The plot subroutine takes a coordinate and a color, and plots it on the screen. The code was originally taken from Lights Out. Plotting individual pixels is slower than blitting an image to VRAM at once, but is useful in some applications. There are two versions here: the original version, and a coordinate-adjusted version to make programming easier.
The following is the annotated source, with adjustments for the coordinates. Routine takes 69 machine cycles including pi and pop instructions.
;---------------; ; Plot Function ; ;---------------; ; plot out a single point on the screen ; uses three registers as "arguments" ; r1 = color $00 = green, $40 = %01000000 = red, $80 = %10000000 = blue, $C0 = %11000000 = background ; r2 = x (to screen) (0-101) ; r3 = y (to screen) (0-57) plot: ; set the color using r1 lr A, 1 outs 1 ; Color is set by bit 6 and 7 only ; set the column using r2 lis 4 as 2 ; adjust the x coordinate com outs 4 ; place inverted coordinate for x on port 4 (bit 7 not used) ; set the row using r3 lis 4 as 3 ; adjust the y coordinate com outs 5 ; place inverted coordinate for y on port 5 (bit 6, 7 not used, those are for sound) ; transfer data to the screen memory by toggling ARM lis 6 sl 4 outs 0 ; %01100000, no controller inputs, set b5 ARM to 1 sl 1 outs 0 ; %11000000, no controller inputs, b5 ARM = 0, b7 (N/C) is set to 1 ; delay until it's fully updated lis 6 ; Value here is 4 in Videocart 21, Bowling .plotDelay: ai $ff bnz .plotDelay pop ; return from the subroutine
The adjustment of x and y is to allow the coordinates 0, 0 to access the upper-left pixel of the screen (at least in MESS, for real it may be better or worse). VRAM in the Channel F actually expands four pixels above and to the left of that pixel. If you need a quicker plotting function, you can sacrifice this convenience and use the original version.
;---------------; ; Plot Function ; ;---------------; ; plot out a single point on the screen ; uses three registers as "arguments" ; r1 = color ; r2 = x (to screen) (4-105) ; r3 = y (to screen) (4-61) plot: ; set the color using r1 lr A, 1 outs 1 ; set the column using r2 lr A, 2 com outs 4 ; set the row using r3 lr A, 3 com outs 5 ; transfer data to the screen memory li $60 outs 0 li $50 outs 0 ; delay until it's fully updated lis 6 .plotDelay: ai $ff bnz .plotDelay pop ; return from the subroutine