Commodore 64 USR function

0 gosub 50
10 poke 785, 0
20 poke 786, 192:rem usr(x) calls 49152
30 let x = usr(6)
40 end
50 let i = 49152
60 read a
70 if a = -1 then return
80 poke i, a
90 let i = i + 1
100 go to 60 main loop
500 data 32, 170, 177:rem jsr $b1aa
510 data 140, 32, 208:rem sty $d020
520 data 140, 33, 208:rem sty $d021
530 data 169, 0:      rem lda #$00
540 data 32, 145, 179:rem jsr $b391
550 data 96:          rem rts
560 data -1
rem this programme will create a custom routine called by the usr(x) keywork in Commodore 64 BASIC
rem the routine accepts a numeric value and essentially poke the border and screen with the same
rem value; the number is returned back between 0 - 255, so print usr(511) will return 255, so it
rem will also act as a modulo 256 function as well, so this may be modified to a custom modulo.