Interrupt system implemented, some new programs.

Source/Emulator/cpu.h

@@ -26,6 +26,30 @@
 #error "Cannot preserve more Data Pointers than the CPU has."
 #endif
 
+// The bits of the Status register that mean something.
+#define STATUS_CARRY     0x01 // An arithmetic result carried out of, or borrowed into, a byte.
+#define STATUS_FAULT     0x02 // The CPU met a byte it could not decode, and stopped.
+#define STATUS_INTERRUPT 0x04 // Hardware interrupts are enabled. Nothing reads this yet.
+#define STATUS_HALT      0x80 // Execution has stopped, either from HALT or from a fault.
+
+// What an interrupt puts on the Stack: the resume address, every Data Pointer, and
+// every register the CPU has. The CALL frame leaves Q and DP3 alone, but that is a
+// convention between a caller and the subroutine it called. An interrupt arrives in
+// code that never agreed to give anything up, so it saves the lot.
+#define INTERRUPT_FRAME_BYTES (2 + DATA_POINTERS * 2 + 4)
+
+// Why the CPU stopped, when the Fault Flag is set. This is not something a program can
+// read, and it is deliberately not a register: when a handler is installed, the vector
+// it arrived through already says what happened, which is why the ISA has no fault
+// cause. This exists for the case where nothing is installed and the machine is dead,
+// so that whatever examines the wreckage can say something better than "it stopped".
+typedef enum {
+    FAULT_NONE = 0,
+    FAULT_BAD_OPCODE,  // A byte that does not decode to an instruction.
+    FAULT_NO_HANDLER,  // Dispatched through a software vector with nothing in it.
+    FAULT_NO_DEVICE_HANDLER  // A device interrupted, and its vector was empty.
+} FaultCause;
+
 // The struct containing the CPU registers.
 typedef struct {
     uint8_t A;
@@ -37,6 +61,9 @@ typedef struct {
     uint16_t StackPointer;
     uint8_t *Program;
     uint8_t *Data;
+    // Set alongside the Fault Flag, and read only by whatever reports the stop.
+    uint8_t Fault;        // A FaultCause.
+    uint8_t FaultVector;  // Which vector was empty, when Fault is FAULT_NO_HANDLER.
 } CPURegisters;
 
 uint8_t executeOperation(uint8_t instruction, CPURegisters *cpu);