with asm

 set romsize 4k

 dim mapx = a
 dim mapy = b
 dim pfy = c
 const mem = $A4

 macro revb
 asm
 lda temp1
 and #$0F
 tax
 lda temp1
 lsr
 lsr
 lsr
 lsr
 tay
 lda rev_tbl_lo,x
 ora rev_tbl_hi,y
 sta temp1
end
end

 COLUPF=$FF
 COLUBK=$00

 playfield:
 ....XX...XX...XXX...XX...XX....
 ....X.X..X.X..X....X....X......
 ....XX...XX...XX....X....X.....
 ....X....X.X..X......X....X....
 ....X....X.X..XXX..XX...XX.....
 ...............................
 .....XX..XX....X....XX..XXX....
 ....X....X.X..X.X..X....X......
 .....X...XX...XXX..X....XX.....
 ......X..X....X.X..X....X......
 ....XX...X....X.X...XX..XXX....
end

PROGRAMLOOP
 if joy0fire then let z = z | 1
 if !joy0fire && z then gosub DRAW_MAP

 drawscreen

 goto PROGRAMLOOP

DRAW_MAP
 if mapy > 39 then mapy = 0 else mapy = mapy + 4
 for pfy = 0 to 40 step 4

    for mapx = 0 to 3
    temp1 = mapy | mapx
    temp1 = map[temp1]
    if mapx & 1 then callmacro revb
    temp2 = pfy | mapx
    mem[temp2] = temp1
    next
 if mapy > 39 then mapy = 0 else mapy = mapy + 4
 next
 z = 0
 return

 data map
 %00001010, %11101000, %10001110, %00000000,
 %00001010, %10001000, %10001010, %00000000,
 %00001110, %11001000, %10001010, %00000000,
 %00001010, %10001000, %10001010, %00000000,
 %00001010, %11101110, %11101110, %00000000,
 %00000000, %00000000, %00000000, %00000000,
 %00001000, %00100110, %01110010, %00011100,
 %00001000, %00101001, %01001010, %00010010,
 %00000101, %01001001, %01110010, %00010010,
 %00000010, %10000110, %01001011, %11011100,
 %00000000, %00000000, %00000000, %00000000,
 %00000000, %00000000, %00000000, %00000000
end

 data rev_tbl_lo
 %00000000, %10000000, %01000000, %11000000
 %00100000, %10100000, %01100000, %11100000
 %00010000, %10010000, %01010000, %11010000
 %00110000, %10110000, %01110000, %11110000
end

 data rev_tbl_hi
 %00000000, %00001000, %00000100, %00001100
 %00000010, %00001010, %00000110, %00001110
 %00000001, %00001001, %00000101, %00001101
 %00000011, %00001011, %00000111, %00001111
end