From 3743c0b0d86f33f545becb0d02594988190c43b9 Mon Sep 17 00:00:00 2001
From: Bradford Lamson-Scribner <brad.lamson@gmail.com>
Date: Sun, 31 May 2020 09:38:10 -0600
Subject: [PATCH] internal/vm/{exec_funcs,opcodes}: add support for executing
 DEY, DEX, TAY, TAX opcodes

---
 internal/vm/exec_funcs.go | 49 +++++++++++++++++++++++++++++++++++++++
 internal/vm/opcodes.go    | 19 +++++++++++----
 2 files changed, 64 insertions(+), 4 deletions(-)

diff --git a/internal/vm/exec_funcs.go b/internal/vm/exec_funcs.go
index 858f8bb..f736bef 100644
--- a/internal/vm/exec_funcs.go
+++ b/internal/vm/exec_funcs.go
@@ -79,3 +79,52 @@ func execINY(a *Appleone, o op) error {
 	a.setNegativeIfOverflow(b)
 	return nil
 }
+
+// A -> X                           N Z C I D V
+//                                  + + - - - -
+func execTAX(a *Appleone, o op) error {
+	a.cpu.x = a.cpu.a
+	a.setZeroIfNeeded(a.cpu.x)
+	a.setNegativeIfOverflow(a.cpu.x)
+	return nil
+}
+
+// A -> Y                           N Z C I D V
+//                                  + + - - - -
+func execTAY(a *Appleone, o op) error {
+	a.cpu.y = a.cpu.a
+	a.setZeroIfNeeded(a.cpu.y)
+	a.setNegativeIfOverflow(a.cpu.y)
+	return nil
+}
+
+// X - 1 -> X                       N Z C I D V
+//                                  + + - - - -
+func execDEX(a *Appleone, o op) error {
+	b := a.cpu.x - 1
+	a.cpu.x = b
+	a.setZeroIfNeeded(b)
+	a.setNegativeIfOverflow(b)
+	return nil
+}
+
+// Y - 1 -> Y                       N Z C I D V
+//                                  + + - - - -
+func execDEY(a *Appleone, o op) error {
+	b := a.cpu.y - 1
+	a.cpu.y = b
+	a.setZeroIfNeeded(b)
+	a.setNegativeIfOverflow(b)
+	return nil
+}
+
+// M -> A                           N Z C I D V
+//                                  + + - - - -
+func execLDA(a *Appleone, o op) error {
+	b, err := o.getData(a)
+	if err != nil {
+		return err
+	}
+	a.cpu.a = b
+	return nil
+}
diff --git a/internal/vm/opcodes.go b/internal/vm/opcodes.go
index 2896aa0..8ba192d 100644
--- a/internal/vm/opcodes.go
+++ b/internal/vm/opcodes.go
@@ -70,6 +70,17 @@ func (o op) getAddr(a *Appleone) (uint16, error) {
 	}
 }
 
+func (o op) getData(a *Appleone) (uint8, error) {
+	if o.addrMode == accumulator {
+		return a.cpu.a, nil
+	}
+	b, err := o.getAddr(a)
+	if err != nil {
+		return 0, err
+	}
+	return a.mem[b], nil
+}
+
 // opcodes represent all of the Apple 1 opcodes available. Each 8 bit opcode is mapped to a corresponding
 // "op" which is just a struct holding metadata about the operation.
 var opcodes = map[uint8]op{
@@ -125,25 +136,25 @@ var opcodes = map[uint8]op{
 	// addressing    assembler    opc  bytes  cyles
 	// --------------------------------------------
 	// implied       TAX          AA   1      2
-	0xAA: newOp("TAX", 0xAA, 1, implied, todo),
+	0xAA: newOp("TAX", 0xAA, 1, implied, execTAX),
 
 	// TAY Transfer Accumulator to Index Y
 	// addressing    assembler    opc  bytes  cyles
 	// --------------------------------------------
 	// implied       TAY          A8   1      2
-	0xA8: newOp("TAY", 0xA8, 1, implied, todo),
+	0xA8: newOp("TAY", 0xA8, 1, implied, execTAY),
 
 	// DEX Decrement Index X by One
 	// addressing    assembler    opc  bytes  cyles
 	// --------------------------------------------
 	// implied       DEC          CA   1      2
-	0xCA: newOp("DEX", 0xCA, 1, implied, todo),
+	0xCA: newOp("DEX", 0xCA, 1, implied, execDEX),
 
 	// DEY Decrement Index Y by One
 	// addressing    assembler    opc  bytes  cyles
 	// --------------------------------------------
 	// implied       DEC          88   1      2
-	0x88: newOp("DEY", 0x88, 1, implied, todo),
+	0x88: newOp("DEY", 0x88, 1, implied, execDEY),
 
 	// LDA Load Accumulator with Memory
 	// addressing    assembler    opc  bytes  cyles