achieve cycle accuracy for official opcodes

Now, the unofficial opcodes can be implemented, and then the PPU and
APU.

1 files changed, 22 insertions, 5 deletions

diff --git a/cpu.c b/cpu.c
index 31f4be2..7ef4985 100644
--- a/cpu.c
+++ b/cpu.c
@@ -60,6 +60,8 @@ uint8_t memory[0x16000];
 
 uint32_t cycles = 0;
 
+bool page_crossed = false;
+
 static uint8_t
 peek(uint16_t addr)
 {
@@ -124,16 +126,26 @@ opcode_mem(enum addressing_mode mode)
 	case AM_ZP_Y:
 		val = (arg + regs.y) % 256;
 		break;
-	case AM_IMM:
 	case AM_REL:
+		val = arg + regs.pc;
+		if ((regs.pc + 1) & 0xFF00 != val & 0xFF00)
+			page_crossed = true;
+		break;
+	case AM_IMM:
 	case AM_ABS:
 		val = arg;
 		break;
 	case AM_ABS_X:
 		val = arg + regs.x;
+
+		if ((arg & 0xFF00) != (val & 0xFF00))
+			page_crossed = true;
 		break;
 	case AM_ABS_Y:
 		val = arg + regs.y;
+
+		if ((arg & 0xFF00) != (val & 0xFF00))
+			page_crossed = true;
 		break;
 	case AM_IND:
 		val = peek(arg) | (peek(((arg + 1) & 0xFF) | (arg & 0xFF00)) << 8);
@@ -143,6 +155,9 @@ opcode_mem(enum addressing_mode mode)
 		break;
 	case AM_IND_Y:
 		val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
+
+		if (((uint16_t)(val - regs.y) & 0xFF00) != (val & 0xFF00))
+			page_crossed = true;
 		break;
 	default:
 		fprintf(stderr, "INVALID ADDRESSING MODE %i\n", mode);
@@ -159,10 +174,6 @@ branch(uint16_t addr, bool cond)
 		return;
 	cycles++;
 
-	addr += regs.pc;
-	if ((regs.pc + 1) & 0xFF00 != addr & 0xFF00)
-		cycles++;
-
 	regs.pc = addr;
 }
 
@@ -763,7 +774,13 @@ interpret(void)
 			opcodes[op].instr(peek(arg));
 		else
 			opcodes[op].instr(arg);
+
 		cycles += opcodes[op].cycles;
+		if (page_crossed) {
+			if (opcodes[op].page_cross)
+				cycles++;
+			page_crossed = false;
+		}
 	}
 loop_exit:
 }