====== Basics ======

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 8x8 pixels.
The Petscii Character '@' uses the first 8 bytes,
the 'a' the second 8 bytes etc.

====== What is it about ======

Contrary to the bitmap mode these 8x8 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. 

==== A 16x16 matrix looks like this ====

{{base:16x16_char_display_small.jpg|}}

==== Stupid effect using 16x16 character mode ====

{{base:16x16stupideffect.jpg|}}

====== Extentions ======

If you code an effect with rotation, for example a vector,
then in a 16x16 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:

{{:base:circular-chars.png|}}

This has the advantage of covering a bigger area.

====== Code ======

===== Code for generating 16x16 rectangular grid =====
This code uses the ca65 assembler.

<code>
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
</code>

===== Code for plotting a pixel to the 16x16 grid =====

The coordinates are passed in the X and Y registers.

<code>
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
</code>