LDY #0
LDX #20
LOOP:
LDA $FE  ;load a random byte
BMI LOOP ;IF GREATER THAN 7F, ROLL AGAIN
CMP #$21 ;ASCII CODE FOR !
BCC LOOP
CMP #$5C
BEQ LOOP ;NO \
CMP #$60
BEQ LOOP ;NO `
;else, must be a good character
STA $0200,Y
INY
DEX
BNE LOOP
BRK    ;end program