mandel-6502/mandel.s

; Our zero-page vars
sx    = $80     ; i16: screen pixel x
sy    = $82     ; i16: screen pixel y
ox    = $84     ; fixed4.12: center point x
oy    = $86     ; fixed4.12: center point y
cx    = $88     ; fixed4.12: c_x
cy    = $8a     ; fixed4.12: c_y
zx    = $8c     ; fixed4.12: z_x
zy    = $8e     ; fixed4.12: z_y

zx_2  = $90     ; fixed4.12: z_x^2
zy_2  = $92     ; fixed4.12: z_y^2
zx_zy = $94     ; fixed4.12: z_x * z_y
dist  = $96     ; fixed4.12: z_x^2 + z_y^2

iter         = $a0 ; u8: iteration count

zoom         = $a1 ; u8: zoom shift level
count_frames = $a2 ; u8
count_pixels = $a3 ; u8
total_ms     = $a4 ; float48
total_pixels = $aa ; float48

z_buffer_active = $b0 ; boolean: 1 if we triggered the lake, 0 if not
z_buffer_start  = $b1 ; u8: index into z_buffer
z_buffer_end    = $b2 ; u8: index into z_buffer
temp            = $b4 ; u16
temp2           = $b6 ; u16
pixel_ptr       = $b8 ; u16
pixel_color     = $ba ; u8
pixel_mask      = $bb ; u8
pixel_shift     = $bc ; u8
pixel_offset    = $bd ; u8
fill_level      = $be ; u8
palette_offset  = $bf ; u8

; FP registers in zero page
FR0    = $d4 ; float48
FRE    = $da
FR1    = $e0 ; float48
FR2    = $e6 ; float48
CIX    = $f2 ; u8 - index into INBUFF
INBUFF = $f3 ; u16 - pointer to ascii
FLPTR  = $fc ; u16 - pointer to user buffer float48

CH1    = $02f2 ; previous character read from keyboard
CH     = $02fc ; current character read from keyboard

LBUFF  = $0580 ; result buffer for FASC routine

; FP ROM routine vectors
FASC   = $D8E6 ; FLOATING POINT TO ASCII (output in INBUFF, last char has high bit set)
IFP    = $D9AA ; INTEGER TO FLOATING POINT CONVERSION (FR0:u16 -> FR0:float48)
FADD   = $DA66 ; ADDITION       (FR0 += FR1)
FSUB   = $DA60 ; SUBTRACTION    (FR0 -= FR1)
FMUL   = $DADB ; MULTIPLICATION (FR0 *= FR1)
FDIV   = $DB28 ; DIVISION       (FR0 /= FR1)
ZF1    = $DA46 ; CLEAR ZERO PAGE FLOATING POINT NUMBER (XX)
FLD0R  = $DD89 ; LOAD FR0 WITH FLOATING POINT NUMBER (YYXX)
FLD1R  = $DD98 ; LOAD FR1 WITH FLOATING POINT NUMBER (YYXX)
FST0R  = $DDA7 ; STORE FR0 IN USER BUFFER (YYXX)
FMOVE  = $DDB6 ; MOVE FR0 TO FR1

; High data
framebuffer_top    = $8000
textbuffer         = $8f00
framebuffer_bottom = $9000
display_list       = $9f00
framebuffer_end    = $a000

height = 184
half_height = height >> 1
width = 160
half_width = width >> 1
stride = width >> 2

DMACTL = $D400
DLISTL = $D402
DLISTH = $D403
WSYNC  = $D40A

; OS shadow registers
SDLSTL = $230
SDLSTH = $231

; interrupt stuff
SYSVBV = $E45F
XITVBV = $E462
SETVBV = $E45C

COLOR0 = $2C4
COLOR1 = $2C5
COLOR2 = $2C6
COLOR3 = $2C7
COLOR4 = $2C8

; Keycodes!
KEY_PLUS  = $06
KEY_MINUS = $0e
KEY_UP    = $8e
KEY_DOWN  = $8f
KEY_LEFT  = $86
KEY_RIGHT = $87

.struct float48
    exponent .byte
    mantissa .byte 6
.endstruct

.import mul_lobyte256
.import mul_hibyte256
.import mul_hibyte512

.data

strings:
str_self:
    .byte "MANDEL-6502"
str_self_end:
str_speed:
    .byte " ms/px"
str_speed_end:
str_run:
    .byte " RUN"
str_run_end:
str_done:
    .byte "DONE"
str_done_end:

