This is a straight-forward detection routine to find out if a Retro Replay is active on the system. It works by writing different bytes to the same address in the $8000-$9FFF range. If there is no cart, all of the writes will go to normal RAM and the latest byte will be the one read back. Otherwise some of them go to different RR RAM banks. Successful readback of both bytes written to the RR RAM will be taken to signal the presence of a Retro Replay.
Find the print routine elsewhere. This routine is well tested and is believed to reliably identify either the presence or nonexistence of an RR.
; Retro Replay detection using RR RAM writes by FMan/Tropyx !to "rrdetect.prg",cbm ; note that this code puts the computer in Ultimax mode ; a few times, so it must be located below $1000 in RAM *=$c00 RRDetect sei lda #66 sta $de01 ; configure the cart ldx #$a4 ldy #$6a lda #$2b ; select RAM bank 1 and sta $de00 ; invoke Ultimax mode to sty $8080 ; write into the RR RAM lda #$23 sta $de00 sta $8080 ; write to normal RAM now lda #$3b sta $de00 ; select RAM bank 3 and stx $8080 ; write another byte there lda #$28 sta $de00 cli cpy $8080 ; see if the original byte survived bne NoRR ; subsequent writes to same address lda #$38 sta $de00 ; do another check with the second cpx $8080 ; byte that was written to bank 3 beq RRFound NoRR lda #<NoRRstr sta dst lda #>NoRRstr sta dst+1 jsr print ; see note above rts RRFound nop ; continue your program using the Retro Replay rts NoRRstr !pet 13,"no retro replay was found.",13,0