board/io3.c: Add SEGA "Type 3" JVS I/O emulator

board/io3.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "jvs/jvs-bus.h"
+
+struct io3_switch_state {
+    /* Note: this struct is host-endian. The IO3 emulator handles the conversion
+       to protocol-endian. */
+
+    uint8_t system;
+    uint16_t p1;
+    uint16_t p2;
+};
+
+struct io3_ops {
+    void (*reset)(void *ctx);
+    void (*write_gpio)(void *ctx, uint32_t state);
+    void (*read_switches)(void *ctx, struct io3_switch_state *out);
+    uint16_t (*read_analog)(void *ctx, uint8_t analog_no);
+    uint16_t (*consume_coins)(void *ctx, uint8_t slot_no);
+};
+
+struct io3 {
+    struct jvs_node jvs;
+    uint8_t addr;
+    const struct io3_ops *ops;
+    void *ops_ctx;
+};
+
+void io3_init(
+        struct io3 *io3,
+        struct jvs_node *next,
+        const struct io3_ops *ops,
+        void *ops_ctx);