Add VIA2 socket support for external I/O integration

Adds optional -via2-socket support to allow external processes to
communicate with the emulator via a UNIX domain socket.
Includes documentation and example usage.

Author: Russell-S-Harper

README.md

@@ -128,6 +128,7 @@ When starting `x16emu` without arguments, it will pick up the system ROM (`rom.b
 * `-sound <device>` can be used to specify the output sound device. If 'none', no audio is generated.
 * `-abufs` can be used to specify the number of audio buffers (defaults to 8 when using the SD card, 32 when using HostFS). If you're experiencing stuttering in the audio, try increasing this number. This will result in additional audio latency though.
 * `-via2` installs the second VIA chip expansion at $9F10.
+* `-via2-socket <socket>` Use a socket to emulate VIA2 I/O. Currently only UNIX / POSIX are supported. The emulator opens this socket as a client, so it must be set up in advance. The -via2 option must be specified along with this option. See [Using -via2-socket](#via2-socket) for sample server and client code.
 * `-midline-effects` enables mid-scanline raster effects at the cost of vastly increased host CPU usage.
 * `-mhz <integer>` sets the emulated CPU's speed. Range is from 1-40. This option is mainly for testing and benchmarking.
 * `-enable-ym2151-irq` connects the YM2151's IRQ pin to the system's IRQ line with a modest increase in host CPU usage.
@@ -450,6 +451,113 @@ When `-debug` is selected the STP instruction (opcode $DB) will break into the d
 
 Keyboard routines only work when the emulator is running normally. Single stepping through keyboard code will not work at present.
 
+<a name="via2-socket"></a>
+Using -via2-socket
+------------------
+
+Reads from and writes to `$9F10` and `$9F11` (VIA ports A and B) are forwarded to the socket.
+
+There are no restrictions on the protocol except that a value of `255` is reserved for "no data yet" when reading.
+
+The following example demonstrates a simple bidirectional chat interface implemented using the `-via2-socket` option.
+
+### Sample server (Python)
+
+Save as `via2.py`:
+
+```python
+#!/usr/bin/env python3
+import os
+import socket
+import subprocess
+import sys
+
+SOCKET_PATH = "/tmp/via2.sock"
+CHT_EOL = b'\x00'
+
+# Remove old socket if present
+try:
+    os.unlink(SOCKET_PATH)
+except FileNotFoundError:
+    pass
+
+# Create UNIX domain socket
+server = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+server.bind(SOCKET_PATH)
+server.listen(1)
+
+print("Waiting for emulator...")
+
+# Launch emulator AFTER socket is ready
+subprocess.Popen([
+    "./x16emu",
+    "-scale", "2",
+    "-via2",
+    "-via2-socket", SOCKET_PATH,
+    "-bas", "VIA2.BAS",
+    "-run"
+], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+
+# Accept connection
+conn, _ = server.accept()
+print("Emulator connected.")
+
+try:
+    while True:
+        # ---- RECEIVE STRING ----
+        buf = bytearray()
+        while True:
+            b = conn.recv(1)
+            if not b:
+                raise ConnectionError("Socket closed")
+            if b == CHT_EOL:
+                break
+            buf += b
+
+        text = buf.decode(errors="replace")
+        print("> " + text)
+
+        if text == "":
+            break
+
+        # ---- SEND RESPONSE ----
+        reply = input("MESSAGE? ").upper().encode() + CHT_EOL
+        conn.sendall(reply)
+
+finally:
+    try:
+        conn.close()
+    except Exception:
+        pass
+    server.close()
+    try:
+        os.unlink(SOCKET_PATH)
+    except FileNotFoundError:
+        pass
+```
+
+### Sample client (Commander X16 BASIC)
+
+Save as `VIA2.BAS`:
+
+```basic
+100 REM VIA2 ACTS AS A BYTE-SERIAL DEVICE VIA SOCKET BRIDGE
+110 T$ = ""
+120 INPUT "MESSAGE (EMPTY LINE TO QUIT)"; T$
+130 REM WRITE T$ TO VIA2
+140 IF T$ = "" THEN GOTO 160
+150 FOR I = 1 TO LEN(T$): POKE $9F10, ASC(MID$(T$, I, 1)): NEXT I
+160 POKE $9F10, 0: REM END OF LINE
+170 IF T$ = "" THEN END
+180 REM READ T$ FROM VIA2
+190 T$ = ""
+200 B = PEEK($9F10)
+210 IF B = 255 THEN FOR I = 1 TO 20: NEXT I: GOTO 200: REM NO DATA YET
+220 IF B = 0 THEN GOTO 240: REM END OF LINE
+230 T$ = T$ + CHR$(B): GOTO 200
+240 PRINT "> "; T$: GOTO 110
+```
+
 CRT File Format
 ---------------
 

src/glue.h

@@ -80,6 +80,7 @@ extern bool warp_mode;
 extern bool grab_mouse;
 extern bool testbench;
 extern bool has_via2;
+extern char *via2_socket;
 extern uint32_t host_sample_rate;
 extern bool enable_midline;
 

