Screen handling

[britlion]

Code:

SUB plotPoint (x as uByte, y as uByte)
ASM

ld d,a ;'X

    ld a, 191
    sub (IX+7) ;' y
    jr c, plotPoint_end    
    ld l,a    

ld h,ScreenTables/256
ld a,(HL)
inc h
ld l,(HL)
ld h,a
; now we're on the right row on screen.

ld a,d
RRCA  ; divide column by 8 for bytes.
RRCA
RRCA
AND 31
add a,l
ld l,a
    
    ld a, d
    and 7
    ld b, a
    inc b
    ld a, 1
    
    plotPoint_loop:
        rrca
    djnz plotPoint_loop    
    ld b, a    
    ld a, (hl)
    or b    
    ld (hl), a

plotPoint_end:
    
END ASM
END SUB

Using screen address tables shaves this down to 6.42 seconds for 50,000 points plotted. That's not too bad, all told.
(screentables.asm is available elsewhere in the forum, I promise!)

Considering that quite a few routines can use the screen lookup tables, 512 bytes of screen tables isn't crazy for a solid speedup (and of course, if you are using a shadow screen, then you could either rewrite a bit, or make screen tables that point to your shadow screen memory).

Not sure requiring rotate tables, which is much bigger, is a win, here for one loop removal.