This is a short reference to the XY-2000 PPU.

These are practical numbers which show plot pixel being used to clear the screen in an x/y loop. 460,000 pixels/second is 7,600 pixels per frame at 60fps. That is almost exactly 30 16×16 sprites. Suggested use is to MEMCOPY a pre-drawn background into the frame-buffer and then draw sprites with PPIXEL (if you want to use PPIXEL for sprites). This way you don't have to draw the whole screen, and you don't have to draw the sprite background. It is hard to say how many sprites you will actually be able to draw in a frame because each sprite has more or less empty background space.

PPIXEL can be called via INT 0x03:

  LDA  $0100       ; AH = 1 (PPU dispatch), AL = 00 (plot_pixel)
  INT  $03         ; plot_pixel(X, Y, C)

It replaces the old plot pixel function in the graphics library (AH $01 INT $18):

; ============================================================================
; AH=01h - Plot MODE 3 Pixel (4bpp packed nibbles)
; ============================================================================
; Input:  X = x coordinate (mode dependant)
;         Y = y coordinate (mode dependant)
;         C = color (0-15)
; Output: CF = 0 on success, 1 if out of bounds
; ============================================================================
int18_plot_pixel_4bpp:
    PUSHA

    ; Bounds check using SCREEN_WIDTH / SCREEN_HEIGHT
    LDA [@SCREEN_WIDTH]
    CMP X, A
    JC @plot4_error
    LDA [@SCREEN_HEIGHT]
    CMP Y, A
    JC @plot4_error

    PUSH X                      ; save x
    PUSH C                      ; save color

    LDA [@SCREEN_WIDTH]
    SHR A                       ; A = stride (width / 2)
    MOV B, A                    ; B = stride
    MOV A, Y                    ; A = y
    MUL A, B                    ; A = y * stride

    POP C                       ; restore color
    POP B                       ; restore x into B
    PUSH B                      ; save x for nibble check
    MOV T, B
    SHR T
    ADD A, T                    ; A = byte offset

    MOV J, A
    LDI #2

    POP A                       ; A = x
    LDTL $01
    AND AL, TL
    CMP AL, $00
    JNZ @plot4_odd

plot4_even:
    LDBL [I:J]
    MOV AL, BL
    UAB                         ; AL = odd pixel, BL = even pixel
    MOV BL, CL                  ; BL = new color
    PAB                         ; AL = (new color << 4) | odd pixel
    MOV BL, AL
    STBL [I:J]
    POPA
    CLC
    RET

plot4_odd:
    LDBL [I:J]
    MOV AL, BL
    UAB                         ; AL = odd pixel, BL = even pixel
    MOV AL, CL                  ; AL = new color
    PAB                         ; AL = (even pixel << 4) | new color
    MOV BL, AL
    STBL [I:J]
    POPA
    CLC
    RET

plot4_error:
    POPA
    SEC
    RET