADC #256 DIV 8 * 4
              STA rast_in + 0
              BCC scr3
              INC rast_in + 1
.scr3         CLC
              LDA rast_out + 0
              ADC rast_out_y
              STA rast_out + 0
              BCC scr4
              INC rast_out + 1
.scr4         RTS