From 5f6ee5837872bd30780cb3cfa990d124b18d3038 Mon Sep 17 00:00:00 2001
From: radumacocian <rmacocian@yahoo.com>
Date: Mon, 30 Jun 2025 14:34:58 +0200
Subject: [PATCH] added half carry flags

---
 cpu/functions.go                 | 23 +++++++++++++++++------
 cpu/operations/read_byte.go      | 10 ++++++++++
 cpu/operations/read_byte_test.go | 25 +++++++++++++++++++++++++
 journal/References.md            |  6 +++++-
 4 files changed, 57 insertions(+), 7 deletions(-)
 create mode 100644 cpu/operations/read_byte.go
 create mode 100644 cpu/operations/read_byte_test.go

diff --git a/cpu/functions.go b/cpu/functions.go
index 50a5481..8031f6a 100644
--- a/cpu/functions.go
+++ b/cpu/functions.go
@@ -6,8 +6,8 @@ import (
 )
 
 type op_context struct {
-	cpu  CPU_struct
-	addr uint
+	cpu       CPU_struct
+	immediate byte
 }
 
 func Noop(context op_context) {
@@ -16,8 +16,8 @@ func Noop(context op_context) {
 
 // LDBCd16 Load the 2 bytes of immediate data into register pair BC.
 func LDBCd16(context op_context) {
-	operations.LD(&context.cpu.C, memory.Read8(context.addr))
-	operations.LD(&context.cpu.B, memory.Read8(context.addr+8))
+	operations.LD(&context.cpu.C, memory.Read8(uint(context.immediate)))
+	operations.LD(&context.cpu.B, memory.Read8(uint(context.immediate)+8))
 }
 
 // LDBCa Store the contents of register A in the memory location specified by register pair BC.
@@ -34,6 +34,11 @@ func INCBC(context op_context) {
 
 // INCB Increment the contents of register B by 1.
 func INCB(context op_context) {
+	if context.cpu.B&0x0F == 0x0F {
+		context.cpu.SetHF(true)
+	} else {
+		context.cpu.SetHF(false)
+	}
 	operations.INC(&context.cpu.B)
 
 	context.cpu.SetZF(context.cpu.B == byte(0))
@@ -42,6 +47,11 @@ func INCB(context op_context) {
 
 // DECB Decrement the contents of register B by 1.
 func DECB(context op_context) {
+	if context.cpu.B&0x0F == 0x00 {
+		context.cpu.SetHF(true)
+	} else {
+		context.cpu.SetHF(false)
+	}
 	operations.DEC(&context.cpu.B)
 
 	context.cpu.SetZF(context.cpu.B == byte(0))
@@ -50,7 +60,7 @@ func DECB(context op_context) {
 
 // LD8B Load the 8-bit immediate operand d8 into register B.
 func LD8B(context op_context) {
-	operations.LD(&context.cpu.B, memory.Read8(context.addr))
+	operations.LD(&context.cpu.B, memory.Read8(uint(context.immediate)))
 }
 
 // RLCA Rotate the contents of register A to the left. The contents of bit 7 are placed in both the CY flag and bit 0 of register A.
@@ -66,5 +76,6 @@ func RLCA(context op_context) {
 
 // LDA16 Store the lower byte of stack pointer SP at the address specified by the 16-bit immediate operand a16, and store the upper byte of SP at address a16 + 1.
 func LDA16(context op_context) {
-
+	operations.LDInMemory8(uint16(context.immediate), operations.GetLowerByte(context.cpu.SP))
+	operations.LDInMemory8(uint16(context.immediate+1), operations.GetHigherByte(context.cpu.SP))
 }
diff --git a/cpu/operations/read_byte.go b/cpu/operations/read_byte.go
new file mode 100644
index 0000000..d915b4c
--- /dev/null
+++ b/cpu/operations/read_byte.go
@@ -0,0 +1,10 @@
+package operations
+
+func GetLowerByte(value uint16) byte {
+	return (byte)(value)
+}
+
+func GetHigherByte(value uint16) byte {
+	return (byte)(value >> 8)
+}
+
diff --git a/cpu/operations/read_byte_test.go b/cpu/operations/read_byte_test.go
new file mode 100644
index 0000000..befeba7
--- /dev/null
+++ b/cpu/operations/read_byte_test.go
@@ -0,0 +1,25 @@
+package operations
+
+import (
+	"testing"
+)
+
+func TestReadLowerByte(t *testing.T) {
+	SP := uint16(0x1234)
+
+	actual := GetLowerByte(SP)
+
+	if actual != 0x34 {
+		t.Errorf("actual %v != expected %v", actual, true)
+	}
+}
+
+func TestReadUpperByte(t *testing.T) {
+	SP := uint16(0x1234)
+
+	actual := GetHigherByte(SP)
+
+	if actual != 0x12 {
+		t.Errorf("actual %v != expected %v", actual, true)
+	}
+}
diff --git a/journal/References.md b/journal/References.md
index 4b354ae..f2f2386 100644
--- a/journal/References.md
+++ b/journal/References.md
@@ -1 +1,5 @@
-[Opcodes Table](https://meganesulli.com/blog/game-boy-opcodes/)
\ No newline at end of file
+[Opcodes Table](https://meganesulli.com/blog/game-boy-opcodes/)
+[Registers & Flags](https://gbdev.io/pandocs/CPU_Registers_and_Flags.html)
+
+- run command: go run . roms/rom.gb
+- test command: go test ./...