From 772ff893afc43ff8327953be549c0ba367e11a3f Mon Sep 17 00:00:00 2001
From: Michael Smith <m@michaelsmith.be>
Date: Fri, 29 Aug 2025 17:42:30 +0200
Subject: [PATCH] Implement CB instructions

---
 gb/cpu.go      | 14 ++++++++++++++
 gb/cpu_test.go | 19 +++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/gb/cpu.go b/gb/cpu.go
index 322b047..e375bf2 100644
--- a/gb/cpu.go
+++ b/gb/cpu.go
@@ -109,6 +109,20 @@ func (cpu *CPU) Step() {
 			cpu.Regs.PC++
 
 			switch cbOpcode {
+			case 0x7E:
+				// BIT 7, [HL]
+				// Read byte pointed to by address HL
+				address := uint16(cpu.Regs.H)<<8 | uint16(cpu.Regs.L)
+				val := cpu.Bus.Read(address)
+
+				// Check if bit 7 is set
+				if (val & 0x80) == 0 {
+					// Set zero flag if bit is not set
+					cpu.SetFlag(Z)
+				}
+				cpu.ClearFlag(N)
+				cpu.SetFlag(H)
+
 			default:
 				fmt.Printf("\nINVALID INSTRUCTION! Unknown CB opcode: %02X\n", cbOpcode)
 				os.Exit(1)
diff --git a/gb/cpu_test.go b/gb/cpu_test.go
index 5eed4fe..770b84e 100644
--- a/gb/cpu_test.go
+++ b/gb/cpu_test.go
@@ -167,3 +167,22 @@ func TestInstruction21(t *testing.T) {
 	// Should increase the stack pointer
 	assert.Equal(t, uint16(0x3), cpu.Regs.PC)
 }
+
+func TestInstructionCB7E(t *testing.T) {
+	cpu := createCPU([]byte{0xCB, 0x7E, 0x00, 0x00})
+
+	cpu.Regs.H = 0xFF
+	cpu.Regs.L = 0x40
+	err := cpu.Bus.Write(0xFF40, 0xFF)
+	assert.Equal(t, err, nil)
+
+	// FIXME(m): This needs bus access to IO, in particular the LCD
+	// at 0xFF40.
+
+	// assert.Equal(t, 0xFF, cpu.Bus.Read(0xFF40))
+
+	// cpu.Step()
+
+	// // Should set the zero flag
+	// assert.True(t, cpu.IsFlagSet(Z))
+}