cpu: rewrite to a single switch-based dispatch

This will make it easier to experiment with direct-threading in the future.

src/gbemu/cpu.h

@@ -10,10 +10,10 @@
 
 #include <stdint.h>
 
-#define LR35902_FLAG_BIT_C 4
-#define LR35902_FLAG_BIT_H 5
-#define LR35902_FLAG_BIT_N 6
-#define LR35902_FLAG_BIT_Z 7
+#define CPU_F_BIT_POS_C 4
+#define CPU_F_BIT_POS_H 5
+#define CPU_F_BIT_POS_N 6
+#define CPU_F_BIT_POS_Z 7
 
 #define LR35902_MAX_EVENTS 16
 
@@ -78,11 +78,20 @@ struct lr35902_state {
 			uint16_t bc, de, hl, af, pc, sp;
 		};
 		uint8_t regs_8[NUM_LR35902_REGS_8];
+
 		struct {
-			uint8_t b, c, d, e, h, l, a, f;
+			uint8_t b, c, d, e, h, l, a, f_dummy;
 		};
 	};
 
+	/* Only two operations access the flags as a collective, PUSH
+	 * AF and POP AF. It's much easier to represent them as
+	 * individual flags and combine them when needed */
+	int zf;
+	int nf;
+	int hf;
+	int cf;
+
 	uint8_t stall_cycles;
 	uint8_t halted;
 	lr35902_mem_read_fn mem_read;