4way.bas

 dim cadd = c
 dim badd = b
 dim fbyte = f
 dim radd = r
 dim wx = x
 dim wy = y

 COLUBK = $20
 COLUPF = $40


main

 if joy0up    then wy = (wy - 1) &  15 : gosub setup_addresses : goto scroll_up
 if joy0down  then wy = (wy + 1) &  15 : gosub setup_addresses : goto scroll_down
left_right
 if joy0right then wx = (wx + 1) & 127 : gosub setup_addresses : goto scroll_right
 if joy0left  then wx = (wx - 1) & 127 : gosub setup_addresses : goto scroll_left
draw
 drawscreen
 goto main


scroll_up
 for p = 40 to 0 step -4
 var4[p]=var0[p]
 var5[p]=var1[p]
 var6[p]=var2[p]
 var7[p]=var3[p]
 next

 rem if badd, bit address is 0 then we're aligned
 rem no need to shift
 if !badd then gosub fill_top : goto left_right
 if badd > 3 then goto shift_top_right

 gosub fill_top
 fbyte = fill(4)

 for i = 1 to badd
 asm
 asl fbyte
 ror var3
 rol var2
 ror var1
 rol var0
end
 next

 goto left_right

shift_top_right
 fbyte = fill(0) : j = 8 - badd

 cadd = (cadd + 1) & 15 : gosub fill_top

 for i = 1 to j
 asm
 lsr fbyte
 ror var0
 rol var1
 ror var2
 rol var3
end
 next

 goto left_right


scroll_down
 for p = 0 to 40 step 4
 var0[p]=var4[p]
 var1[p]=var5[p]
 var2[p]=var6[p]
 var3[p]=var7[p]
 next

 rem current row + (11*16 bytes/row) to fill in the bottom
 radd = radd + 176

 rem if badd, thr bit address is 0 then we're aligned
 rem no need to shift
 if !badd then  gosub fill_bot : goto left_right

 rem if badd > 3 then shifting right needs fewer shifts
 if badd > 3 then goto shift_bot_right

 gosub fill_bot
 fbyte = fill(4)

 for i = 1 to badd
 asm
 asl fbyte
 ror var47
 rol var46
 ror var45
 rol var44
end
 next

 goto left_right

shift_bot_right
 rem get the fill byte then bump the column to fill
 rem the line by shifting right
 fbyte = fill(0) : cadd = (cadd + 1) & 15 : j = 8 - badd

 gosub fill_bot

 for i = 1 to j
 asm
 lsr fbyte
 ror var44
 rol var45
 ror var46
 rol var47
end
 next

 goto left_right


scroll_left
 for i = 0 to 44 step 4
 fbyte = (i * 4 + radd) | cadd : fbyte = world[fbyte] & setbits[badd]
 if fbyte then fbyte = 1
 asm
 ldx i
 ror fbyte
 ror var0,x
 rol var0+1,x
 ror var0+2,x
 rol var0+3,x
end
 next

 goto draw


scroll_right
 rem the fill in pixel is the current wx + 31, 32 pixels/screen,
 rem wx is at screen 0
 fbyte = (wx + 31) & 127 : cadd = fbyte / 8 : badd = fbyte & 7
 for i = 0 to 44 step 4
 fbyte = (i * 4 + radd) | cadd : fbyte = world[fbyte] & setbits[badd]
 if fbyte then fbyte = 1
 asm
 ldx i
 ror fbyte
 ror var0+3,x
 rol var0+2,x
 ror var0+1,x
 rol var0,x
end
 next

 goto draw

setup_addresses
 cadd = wx / 8 : badd = wx & 7 : radd = wy * 4 * 4
 return

fill_bot
 var44 = fill(0)
 var45 = fill(1) : var45 = rev(var45)
 var46 = fill(2)
 var47 = fill(3) : var47 = rev(var47)
 return

fill_top
 var0 = fill(0)
 var1 = fill(1) : var1 = rev(var1)
 var2 = fill(2)
 var3 = fill(3) : var3 = rev(var3)
 return

 rem function fill
 rem asm
 rem clc
 rem adc cadd
 rem and #$0F
 rem adc radd
 rem tax
 rem lda world,x
 rem rts
 rem end

 rem adds the parameter to the (byte)column with
 rem wrapping and gets that byte in the current (world) row
 function fill()
 temp1 = ((temp1 + cadd) & 15) | radd
 temp1 = world[temp1]
 return

 function rev()
 temp2 = temp1 & $0F
 temp1 = temp1 / 16
 temp1 = revhi[temp2] | revlo[temp1]
 return

 data revhi
 $00, $80, $40, $C0, $20, $A0, $60, $E0
 $10, $90, $50, $D0, $30, $B0, $70, $F0
end

 data revlo
 $00, $08, $04, $0C, $02, $0A, $06, $0E
 $01, $09, $05, $0D, $03, $0B, $07, $0F
end

 data setbits
 $80, $40, $20, $10, $08, $04, $02, $01
end

 data world
 %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
 %10000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %00000000, %00000000, %00000000, %00000000, %00100000, %01000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000011, %10000000, %00000000, %00000000, %10000000, %00100000, %10000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000100, %10000000, %00000000, %00000000, %11000000, %00010001, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10001000, %10000000, %00000000, %00000001, %00100000, %00001010, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10010000, %10001000, %00000100, %00000010, %00010000, %00000100, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10011111, %10001000, %00000100, %00000100, %00001000, %00000100, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %10001000, %10000100, %00001000, %00000100, %00001000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %10001001, %01000100, %00010111, %11111110, %00001000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %10001010, %00100100, %00100000, %00000010, %00010000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %10001100, %00010100, %01000000, %00000010, %00100000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %00001000, %00001100, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %00000000, %00000100, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %10000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000001
 %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
end