str_self_len = str_self_end - str_self
str_speed_len = str_speed_end - str_speed
str_run_len = str_run_end - str_run
str_done_len = str_done_end - str_done
speed_precision = 6

speed_start = 40 - str_done_len - str_speed_len - speed_precision - 1
speed_len = 14 + str_speed_len


char_map:
    ; Map ATASCII string values to framebuffer font entries
    ; Sighhhhh
    .repeat 32, i
        .byte i + 64
    .endrepeat
    .repeat 64, i
        .byte i
    .endrepeat
    .repeat 32, i
        .byte 96 + i
    .endrepeat

hex_chars:
    .byte "0123456789abcdef"

aspect:
    ; aspect ratio!
    ; pixels at 320w are 5:6 (narrow)
    ; pixels at 160w are 5:3 (wide)
    ;
    ; cy = (sy << (8 - zoom)) * (96 / 128 = 3 / 4)
    ; cx = (sx << (8 - zoom)) * ((3 / 4) * (5 / 3) = 5 / 4)
    ;
    ; so vertical range -92 .. 91.9 is -2.15625 .. 2.15624
    ; &horizontal range -80 .. 79.9 is -3.125 .. 3.124
    ;
    ; 184h is the equiv of 220.8h at square pixels
    ; 320 / 220.8 = 1.45 display aspect ratio
aspect_x: ; fixed4.16 5/4
    .word 5 << (12 - 2)

aspect_y: ; fixed4.16 3/4
    .word 3 << (12 - 2)

ms_per_frame: ; float48 16.66666667
    .byte 64  ; exponent/sign
    .byte $16 ; BCD digits
    .byte $66
    .byte $66
    .byte $66
    .byte $67

display_list_start:
    ; 24 lines overscan
    .repeat 3
        .byte $70 ; 8 blank lines
    .endrep

    ; 8 scan lines, 1 row of 40-column text
    .byte $42
    .addr textbuffer

    ; 184 lines graphics
    ; ANTIC mode e (160px 2bpp, 1 scan line per line)
    .byte $4e
    .addr framebuffer_top
    .repeat half_height - 1
        .byte $0e
    .endrep
    .byte $4e
    .addr framebuffer_bottom
    .repeat half_height - 1
        .byte $0e
    .endrep

    .byte $41 ; jump and blank
    .addr display_list
display_list_end:
display_list_len = display_list_end - display_list_start

color_map:
    .byte 0
    .repeat 85
        .byte 1
        .byte 2
        .byte 3
    .endrepeat

palette:
    .byte $00
    .byte $46
    .byte $78
    .byte $b4
.code

z_buffer_len = 16
z_buffer_mask = z_buffer_len - 1
z_buffer:
    ; the last N zx/zy values
    .repeat z_buffer_len
        .word 0
        .word 0
    .endrepeat

.export start

max_fill_level = 6
fill_masks:
    .byte %00011111
    .byte %00001111
    .byte %00000111
    .byte %00000011
    .byte %00000001
    .byte %00000000

; 2 + 9 * byte cycles
.macro add bytes, dest, arg1, arg2
    clc ; 2 cyc
    .repeat bytes, byte ; 9 * byte cycles
        lda arg1 + byte
        adc arg2 + byte
        sta dest + byte
    .endrepeat
.endmacro

.macro add16 dest, arg1, arg2
    add 2, dest, arg1, arg2
.endmacro

.macro add32 dest, arg1, arg2
    add 4, dest, arg2, dest
.endmacro

.macro add_carry dest
    lda dest
    adc #0
    sta dest
.endmacro

; 2 + 9 * byte cycles
.macro sub bytes, dest, arg1, arg2
    sec ; 2 cyc
    .repeat bytes, byte ; 9 * byte cycles
        lda arg1 + byte
        sbc arg2 + byte
        sta dest + byte
    .endrepeat
.endmacro

