For developers wanting to explore FCM (full-colour mode), it can be quite a daunting journey initially, so here’s some pointers along with an ‘experimentation’ walkthrough that will let you peek and poke your way to a better understanding of FCM 
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Perhaps there was a time (earlier in the project) when the bit's sole purpose was to alter the purpose of screen-byte-pair to specify an a new x-position, and in those days it made sense to call the bit
GOTOXBut now, setting this bit does way more than that, it modifies the purpose of other bit-fields do to other things (y-offsets for chars, rowmask, foreground/background rendering, etc)
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I need to divide this by 64 bytes ($40) to get the char index
(since each pixel within the 8x8 character needs one byte in FCM mode)I.e., $4,0000 / $40 = $1000
So you need to set:
Screen RAM byte 0 to $00
Screen RAM byte 1 to $10
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What’s all this about 16-bit character numbers?!
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The docs for CHR16 say it enables "16-bit character numbers", when in reality, it enables “13-bit character numbers” (i.e., bits 0-12 defined in the two fields above)
Whether it's worth changing the name to CHR13 at this stage, I'd lean towards yes, it ought to change to reflect the reality and not mislead, but maybe others might feel the name is locked in now that it has gone to print 😄
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