From e7ed5560234ccb8f5b3d2c00e06ad617b652c7b9 Mon Sep 17 00:00:00 2001
From: Stefan Arentz <stefan@arentz.ca>
Date: Sun, 13 Nov 2016 15:29:49 -0500
Subject: [PATCH] Fixes #2 - Keep the stack pointer as a byte instead of word

---
 cpu.c | 49 +++++++++++++++++++++++++++++++++++++++----------
 cpu.h | 15 +++++++++++++--
 ins.c | 16 ++++++----------
 3 files changed, 58 insertions(+), 22 deletions(-)

diff --git a/cpu.c b/cpu.c
index 0e71064..bded4c8 100644
--- a/cpu.c
+++ b/cpu.c
@@ -39,14 +39,47 @@ typedef void (*cpu_instruction_handler_t)(struct cpu_t *cpu);
 typedef void (*cpu_instruction_handler_byte_t)(struct cpu_t *cpu, uint8_t oper);
 typedef void (*cpu_instruction_handler_word_t)(struct cpu_t *cpu, uint16_t oper);
 
-static uint8_t _mem_get_byte(struct cpu_t *cpu, uint16_t addr) {
+// The following get and set memory directly. There are no checks, so
+// make sure you are doing the right thing. Mainly used for managing
+// the stack, reading instructions, reading vectors and tracing code.
+
+uint8_t _mem_get_byte(struct cpu_t *cpu, uint16_t addr) {
    return cpu->memory[addr];
 }
 
-static uint16_t _mem_get_word(struct cpu_t *cpu, uint16_t addr) {
+uint16_t _mem_get_word(struct cpu_t *cpu, uint16_t addr) {
    return *((uint16_t*) &cpu->memory[addr]);
 }
 
+void _mem_set_byte(struct cpu_t *cpu, uint16_t addr, uint8_t v) {
+  cpu->memory[addr] = v;
+}
+
+void _mem_set_word(struct cpu_t *cpu, uint16_t addr, uint8_t v) {
+  *((uint16_t*) &cpu->memory[addr]) = v;
+}
+
+// Stack management.
+
+void _cpu_push_byte(struct cpu_t *cpu, uint8_t b) {
+  _mem_set_byte(cpu, 0x0100 + cpu->state.sp, b);
+  cpu->state.sp -= 1;
+}
+
+void _cpu_push_word(struct cpu_t *cpu, uint16_t w) {
+  _cpu_push_byte(cpu, (uint8_t) (w >> 8));
+  _cpu_push_byte(cpu, (uint8_t) w);
+}
+
+uint8_t _cpu_pull_byte(struct cpu_t *cpu) {
+  cpu->state.sp += 1;
+  return _mem_get_byte(cpu, 0x0100 + cpu->state.sp);
+}
+
+uint16_t _cpu_pull_word(struct cpu_t *cpu) {
+  return (uint16_t) _cpu_pull_byte(cpu) | ((uint16_t) _cpu_pull_byte(cpu) << 8);
+}
+
 #if 1
 static void cpu_format_instruction(struct cpu_t *cpu, char *buffer) {
    *buffer = 0x00;
@@ -146,7 +179,7 @@ static void cpu_format_instruction(struct cpu_t *cpu, char *buffer) {
 
 static void cpu_format_state(struct cpu_t *cpu, char *buffer) {
    sprintf(buffer, "A=%.2X X=%.2X Y=%.2X S=%.2X SP=%.4X %c%c%c%c%c%c%c%c",
-           cpu->state.a, cpu->state.x, cpu->state.y, cpu->state.s, cpu->state.sp,
+           cpu->state.a, cpu->state.x, cpu->state.y, cpu->state.s, 0x0100 + cpu->state.sp,
 
            cpu->state.n ? 'N' : '-',
            cpu->state.v ? 'V' : '-',
@@ -160,9 +193,9 @@ static void cpu_format_state(struct cpu_t *cpu, char *buffer) {
 
 static void cpu_format_stack(struct cpu_t *cpu, char *buffer) {
    *buffer = 0x00;
-   for (uint16_t sp = cpu->state.sp; sp != 0x01ff; sp++) {
+   for (uint16_t sp = cpu->state.sp; sp != 0xff; sp++) {
       char tmp[8];
-      sprintf(tmp, " %.2X", _mem_get_byte(cpu, sp));
+      sprintf(tmp, " %.2X", _mem_get_byte(cpu, 0x0100 + sp));
       buffer = strcat(buffer, tmp);
    }
 }
@@ -316,11 +349,7 @@ void cpu_reset(struct cpu_t *cpu) {
    cpu->state.i = 1;
    cpu->state.z = 0;
    cpu->state.c = 0;
-   cpu->state.sp = 0x01ff;
-}
-
-void cpu_brk(struct cpu_t *cpu) {
-
+   cpu->state.sp = 0xff;
 }
 