.macro sub16 dest, arg1, arg2
    sub 2, dest, arg1, arg2
.endmacro

.macro sub32 dest, arg1, arg2
    sub 4, dest, arg1, arg2
.endmacro

.macro shl bytes, arg
    asl arg
    .repeat bytes-1, i
        rol arg + 1 + i
    .endrepeat
.endmacro

.macro shl16 arg
    shl 2, arg
.endmacro

.macro shl24 arg
    shl 3, arg
.endmacro

.macro shl32 arg
    shl 4, arg
.endmacro

; 6 * bytes cycles
.macro copy bytes, dest, arg
    .repeat bytes, byte ; 6 * bytes cycles
        lda arg + byte  ; 3 cyc
        sta dest + byte ; 3 cyc
    .endrepeat
.endmacro

.macro copy16 dest, arg
    copy 2, dest, arg
.endmacro

.macro copy32 dest, arg
    copy 4, dest, arg
.endmacro

.macro copyfloat dest, arg
    copy 6, dest, arg
.endmacro

; 2 + 8 * byte cycles
.macro neg bytes, arg
    sec ; 2 cyc
    .repeat bytes, byte ; 8 * byte cycles
        lda #00         ; 2 cyc
        sbc arg + byte  ; 3 cyc
        sta arg + byte  ; 3 cyc
    .endrepeat
.endmacro

; 18 cycles
.macro neg16 arg
    neg 2, arg
.endmacro

; 34 cycles
.macro neg32 arg
    neg 4, arg
.endmacro

; 518 - 828 cyc
.macro imul16 dest, arg1, arg2
    copy16 FR0, arg1  ; 12 cyc
    copy16 FR1, arg2  ; 12 cyc
    jsr imul16_func   ; 470-780 cyc
    copy32 dest, FR2  ; 24 cyc
.endmacro

.macro shift_round_16 arg, shift
    .repeat shift
        shl32 arg
    .endrepeat
    round16 arg
.endmacro

.macro imul16_round dest, arg1, arg2, shift
    copy16 FR0, arg1  ; 12 cyc
    copy16 FR1, arg2  ; 12 cyc
    jsr imul16_func   ; 470-780 cyc
    shift_round_16 FR2, shift
    copy16 dest, FR2 + 2  ; 12 cyc
.endmacro

; Adapted from https://everything2.com/title/Fast+6502+multiplication
.macro imul8 dest, arg1, arg2
    .local under256
    .local next
    .local small_product
    .scope
        mul_factor_a   = arg1
        mul_factor_x   = arg2
        mul_product_lo = dest
        mul_product_hi = dest + 1

        lda mul_factor_a      ; setup: 6 cycles
        ;ldx mul_factor_x

        clc                   ; (a + x)^2/2: 23 cycles
        adc mul_factor_x
        tax
        bcc under256
        lda mul_hibyte512,x
        bcs next
    under256:
        lda mul_hibyte256,x
        sec
    next:
        sta mul_product_hi
        lda mul_lobyte256,x

        ldx mul_factor_a      ; - a^2/2: 20 cycles
        sbc mul_lobyte256,x
        sta mul_product_lo
        lda mul_product_hi
        sbc mul_hibyte256,x
        sta mul_product_hi

        ldx mul_factor_x      ; + x & a & 1: 22 cycles
        txa                   ; (this is a kludge to correct a
        and mul_factor_a      ; roundoff error that makes odd * odd too low)
        and #1

        clc
        adc mul_product_lo
        bcc small_product
        inc mul_product_hi
    small_product:
        sec                   ; - x^2/2: 25 cycles
        sbc mul_lobyte256,x
        sta mul_product_lo
        lda mul_product_hi
        sbc mul_hibyte256,x
        sta mul_product_hi
    .endscope
.endmacro

.proc imul16_func
    arg1 = FR0   ; 16-bit arg (clobbered)
    arg2 = FR1   ; 16-bit arg (clobbered)
    result = FR2 ; 32-bit result
    inter = temp2

