Various bug fixes to assembler, added more data pointers.

Source/Emulator/cpu.h

@@ -8,6 +8,24 @@
 
 #include <stdint.h>
 
+// How many Data Pointers the CPU has. The instructions that name one take a full
+// byte to do it, so the encoding would allow up to 256. The limit here is the size
+// of the register file and the cost of saving pointers across a CALL, not the
+// instruction format. Must be a power of two, so that the selector can be masked
+// down to a valid pointer.
+#define DATA_POINTERS 4
+
+// How many Data Pointers survive a CALL. The low numbered pointers are saved and
+// restored around a subroutine; the rest are left alone, so a subroutine can use
+// one to hand a pointer back to its caller the way Q hands back a byte. This is
+// deliberately independent of DATA_POINTERS: adding more pointers should not make
+// every CALL more expensive.
+#define PRESERVED_DATA_POINTERS 3
+
+#if PRESERVED_DATA_POINTERS > DATA_POINTERS
+#error "Cannot preserve more Data Pointers than the CPU has."
+#endif
+
 // The struct containing the CPU registers.
 typedef struct {
     uint8_t A;
@@ -15,7 +33,7 @@ typedef struct {
     uint8_t Q;
     uint8_t Status;
     uint16_t ProgramCounter;
-    uint16_t DataPointer;
+    uint16_t DataPointer[DATA_POINTERS];
     uint16_t StackPointer;
     uint8_t *Program;
     uint8_t *Data;
@@ -25,4 +43,6 @@ uint8_t executeOperation(uint8_t instruction, CPURegisters *cpu);
 
 void initializeCPU(CPURegisters *cpu, uint8_t *programMemory, uint8_t *dataMemory);
 
+void stepCPU(CPURegisters *cpu);
+
 #endif // CPU_H