Added C128 in C64 mode accelerator code and documentation.

libsrc/c64/acc_detect_c128.s

@@ -0,0 +1,33 @@
+;
+; Marco van den Heuvel, 2018-04-20
+;
+
+; unsigned char detect_c128 (void);
+;
+;/* Check for the presence of a C128 in C64 mode.
+; *
+; * Possible return values:
+; * 0x00  : C128 in C64 mode not present
+; * 0x01  : C128 in C64 mode present
+; */
+
+        .export         _detect_c128
+
+        .include        "accelerator.inc"
+
+_detect_c128:
+        lda     #$00
+        tax
+
+; Make sure the CPU is a 8502
+        .byte   $1A                   ; NOP on 8502, INA on 65(S)C(E)02, 4510 and 65816
+        bne     not_found
+
+; Make sure a C128 VICIIe is present
+        ldy     C128_VICIIE_CLK
+        cpy     #$FF
+        beq     not_found
+found:
+        lda     #$01
+not_found:
+        rts