It uses an obscure VIA mode that selects handshaking of CB2 when writing or reading from Port B.
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*=$500 ;Will return Carry if Real Machine is_real lda #00 sta $30F lda #$FF sta $030C ;Set CB2 to Pulse Mode lda #%10111101 sta $030c lda #$F3 ;Random value sta $030F lda $0300 lda #$dd sta $030C lda #$DF sta $030C lda $030F cmp #$f3 beq skip1 clc skip1 rts
I use it to write a value to the AY sound chip.
The routine then reads back data from the same AY register (0) and if it isn't the same then Euphoric is being used.