From 0a58a7857fb6f73399fe43c8644d35f4cffa3d66 Mon Sep 17 00:00:00 2001
From: Henrik Hautakoski <henrik.hautakoski@gmail.com>
Date: Fri, 26 Apr 2019 13:32:57 +0200
Subject: [PATCH] cpu: remove CPU_FLAGS_HALT (and flag variable from struct
 cpu_state).

we can get handle cpu halts without this flag. CPU's dont actually have these. exceptions are actually implemented as interrupts in real hardware.
---
 vm/cpu.c | 19 ++++++-------------
 vm/cpu.h |  5 -----
 2 files changed, 6 insertions(+), 18 deletions(-)

diff --git a/vm/cpu.c b/vm/cpu.c
index 2d002b6..f8f6669 100644
--- a/vm/cpu.c
+++ b/vm/cpu.c
@@ -84,7 +84,7 @@ static void execute(struct cpu_state *state, struct instr *instr) {
 		break;
 	default :
 		fprintf(stderr, "Invalid instruction (%.2X)\n", instr->opcode);
-		state->flags |= CPU_FLAGS_HALT;
+		state->pc = state->instr_cnt;
 		break;
 	}
 }
@@ -92,7 +92,6 @@ static void execute(struct cpu_state *state, struct instr *instr) {
 void cpu_init(struct cpu_state *state)
 {
 	state->pc = 0;
-	state->flags = 0;
 	memset(state->reg, 0, sizeof(state->reg[0]) * CPU_NUM_REGS);
 
 	state->instr_mem = NULL;
@@ -119,19 +118,13 @@ void cpu_instr_unload(struct cpu_state *state) {
 
 void cpu_set_pc(struct cpu_state *state, uint16_t addr) {
 
-	if (addr > state->instr_cnt / 2) {
+	if (addr > state->instr_cnt / 2)
 		fprintf(stderr, "Runtime error: Invalid instruction address %ui\n", addr);
-		state->flags |= CPU_FLAGS_HALT;
-		return;
-	}
 	state->pc = addr;
 }
 
 static unsigned char* instr_fetch(struct cpu_state *state) {
 
-	if (state->pc + 1 >= state->instr_cnt >> 1)
-		state->flags |= CPU_FLAGS_HALT;
-
 	return state->instr_mem + (state->pc++ << 1);
 }
 
@@ -140,13 +133,13 @@ int cpu_tick(struct cpu_state *state) {
 	struct instr instr;
 	unsigned char* next;
 
-	if (state->flags & CPU_FLAGS_HALT) {
-		return 1;
-	}
-
 	// Fetch next instruction
 	next = instr_fetch(state);
 
+	// We abort if we move past the last program address.
+	if (state->pc > state->instr_cnt >> 1)
+		return 1;
+
 	// decode instruction.
 	instr_decode(next, &instr);
 
diff --git a/vm/cpu.h b/vm/cpu.h
index d5aa989..f4d4912 100644
--- a/vm/cpu.h
+++ b/vm/cpu.h
@@ -25,8 +25,6 @@
 
 #define CPU_NUM_REGS 16
 
-#define CPU_FLAGS_HALT (1<<0)
-
 struct cpu_state {
 	// Registers r0, r15
 	int16_t reg[CPU_NUM_REGS];
@@ -34,9 +32,6 @@ struct cpu_state {
 	// Program counter
 	uint16_t pc;
 
-	// flags
-	unsigned char flags;
-
 	// Instruction
 	unsigned char *instr_mem;
 	unsigned long instr_cnt;