    ; h1l1 * h2l2
    ; (h1*256 + l1) * (h2*256 + l2)
    ; h1*256*(h2*256 + l2) + l1*(h2*256 + l2)
    ; h1*h2*256*256 + h1*l2*256 + h2*l1*256 + l1*l2

    imul8 result, arg1, arg2
    lda #0
    sta result + 2
    sta result + 3

    imul8 inter, arg1 + 1, arg2
    add16 result + 1, result + 1, inter
    add_carry result + 3

    imul8 inter, arg1, arg2 + 1
    add16 result + 1, result + 1, inter
    add_carry result + 3

    imul8 inter, arg1 + 1, arg2 + 1
    add16 result + 2, result + 2, inter

    ; In case of negative inputs, adjust high word
    ; https://stackoverflow.com/a/28827013
    lda arg1 + 1
    bpl arg1_pos
    sub16 result + 2, result + 2, arg2
arg1_pos:
    lda arg2 + 1
    bpl arg2_pos
    sub16 result + 2, result + 2, arg1
arg2_pos:

    rts ; 6 cyc
.endproc

.macro round16 arg
    ; Round top 16 bits of 32-bit fixed-point number in-place
    .local increment
    .local high_half
    .local check_sign
    .local next

    ; low word > $8000: round up
    ;          = $8000: round up   if positive
    ;                   round down if negative
    ;          < $8000: round down

    lda arg + 1
    cmp #$80
    beq high_half
    bpl increment
    bmi next

high_half:
    lda arg
    beq check_sign
    bpl increment
    bmi next

check_sign:
    lda arg + 3
    bmi next

increment:       ; 5-10 cyc
    inc arg + 2  ; 5 cyc
    bne next     ; 2 cyc
    inc arg + 3  ; 5 cyc

next:

.endmacro

.proc mandelbrot
    ; input:
    ; cx: position scaled to 4.12 fixed point - -8..+7.9
    ; cy: position scaled to 4.12
    ;
    ; output:
    ; iter: iteration count at escape or 0

    ; zx = 0
    ; zy = 0
    ; zx_2 = 0
    ; zy_2 = 0
    ; zx_zy = 0
    ; dist = 0
    ; iter = 0
    lda #00
    ldx #(iter - zx + 1)
initloop:
    sta zx - 1,x
    dex
    bne initloop
    sta z_buffer_start
    sta z_buffer_end

loop:
    ; iter++ & max-iters break
    inc iter
    bne keep_going
    jmp exit_path
keep_going:

    .macro quick_exit arg, max
        .local positive
        .local negative
        .local nope_out
        .local first_equal
        .local all_done

        ; check sign bit
        lda arg + 1
        bmi negative

    positive:
        cmp #((max) << 4)
        bmi all_done ; 'less than'
        jmp exit_path

    negative:
        cmp #(256 - ((max) << 4))
        beq first_equal ; 'equal' on first byte
        bpl all_done    ; 'greater than'

    nope_out:
        jmp exit_path
    
    first_equal:
        lda arg
        beq nope_out  ; 2nd byte 0 shows it's really 'equal'

    all_done:
    .endmacro

    ; 4.12: (-8 .. +7.9)
    ; zx = zx_2  - zy_2  + cx
    sub16 zx, zx_2, zy_2
    add16 zx, zx, cx
    quick_exit zx, 2

    ; zy = zx_zy + zx_zy + cy
    add16 zy, zx_zy, zx_zy
    add16 zy, zy, cy
    quick_exit zy, 2

    ; zx_2 = zx * zx
    imul16_round zx_2, zx, zx, 4

    ; zy_2 = zy * zy
    imul16_round zy_2, zy, zy, 4

    ; zx_zy = zx * zy
    imul16_round zx_zy, zx, zy, 4

    ; dist = zx_2 + zy_2
    add16 dist, zx_2, zy_2
    quick_exit dist, 4

    ; if may be in the lake, look for looping output with a small buffer
    ; as an optimization vs running to max iters
    lda z_buffer_active
    beq skip_z_buffer

    ldx z_buffer_start
    cpx z_buffer_end
    beq z_nothing_to_read

z_buffer_loop:
    .macro z_compare arg
        .local compare_no_match
        lda z_buffer,x
        inx
        cmp arg
        bne compare_no_match
        iny
    compare_no_match:
    .endmacro
    .macro z_advance
        .local skip_reset_x
        cpx #(z_buffer_len * 4)
        bmi skip_reset_x
        ldx #0
    skip_reset_x:
    .endmacro
    .macro z_store arg
        lda arg
        sta z_buffer,x
        inx
    .endmacro

