From 12c42d956354c70e2aa36d03696560eb3172e40a Mon Sep 17 00:00:00 2001
From: Jason Hilder <jhilder95@gmail.com>
Date: Thu, 16 Apr 2026 07:34:21 +0200
Subject: [PATCH] Initial commit for basics need a refresher.

---
 cmd/asm/main.go     |  7 ++++
 cmd/sim/main.go     | 26 ++++++++++++++
 go.mod              |  3 ++
 internal/bus/bus.go | 20 +++++++++++
 internal/cpu/cpu.go | 85 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 141 insertions(+)
 create mode 100644 cmd/asm/main.go
 create mode 100644 cmd/sim/main.go
 create mode 100644 go.mod
 create mode 100644 internal/bus/bus.go
 create mode 100644 internal/cpu/cpu.go

diff --git a/cmd/asm/main.go b/cmd/asm/main.go
new file mode 100644
index 0000000..7bd7bfd
--- /dev/null
+++ b/cmd/asm/main.go
@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hi compiler")
+}
diff --git a/cmd/sim/main.go b/cmd/sim/main.go
new file mode 100644
index 0000000..13a706d
--- /dev/null
+++ b/cmd/sim/main.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"fmt"
+
+	"jh/gomos/internal/bus"
+	"jh/gomos/internal/cpu"
+)
+
+func main() {
+	fmt.Println("--- 6502 Simulator Starting ---")
+
+	// Initialize the Bus
+	b := bus.New()
+
+	// Initialize CPU and "plug in" the Bus
+	c := &cpu.CPU{Bus: b}
+	c.Reset()
+
+	// Run for 10 cycles to verify the "Fetch" works
+	for range 10 {
+		c.Step()
+	}
+
+	fmt.Println("--- Simulation Complete ---")
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..f728866
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module jh/gomos
+
+go 1.26.1
diff --git a/internal/bus/bus.go b/internal/bus/bus.go
new file mode 100644
index 0000000..864b7d4
--- /dev/null
+++ b/internal/bus/bus.go
@@ -0,0 +1,20 @@
+package bus
+
+type MainBus struct {
+	// add ram etc here
+}
+
+func New() *MainBus {
+	return &MainBus{}
+}
+
+// Just a test read for now, need to figure wtf is happening...
+func (b *MainBus) Read(address uint16) uint8 {
+	// Just testing with NOPs
+	return 0xEA
+}
+
+func (b *MainBus) Write(address uint16, data uint8) {
+	// Need to write data to the actual ram.
+}
+
diff --git a/internal/cpu/cpu.go b/internal/cpu/cpu.go
new file mode 100644
index 0000000..9bb6795
--- /dev/null
+++ b/internal/cpu/cpu.go
@@ -0,0 +1,85 @@
+package cpu
+
+import "fmt"
+
+const ARCH = "MOS6502"
+
+// bits for the status register
+const (
+	FlagCarry     = 1 << 0 // Bit 0
+	FlagZero      = 1 << 1 // Bit 1
+	FlagInterrupt = 1 << 2 // Bit 2
+	FlagDecimal   = 1 << 3 // Bit 3
+	FlagBreak     = 1 << 4 // Bit 4
+	FlagUnused    = 1 << 5 // Bit 5
+	FlagOverflow  = 1 << 6 // Bit 6
+	FlagNegative  = 1 << 7 // Bit 7
+)
+
+type Bus interface {
+	Read(address uint16) uint8
+}
+
+type CPU struct {
+	A      uint8  // Accumulator
+	X      uint8  // X Register
+	Y      uint8  // Y Register
+	PC     uint16 // Program Counter
+	SP     uint8  // Stack Pointer
+	Status uint8  // Status Flags
+	Bus    Bus    // Connection to the Bus
+}
+
+func (c *CPU) Reset() {
+	c.A = 0
+	c.X = 0
+	c.Y = 0
+	c.SP = 0xFD
+	c.Status = 0x24 
+	c.PC = 0x0000
+}
+
+// Small helper func to flip the status register bits
+func (c *CPU) setFlag(flag uint8, set bool) {
+	if set {
+		c.Status |= flag
+	} else {
+		c.Status &= ^flag
+	}
+}
+
+// fetch -> decode -> execute cycle
+func (c *CPU) Step() {
+	// fetch with program counter and bus memory
+	opcode := c.Bus.Read(c.PC)
+
+	fmt.Printf("Executing at PC: %04X | Opcode: %02X\n", c.PC, opcode)
+
+	// decode + exe need to split this out to handle all instructions etc
+	// Look at : https://www.ahl27.com/posts/2023/01/6502-emu1/
+	switch opcode {
+	case 0xEA: //NOP
+		// does nothing its a no op
+	default:
+		fmt.Printf("Unknown opcode: %02X\n", opcode)
+	}
+
+	// Move the Program counter
+	c.PC++
+}
+
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