The VIC has the ability to display not only the charsets build into the rom, but also custom made ones. For each character 8 bytes are reserved so they represent 8×8 pixels. The Petscii Character '@' uses the first 8 bytes, the 'a' the second 8 bytes etc.

Contrary to the bitmap mode these 8×8 pixel blocks can be freely positioned on the screen. Advantageously the blocks can be ordered in a way so that the y coordinate equals the byte offset in the charset. Character '@' represents the y coordinates 0-7, character 'a' the coordinated 8-15 etc.

If you code an effect with rotation, for example a vector, then in a 16×16 square the edge chars are often not used at all. If so, it is better to order the chars in a circular grid like this:

This has the advantage of covering a bigger area.

This code uses the ca65 assembler.

ichar  
       ldx #0
initic 
       txa
       ldy #0
i2
       sta scrn,y
       clc
       adc #icharhigh
       iny
       cpy #icharwidth
       bne i2

       lda i2+1
       clc
       adc #scrwidth
       sta i2+1
       bcc *+5
       inc i2+2


       inx
       cpx #icharhigh
       bne initic
       rts

The coordinates are passed in the X and Y registers.

plotpixel   lda #<charset
            sta $fb

            ; ptr = (x / 8) * 128
            txa
            lsr                     ; x / 8
            lsr
            lsr

            lsr                     ; * 128 (16-bit)
            ror $fb
            adc #>charset
            sta $fc

            ; mask = 2 ^ (x & 3)
            txa
            and #%00000111
            tax
            lda ($fb),y
            ora bitmask,x
            sta ($fb),y
            rts

bitmask     .byte $80,$40,$20,$10,$08,$04,$02,$01