 void cpu_irq(struct cpu_t *cpu) {
diff --git a/cpu.h b/cpu.h
index 67682fa..5a77ca2 100644
--- a/cpu.h
+++ b/cpu.h
@@ -35,8 +35,7 @@ struct iom_t {
 };
 
 struct cpu_state_t {
-  uint8_t a, x, y, s;
-  uint16_t sp;
+  uint8_t a, x, y, s, sp;
   uint16_t pc;
   uint8_t n, v, b, d, i, z, c;
 };
@@ -48,6 +47,18 @@ struct cpu_t {
   struct iom_t *iom;
 };
 
+// Private. How do we keep them private?
+uint8_t _mem_get_byte(struct cpu_t *cpu, uint16_t addr);
+uint16_t _mem_get_word(struct cpu_t *cpu, uint16_t addr);
+void _mem_set_byte(struct cpu_t *cpu, uint16_t addr, uint8_t v);
+void _mem_set_word(struct cpu_t *cpu, uint16_t addr, uint8_t v);
+
+// Private. How do we keep them private?
+void _cpu_push_byte(struct cpu_t *cpu, uint8_t b);
+void _cpu_push_word(struct cpu_t *cpu, uint16_t w);
+uint8_t _cpu_pull_byte(struct cpu_t *cpu);
+uint16_t _cpu_pull_word(struct cpu_t *cpu);
+
 typedef uint8_t (*iom_read_handler_t)(struct cpu_t *cpu, void *obj, uint16_t addr);
 typedef void (*iom_write_handler_t)(struct cpu_t *cpu, void *obj, uint16_t addr, uint8_t b);
 
diff --git a/ins.c b/ins.c
index ea617ac..f1ca748 100644
--- a/ins.c
+++ b/ins.c
@@ -436,8 +436,7 @@ static void jmp_ind(struct cpu_t *cpu, uint16_t oper) {
 /* JSR */
 
 static void jsr_abs(struct cpu_t *cpu, uint16_t oper) {
-  cpu->state.sp -= 2;
-  mem_set_word(cpu, cpu->state.sp, cpu->state.pc-1);
+  _cpu_push_word(cpu, cpu->state.pc - 1);
   cpu->state.pc = oper;
 }
 
@@ -613,13 +612,11 @@ static void ora_indy(struct cpu_t *cpu, uint8_t oper) {
 /* P** */
 
 static void pha(struct cpu_t *cpu) {
-  cpu->state.sp--;
-  mem_set_byte(cpu, cpu->state.sp, cpu->state.a);
+  _cpu_push_byte(cpu, cpu->state.a);
 }
 
 static void pla(struct cpu_t *cpu) {
-  cpu->state.a = mem_get_byte(cpu, cpu->state.sp);
-  cpu->state.sp++;
+  cpu->state.a = _cpu_pull_byte(cpu);
   update_zn(cpu, cpu->state.a);
 }
 
@@ -686,8 +683,7 @@ static void ror_absx(struct cpu_t *cpu, uint16_t oper) {
 /* RTS */
 
 static void rts(struct cpu_t *cpu) {
-  cpu->state.pc = mem_get_word(cpu, cpu->state.sp)+1;
-  cpu->state.sp += 2;
+  cpu->state.pc = _cpu_pull_word(cpu) + 1;
 }
 
 /* SBC */
@@ -799,7 +795,7 @@ static void tay(struct cpu_t *cpu) {
 }
 
 static void tsx(struct cpu_t *cpu) {
-  cpu->state.x = cpu->state.sp & 0x00ff;
+  cpu->state.x = cpu->state.sp;
   update_zn(cpu, cpu->state.x);
 }
 
@@ -809,7 +805,7 @@ static void txa(struct cpu_t *cpu) {
 }
 
 static void txs(struct cpu_t *cpu) {
-  cpu->state.sp = 0x0100 + cpu->state.x;
+  cpu->state.sp = cpu->state.x;
 }
 
 static void tya(struct cpu_t *cpu) {