    ; Compare the previously stored z values
    ldy #0
    z_compare zx
    z_compare zx + 1
    z_compare zy
    z_compare zy + 1

    cpy #4
    bne z_no_matches
    jmp z_exit

z_no_matches:
    z_advance

    cpx z_buffer_end
    bne z_buffer_loop

z_nothing_to_read:

    ; Store and expand
    z_store zx
    z_store zx + 1
    z_store zy
    z_store zy + 1
    z_advance
    stx z_buffer_end

    ; Increment the start roller if necessary (limit size)
    lda iter
    cmp #(z_buffer_len * 4)
    bmi skip_inc_start
    lda z_buffer_start
    clc
    adc #4
    tax
    z_advance
    stx z_buffer_start
skip_inc_start:

skip_z_buffer:

    jmp loop

z_exit:
    lda #0
    sta iter

exit_path:
    ldx #0
    lda iter
    bne next
    inx
next:
    stx z_buffer_active
    rts

.endproc

.macro scale_zoom dest
    ; clobbers X, flags
    .local cont
    .local enough

    ; cx = (sx << (8 - zoom))
    ldx zoom
cont:
    cpx #8
    beq enough
    shl16 dest
    inx
    jmp cont
enough:
.endmacro

.macro zoom_factor dest, src, zoom, aspect
    ; clobbers A, X, flags, etc
    copy16 dest, src
    scale_zoom dest

    ; cy = cy * (3 / 4)
    ; cx = cx * (5 / 4)
    imul16_round dest, dest, aspect, 4
.endmacro

.proc pset
    ; screen coords in signed sx,sy
    ; iter holds the target to use
    ; @todo implement

    ; iter -> color
    ldx iter
    lda color_map,x
    sta pixel_color
    lda #(255 - 3)
    sta pixel_mask

    ; sy -> line base address in temp
    lda sy
    bpl positive

negative:
    ; temp1 = top half
    lda #.lobyte(framebuffer_top + stride * half_height)
    sta pixel_ptr
    lda #.hibyte(framebuffer_top + stride * half_height)
    sta pixel_ptr + 1
    jmp point

positive:

    lda #.lobyte(framebuffer_bottom)
    sta pixel_ptr
    lda #.hibyte(framebuffer_bottom)
    sta pixel_ptr + 1

point:

    ; pixel_ptr += sy * stride
    ;    temp * 40
    ; =  temp * 32  +  temp * 8
    ; = (temp << 5) + (temp << 3)
    copy16 temp, sy
    shl16 temp
    shl16 temp
    shl16 temp
    add16 pixel_ptr, pixel_ptr, temp
    shl16 temp
    shl16 temp
    add16 pixel_ptr, pixel_ptr, temp

    ; Ok so temp1 points to the start of the line, which is 40 bytes.
    ; Get the byte and bit offsets
    lda sx
    clc
    adc #half_width
    sta temp

    ; pixel_shift = temp & 3
    ; pixel_color <<= pixel_shift (shifting in zeros)
    ; pixel_mask <<= pixel_shift (shifting in ones)
    and #3
    sta pixel_shift
    lda #3
    sec
    sbc pixel_shift
    tax
shift_loop:
    beq shift_done
    asl pixel_color
    asl pixel_color
    sec
    rol pixel_mask
    sec
    rol pixel_mask
    dex
    jmp shift_loop
shift_done:

    ; pixel_offset = temp >> 2
    lda temp
    lsr a
    lsr a
    sta pixel_offset
    tay

    ; read, mask, or, write
    lda (pixel_ptr),y
    and pixel_mask
    ora pixel_color
    sta (pixel_ptr),y

    rts
.endproc

.macro draw_text_indirect col, len, strptr
    ; clobbers A, X
    .local loop
    .local done
    ldx #0
loop:
    cpx #len
    beq done
    txa
    tay
    lda (strptr),y
    tay
    lda char_map,y
    sta textbuffer + col,x
    inx
    jmp loop
done:
.endmacro

