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48-Pixel / 8-Character Display


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While planning my Nyan Cat game, I decided I wanted the main menu to allow up to 8 characters per line.  This could easily be done with each word hard coded into the ROM, but I wanted to allow each character to be set on-the-fly.  This takes less ROM when a lot of messages are needed, but it also allows this kernel to be used for 8-digit scores, if desired.  I am considering remaking the score display for my game to utilize this.

I have been working on this display for quite a while now, and I am glad to say it is finished!


It uses 3 pre-shifted graphics tables for the font.  I am using a font that is 7 pixels high, allowing 36 unique characters to fit in each page.  This works perfectly for me, as I am using 10 numbers, plus 25 letters (letter-O is shared with zero), plus a space, but this could be adjusted as necessary.  If this was only used as a score display, all 3 graphics tables would easily fit within a single page.

The kernel and pre-kernel subroutines take up a total of 235 bytes.  I split them into 2 separate routines so that I could more easily utilize them in different areas of my game, but there is no reason why they couldn't be combined.

32 bytes of RAM are used in this routine, 8 for pointers, 21 for graphics buffers, and 3 for temp variables.


The graphics tables are all upside-down.  Any bits not used are cleared.

XXXXX___ Table #0 has the data shifted to the far left.

XXX___XX Table #1 has the first 2 bits shifted to the far right, and the last 3 bits shifted to the far left (so it "wraps" around the end of the byte to the beginning).

__XXXXX_ Table #2 has the data shifted almost all the way to the right.

Edit: As-is, the code looks for a label FontGfx0, which would be Table #0.  It assumes that Table #1 and #2 are in the following 2 pages.


The way text is selected for the routine is by loading Y with the offset of the first of the 8 characters to display.  You would set up a table as such:

	HEX ## ## ## ## ## ## ## ##
	HEX ## ## ## ## ## ## ## ##
	HEX ## ## ## ## ## ## ## ##

The reason for this is that you can save some ROM by overlapping messages with each other.

Edit: As-is, the code assumes this table is called FontText.


Actually using these routines would look like this:

	sta WSYNC

	ldy #0		; start with character #0
	jsr CharLoad	; prepare for kernel
	jsr CharKernel	; draw kernel

	SLEEP 9		; or change color, for example
	ldy #8		; start with character #8
	jsr CharLoad	; prepare for kernel
	jsr CharKernel	; draw kernel


I have included both routines, as well as an example binary.

8_char_display.bin 8_char_routine.asm

Edited by JeremiahK
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