From 0581ed6b441d8df65967137659f92e392a055698 Mon Sep 17 00:00:00 2001
From: Peter Evans <pevans@activecampaign.com>
Date: Sun, 21 Jan 2018 01:09:42 -0600
Subject: [PATCH] Fix stack in several ways

1. The stack should descend, not ascend;
2. The stack should be pushed byte for byte; meaning, when pushing P or
A, those should consume one byte, not two;
3. The MSB should be pushed first when doing JSR and BRK, which makes
some sense if you were reading the stack from $0100 - $01FF.
---
 include/mos6502.h  |  4 ++--
 src/apple2.c       |  2 +-
 src/mos6502.c      | 14 +++++++-------
 src/mos6502.exec.c | 10 ++++++++--
 4 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/include/mos6502.h b/include/mos6502.h
index 95bba8c..9fb056f 100644
--- a/include/mos6502.h
+++ b/include/mos6502.h
@@ -136,13 +136,13 @@ extern int mos6502_instruction(vm_8bit);
 extern mos6502 *mos6502_create(vm_segment *, vm_segment *);
 extern mos6502_instruction_handler mos6502_get_instruction_handler(vm_8bit);
 extern vm_16bit mos6502_get16(mos6502 *, size_t);
-extern vm_16bit mos6502_pop_stack(mos6502 *);
 extern vm_8bit mos6502_get(mos6502 *, size_t);
+extern vm_8bit mos6502_pop_stack(mos6502 *);
 extern void mos6502_execute(mos6502 *);
 extern void mos6502_free(mos6502 *);
 extern void mos6502_last_executed(mos6502 *, vm_8bit *, vm_8bit *, vm_16bit *);
 extern void mos6502_modify_status(mos6502 *, vm_8bit, int, int);
-extern void mos6502_push_stack(mos6502 *, vm_16bit);
+extern void mos6502_push_stack(mos6502 *, vm_8bit);
 extern void mos6502_set(mos6502 *, size_t, vm_8bit);
 extern void mos6502_set16(mos6502 *, size_t, vm_16bit);
 extern void mos6502_set_memory(mos6502 *, vm_segment *, vm_segment *);
diff --git a/src/apple2.c b/src/apple2.c
index deb6857..63da6f2 100644
--- a/src/apple2.c
+++ b/src/apple2.c
@@ -254,7 +254,7 @@ apple2_reset(apple2 *mach)
 {
     mach->cpu->P = MOS_INTERRUPT;
     mach->cpu->PC = vm_segment_get16(mach->main, 0xFFFC);
-    mach->cpu->S = 0;
+    mach->cpu->S = 0xff;
 
     // Switch video mode back to 40 column text
     apple2_set_display(mach, DISPLAY_DEFAULT);
diff --git a/src/mos6502.c b/src/mos6502.c
index efc3424..bb586e6 100644
--- a/src/mos6502.c
+++ b/src/mos6502.c
@@ -163,7 +163,7 @@ mos6502_create(vm_segment *rmem, vm_segment *wmem)
     cpu->X = 0;
     cpu->Y = 0;
     cpu->P = 0;
-    cpu->S = 0;
+    cpu->S = 0xff;
 
     return cpu;
 }
@@ -189,29 +189,29 @@ mos6502_free(mos6502 *cpu)
  * 256, addresses.
  */
 void
-mos6502_push_stack(mos6502 *cpu, vm_16bit addr)
+mos6502_push_stack(mos6502 *cpu, vm_8bit addr)
 {
-    mos6502_set16(cpu, 0x100 + cpu->S, addr);
+    mos6502_set(cpu, 0x100 + cpu->S, addr);
 
     // And finally we need to increment S by 2 (since we've used two
     // bytes in the stack).
-    cpu->S += 2;
+    cpu->S--;
 }
 
 /*
  * Pop an address from the stack and return that.
  */
-vm_16bit
+vm_8bit
 mos6502_pop_stack(mos6502 *cpu)
 {
     // The first thing we want to do here is to decrement S by 2, since
     // the value we want to return is two positions back.
-    cpu->S -= 2;
+    cpu->S++;
 
     // We need to use a bitwise-or operation to combine the hi and lo
     // bytes we retrieve from the stack into the actual position we
     // would use for the PC register.
-    return mos6502_get16(cpu, 0x0100 + cpu->S);
+    return mos6502_get(cpu, 0x0100 + cpu->S);
 }
 
 /*
diff --git a/src/mos6502.exec.c b/src/mos6502.exec.c
index 545542a..14146a7 100644
--- a/src/mos6502.exec.c
+++ b/src/mos6502.exec.c
@@ -14,7 +14,8 @@
  */
 DEFINE_INST(brk)
 {
-    mos6502_push_stack(cpu, cpu->PC);
+    mos6502_push_stack(cpu, cpu->PC >> 8);
+    mos6502_push_stack(cpu, cpu->PC & 0xff);
     mos6502_push_stack(cpu, cpu->P);
     cpu->P |= MOS_INTERRUPT;
     cpu->PC += 2;
@@ -36,7 +37,10 @@ DEFINE_INST(jmp)
  */
 DEFINE_INST(jsr)
 {
-    mos6502_push_stack(cpu, cpu->PC + 3);
+    vm_16bit pc3 = cpu->PC + 3;
+
+    mos6502_push_stack(cpu, pc3 >> 8);
+    mos6502_push_stack(cpu, pc3 & 0xff);
     cpu->PC = cpu->eff_addr;
 }
 
@@ -57,6 +61,7 @@ DEFINE_INST(rti)
 {
     cpu->P = mos6502_pop_stack(cpu);
     cpu->PC = mos6502_pop_stack(cpu);
+    cpu->PC |= mos6502_pop_stack(cpu) << 8;
 }
 
 /*
@@ -66,4 +71,5 @@ DEFINE_INST(rti)
 DEFINE_INST(rts)
 {
     cpu->PC = mos6502_pop_stack(cpu);
+    cpu->PC |= mos6502_pop_stack(cpu) << 8;
 }