src/main.c

@@ -115,6 +115,7 @@ bool set_system_time = false;
 bool has_serial = false;
 bool no_ieee_intercept = false;
 bool has_via2 = false;
+char *via2_socket = NULL;
 gif_recorder_state_t record_gif = RECORD_GIF_DISABLED;
 char *gif_path = NULL;
 char *wav_path = NULL;
@@ -514,6 +515,11 @@ usage()
 	printf("\tSet the real-time-clock to the current system time and date.\n");
 	printf("-via2\n");
 	printf("\tInstall the second VIA chip expansion at $9F10\n");
+	printf("-via2-socket <socket>\n");
+	printf("\tUse a socket to emulate VIA2 I/O. Currently only UNIX / POSIX are\n");
+	printf("\tsupported. The emulator opens this socket as a client, so it must be\n");
+	printf("\tset up in advance. The -via2 option must be specified along with\n");
+	printf("\tthis option.\n");
 	printf("-testbench\n");
 	printf("\tHeadless mode for unit testing with an external test runner\n");
 	printf("-mhz <integer>\n");
@@ -1088,6 +1094,15 @@ main(int argc, char **argv)
 			argc--;
 			argv++;
 			has_via2 = true;
+		} else if (!strcmp(argv[0], "-via2-socket")) {
+			argc--;
+			argv++;
+			if (!argc || argv[0][0] == '-') {
+				usage();
+			}
+			via2_socket = argv[0];
+			argc--;
+			argv++;
 		} else if (!strcmp(argv[0], "-version")){
 			printf("%s", VER_INFO);
 #ifdef GIT_REV
@@ -1310,6 +1325,7 @@ void main_shutdown() {
 		cartridge_unload();
 	}
 	files_shutdown();
+	via2_shutdown();
 
 #ifdef PERFSTAT
 	for (int pc = 0xc000; pc < sizeof(stat)/sizeof(*stat); pc++) {

src/via.c

@@ -25,6 +25,17 @@ typedef struct {
 
 static via_t via[2];
 
+#define VIA2_SOCKET_ERROR 0xFF
+
+#ifdef _WIN32
+// Windows not yet supported
+#else // UNIX / POSIX
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+static int via2_socket_fd = -1;
+#endif
+
 // only internal logic is handled here, see via1/2 calls for external
 // operations specific to each unit
 
@@ -335,18 +346,56 @@ void
 via2_init()
 {
 	via_init(&via[1]);
+	if (via2_socket) {
+#ifdef _WIN32
+		// Windows not yet supported
+		via2_socket = NULL;
+		activity_led = VIA2_SOCKET_ERROR;
+#else // UNIX / POSIX
+		struct sockaddr_un addr;
+		memset(&addr, 0, sizeof(addr));
+		addr.sun_family = AF_UNIX;
+		strncpy(addr.sun_path, via2_socket, sizeof(addr.sun_path) - 1);
+		if ((via2_socket_fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0 || connect(via2_socket_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+			via2_shutdown();
+			// Clear to prevent socket I/O
+			via2_socket = NULL;
+			activity_led = VIA2_SOCKET_ERROR;
+		}
+#endif
+	}
 }
 
 uint8_t
 via2_read(uint8_t reg, bool debug)
 {
-	return via_read(&via[1], reg, debug);
+	uint8_t byte = via_read(&via[1], reg, debug);
+	if (via2_socket && reg <= 1) {
+#ifdef _WIN32
+		// Windows not yet supported
+#else // UNIX / POSIX
+		if (recv(via2_socket_fd, &byte, 1, MSG_DONTWAIT) <= 0) {
+			// Use 0xFF for no data
+			byte = 0xFF;
+		}
+#endif
+	}
+	return byte;
 }
 
 void
 via2_write(uint8_t reg, uint8_t value)
 {
 	via_write(&via[1], reg, value);
+	if (via2_socket && reg <= 1) {
+#ifdef _WIN32
+		// Windows not yet supported
+#else // UNIX / POSIX
+		if (send(via2_socket_fd, &value, 1, 0) <= 0) {
+			activity_led = VIA2_SOCKET_ERROR;
+		}
+#endif
+	}
 }
 
 void
@@ -360,3 +409,14 @@ via2_irq()
 {
 	return (via[1].registers[13] & via[1].registers[14]) != 0;
 }
+
+void
+via2_shutdown()
+{
+#ifdef _WIN32
+	// Windows not yet supported
+#else // UNIX / POSIX
+	close(via2_socket_fd);
+	via2_socket_fd = -1;
+#endif
+}

src/via.h

@@ -19,5 +19,6 @@ uint8_t via2_read(uint8_t reg, bool debug);
 void via2_write(uint8_t reg, uint8_t value);
 void via2_step(unsigned clocks);
 bool via2_irq();
+void via2_shutdown();
 
 #endif