The STOP kernal routine is located at $FFE1 (65505).
This routine does an indirect jump to vector $0328/$0329 (808/809), which by default points to $F6ED (63213). It checks for the STOP-key to be pressed by looking if address $91 (145) in the zeropage has value $7f (127)) and if so, it calls CLRCHN to restore all I/O devices to default. Before returning, it also clears the keyboard buffer by storing #$00 in address $C6 (198).
FFE1 6C 28 03 JMP ($0328) ; (F6ED) check stop key ↓ F6ED A5 91 LDA $91 ; check STOP-key indicator F6EF C9 7F CMP #$7F ; #$7F is STOP-key value F6F1 D0 07 BNE $F6FA ; if not this value, return F6F3 08 PHP ; push processor status to stack F6F4 20 CC FF JSR $FFCC ; restore I/O devices to default F6F7 85 C6 STA $C6 ; clear keyboard buffer (A always equals 0 when returning from $FFCC) F6F9 28 PLP ; restore processor status from stack F6FA 60 RTS