.macro draw_text col, len, cstr
    ; clobbers A, X
    .local loop
    .local done
    ldx #0
loop:
    cpx #len
    beq done
    ldy cstr,x
    lda char_map,y
    sta textbuffer + col,x
    inx
    jmp loop
done:
.endmacro

.proc vblank_handler
    inc count_frames
    inc palette_offset
    jsr update_palette
    jmp XITVBV
.endproc

.proc update_palette
    lda palette
    sta COLOR4

    clc
    lda palette_offset
    and #$f0
    adc palette + 1
    sta COLOR0

    clc
    lda palette_offset
    and #$f0
    adc palette + 2
    sta COLOR1

    clc
    lda palette_offset
    and #$f0
    adc palette + 3
    sta COLOR2
.endproc

.proc update_speed
    ; convert frames (u16) to fp
    ; add to frames_total
    ; convert pixels (u16) to fp
    ; add to pixels_total
    ; (frames_total * 16.66666667) / pixels_total
    ; convert to ATASCII
    ; draw text
.endproc

.proc keycheck
    ; clobbers all
    ; returns 255 in A if state change or 0 if no change

    ; check keyboard buffer
    lda CH
    cmp #$ff
    beq skip_char

    ; Clear the keyboard buffer and re-enable interrupts
    ldx #$ff
    stx CH

    tay

    lda zoom
    cpy #KEY_PLUS
    beq plus
    cpy #KEY_MINUS
    beq minus

    ; temp = $0010 << (8 - zoom)
    lda #$10
    sta temp
    lda #$00
    sta temp + 1
    scale_zoom temp

    cpy #KEY_UP
    beq up
    cpy #KEY_DOWN
    beq down
    cpy #KEY_LEFT
    beq left
    cpy #KEY_RIGHT
    beq right

skip_char:
    lda #0
    rts

plus:
    cmp #8
    bpl skip_char
    inc zoom
    jmp done
minus:
    cmp #1
    bmi skip_char
    dec zoom
    jmp done
up:
    sub16 oy, oy, temp 
    jmp done
down:
    add16 oy, oy, temp
    jmp done
left:
    sub16 ox, ox, temp
    jmp done
right:
    add16 ox, ox, temp
done:
    lda #255
    rts

.endproc

.proc clear_screen
    ; zero the range from framebuffer_top to display_list
    lda #.lobyte(framebuffer_top)
    sta temp
    lda #.hibyte(framebuffer_top)
    sta temp + 1

zero_page_loop:
    lda #0
    ldy #0
zero_byte_loop:
    sta (temp),y
    iny
    bne zero_byte_loop

    inc temp + 1
    lda temp + 1
    cmp #.hibyte(display_list)
    bne zero_page_loop

    rts
.endproc

.proc status_bar
    ; Status bar
    draw_text 0, str_self_len, str_self
    draw_text 40 - str_run_len, str_run_len, str_run

    rts
.endproc

.proc start

    ; ox = 0; oy = 0; zoom = 0
    ; count_frames = 0; count_pixels = 0
    lda #0
    sta ox
    sta ox + 1
    sta oy
    sta oy + 1
    sta count_frames
    sta count_pixels

    ; total_ms = 0.0; total_pixels = 0.0
    ldx #total_ms
    jsr ZF1
    ldx #total_pixels
    jsr ZF1

    ; zoom = 2x
    lda #1
    sta zoom

    ; Disable display DMA
    lda #0
    sta DMACTL

    jsr clear_screen

    ; Copy the display list into properly aligned memory
    ; Can't cross 1024-byte boundaries :D
    ldx #0
copy_byte_loop:
    lda display_list_start,x
    sta display_list,x
    inx
    cpx #display_list_len
    bne copy_byte_loop

    ; Set up the display list
    lda #.lobyte(display_list)
    sta DLISTL ; actual register
    sta SDLSTL ; shadow register the OS will copy in
    lda #.hibyte(display_list)
    sta DLISTH ; actual register
    sta SDLSTH ; shadow register the OS will copy in

