See BeebWiki

.ForceHardReset
LDA #&8F:LDX #&0C
LDY #&FF:JSR OSBYTE ; Claim NMIs
LDA #&40:STA &D00   ; Disable NMIs
SEI:CLD             ; Disable IRQs, ensure binary mode
LDX #&FF:TXS        ; Reset stack
LDX #&0F:STX &FE42  ; SysVIA PostB=iiiioooo
.LDA08
DEX:STX &FE40       ; Write I/O latch
CPX #&09:BCS LDA08  ; Loop to set up internal I/O
INX                 ; X=9
.LDA11
TXA:JSR LF02A       ; KeyScan for key X, test links and CTRL
CPX #&80:ROR &FC    ; Rotate keystate in bit 7 into &FC
TAX:DEX:BNE LDA11   ; Loop for key 9 to key 1
STX &028D           ; Clear last BREAK flag
ROL &FC             ; CTRL is now in carry, &FC is keyboard links
SEC                 ; Set CTRL pressed
JMP LDA22           ; Jump to RESET code