===== C64 Game System (C64GS) detection =====

If you're developing a cartridge-based game and want to setup the controls differently on C64GS (since it doesn't have any physical keyboard), here is how to detect it.

<code>
check_c64gs			; returns 1 in x-register if true, otherwise 0.

                lda $01		; save $01 temporarily
                pha

		lda #$36	; kernal will now be visible at $e000
		sta $01

		ldx #$00

		lda #$43	; check bytes from kernal
		cmp $fc00	; equals 'C' from 'COPYRIGHT 1990' in C64GS ROM
		bne cc_break
		cmp $fc0f	; equals 'C' from 'COMMODORE'
		bne cc_break
		cmp $fc1c	; equals first 'C' from 'ELECTRONICS'
		bne cc_break

		inx

cc_break	pla		; load old $01-value again
		sta $01

		rts
</code>

How to use: Call the routine with 'jsr check_c64gs' and check the x-register afterwards.