Interrupt system implemented, some new programs.

Source/Emulator/io.c

@@ -6,6 +6,37 @@
 #include "io.h"
 #include <stdio.h>
 
+// One bit per port, so a device can ask for attention without anything having to poll
+// it. Eight ports to the byte, low bit first.
+#define INTERRUPT_LINE_BYTES 32
+
+static uint8_t pendingInterrupts[INTERRUPT_LINE_BYTES];
+
+void raiseInterrupt(uint8_t port) {
+    pendingInterrupts[port >> 3] |= (uint8_t)(1u << (port & 7));
+}
+
+void clearInterrupt(uint8_t port) {
+    pendingInterrupts[port >> 3] &= (uint8_t)~(1u << (port & 7));
+}
+
+int nextPendingInterrupt(void) {
+    // Lowest numbered port wins. This is a scan rather than a priority encoder, which
+    // means there is no arbitration to explain and a programmer can work out what
+    // happens next by reading the port numbers.
+    for (int group = 0; group < INTERRUPT_LINE_BYTES; group++) {
+        if (pendingInterrupts[group] == 0) {
+            continue;
+        }
+        for (int bit = 0; bit < 8; bit++) {
+            if (pendingInterrupts[group] & (1u << bit)) {
+                return group * 8 + bit;
+            }
+        }
+    }
+    return -1;
+}
+
 uint8_t OutputHandler(uint8_t DataByte, uint8_t Address) {
     // This function sends the DataByte to the appropriate place based on the Port Address.
     switch(Address) {
@@ -15,6 +46,14 @@ uint8_t OutputHandler(uint8_t DataByte, uint8_t Address) {
             // Later, I'll want to use a buffer for this for performance, probably.
             putchar(DataByte);
         break;
+        case 0x10:
+            // A test device, and about the simplest one that can exist: writing to it
+            // puts its own line up. It stands in for the shape a real device has, where
+            // the CPU asks for something and is interrupted once the answer is ready,
+            // with the waiting taken out so that a test runs the same way every time.
+            // The byte written is ignored; only the asking matters.
+            raiseInterrupt(0x10);
+        break;
         default:
             // Writes to unused Output Ports are ignored.
             return 1;