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libsrc/common/getcpu.s

@@ -0,0 +1,37 @@
+;
+; Ullrich von Bassewitz, 02.04.1999
+;
+; unsigned char getcpu (void);
+;
+
+	.export		_getcpu
+
+; ---------------------------------------------------------------------------
+; Subroutine to detect an 816. Returns
+;
+;   - carry clear and 0 in A for a NMOS 6502 CPU
+;   - carry set and 1 in A for some CMOS 6502 CPU
+;   - carry set and 2 in A for a 65816
+;
+; This function uses a $1A opcode which is a INA on the 816 and ignored
+; (interpreted as a NOP) on a NMOS 6502. There are several CMOS versions
+; of the 6502, but all of them interpret unknown opcodes as NOP so this is
+; just what we want.
+
+.p816				; Enable 65816 instructions
+
+_getcpu:
+	lda	#0
+      	inc	a	      	; .byte $1A
+      	cmp	#1
+       	bcc    	@L9
+
+; This is at least a 65C02, check for a 65816
+
+      	xba		    	; .byte $eb, put $01 in B accu
+      	dec	a     		; .byte $3a, A=$00 if 65C02
+       	xba    	       	       	; .byte $eb, get $01 back if 65816
+       	inc    	a      	       	; .byte $1a, make $01/$02
+@L9:   	ldx	#0		; Load high byte of word
+	rts
+