    ; Re-enable display DMA
    lda #$22
    sta DMACTL

    ; Initialize the palette
    lda #0
    sta palette_offset
    jsr update_palette

    ; install the vblank handler
    lda #7 ; deferred
    ldx #.hibyte(vblank_handler)
    ldy #.lobyte(vblank_handler)
    jsr SETVBV

main_loop:
    jsr clear_screen
    jsr status_bar

    lda #0
    sta fill_level

fill_loop:

    ; sy = -92 .. 91
    lda #(256-half_height)
    sta sy
    lda #(256-1)
    sta sy + 1

loop_sy:
    ; sx = -80 .. 79
    lda #(256-half_width)
    sta sx
    lda #(256-1)
    sta sx + 1

loop_sx:
    ; check the fill mask
    ldy #0

loop_skip_level:
    cpy fill_level
    beq current_level

    lda fill_masks,y
    and sx
    bne not_skipped_mask1

    lda fill_masks,y
    and sy
    beq skipped_mask

not_skipped_mask1:
    iny
    jmp loop_skip_level

current_level:
    lda fill_masks,y
    and sx
    bne skipped_mask

    lda fill_masks,y
    and sy
    beq not_skipped_mask

skipped_mask:
    jmp skipped

not_skipped_mask:

    ; run the fractal!
    zoom_factor cx, sx, zoom, aspect_x
    add16 cx, cx, ox
    zoom_factor cy, sy, zoom, aspect_y
    add16 cy, cy, oy
    jsr mandelbrot
    jsr pset

    jsr keycheck
    beq no_key
    ; @fixme clear the pixel stats
    jmp main_loop

no_key:
    ; check if we should update the counters
    ;
    ; count_pixels >= width? update!
    inc count_pixels
    lda count_pixels
    cmp #width
    bmi update_status

    ; count_frames >= 120? update!
    lda count_frames
    cmp #120 ; >= 2 seconds
    bmi skipped

update_status:
    ; FR0 = (float)count_pixels & clear count_pixels
    lda count_pixels
    sta FR0
    lda #0
    sta FR0 + 1
    sta count_pixels
    jsr IFP

    ; FR1 = total_pixels
    ldx #.lobyte(total_pixels)
    ldy #.hibyte(total_pixels)
    jsr FLD1R

    ; FR0 += FR1
    jsr FADD

    ; total_pixels = FR0
    ldx #.lobyte(total_pixels)
    ldy #.hibyte(total_pixels)
    jsr FST0R


    ; FR0 = (float)count_frames & clear count_frames
    ; warning: this should really disable interrupts @TODO
    lda count_frames
    sta FR0
    lda #0
    sta FR0 + 1
    sta count_frames
    jsr IFP

    ; FR0 *= ms_per_frame
    ldx #.lobyte(ms_per_frame)
    ldy #.hibyte(ms_per_frame)
    jsr FLD1R
    jsr FMUL

    ; FR0 += total_ms
    ldx #total_ms
    ldy #0
    jsr FLD1R
    jsr FADD

    ; total_ms = FR0
    ldx #total_ms
    ldy #0
    jsr FST0R

    ; FR0 /= total_pixels
    ldx #total_pixels
    ldy #0
    jsr FLD1R
    jsr FDIV

    ; convert to ASCII in INBUFF
    jsr FASC

    ; print the first 6 digits
    draw_text_indirect speed_start, speed_precision, INBUFF
    draw_text speed_start + speed_precision, str_speed_len, str_speed

skipped:

    clc
    lda sx
    adc #1
    sta sx
    lda sx + 1
    adc #0
    sta sx + 1

    lda sx
    cmp #half_width
    beq loop_sx_done
    jmp loop_sx

loop_sx_done:

    clc
    lda sy
    adc #1
    sta sy
    lda sy + 1
    adc #0
    sta sy + 1

    lda sy
    cmp #half_height
    beq loop_sy_done
    jmp loop_sy

loop_sy_done:

fill_loop_done:
    inc fill_level
    lda fill_level
    cmp #max_fill_level
    beq loop
    jmp fill_loop

loop:
    ; finished
    draw_text 40 - str_done_len, str_done_len, str_done
    jsr keycheck
    beq loop
    jmp main_loop

.endproc