From 726666109718e09032f9942bcd75ef7e2524b514 Mon Sep 17 00:00:00 2001
From: lordtet <lordtet@foxcarries.me>
Date: Wed, 30 Jul 2025 17:17:12 -0400
Subject: [PATCH] Boilerplate + Protected mode

Added make debug to Makefile
Added the gdb script from the kernel for easy debugging
now makes it to protected mode and shows a green box when done
Added a gdt (of course)
---
 Makefile       |  9 +++++++--
 connect_gdb.sh |  5 +++++
 src/boot.asm   | 15 --------------
 src/boot.s     | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/gdt.s      | 26 ++++++++++++++++++++++++
 5 files changed, 93 insertions(+), 17 deletions(-)
 create mode 100755 connect_gdb.sh
 delete mode 100644 src/boot.asm
 create mode 100644 src/boot.s
 create mode 100644 src/gdt.s

diff --git a/Makefile b/Makefile
index 1d754c8..75d4cc0 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 # Makefile
 SRC_DIR = src
 BUILD_DIR = build
-BOOTLOADER_SRC = $(SRC_DIR)/boot.asm
+BOOTLOADER_SRC = $(SRC_DIR)/boot.s
 BOOTLOADER_IMG = $(BUILD_DIR)/boot.img
 
 QEMU = qemu-system-i386
@@ -12,8 +12,13 @@ $(BUILD_DIR):
 	mkdir -p $(BUILD_DIR)
 
 $(BOOTLOADER_IMG): $(BOOTLOADER_SRC) | $(BUILD_DIR)
-	nasm -f bin $(BOOTLOADER_SRC) -o $(BOOTLOADER_IMG)
+	nasm -f bin $(BOOTLOADER_SRC) -i $(SRC_DIR) -o $(BOOTLOADER_IMG)
 
+.PHONY: debug
+debug:
+	$(QEMU) $(QEMU_FLAGS) -s -S -drive format=raw,file=$(BOOTLOADER_IMG)
+
+.PHONY: run
 run: all
 	$(QEMU) -drive format=raw,file=$(BOOTLOADER_IMG) -display gtk
 
diff --git a/connect_gdb.sh b/connect_gdb.sh
new file mode 100755
index 0000000..7d72346
--- /dev/null
+++ b/connect_gdb.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+i686-elf-gdb build/ukern.elf \
+    -ex "target remote localhost:1234" \
+    -ex "break start" \
diff --git a/src/boot.asm b/src/boot.asm
deleted file mode 100644
index c53fb71..0000000
--- a/src/boot.asm
+++ /dev/null
@@ -1,15 +0,0 @@
-; boot.asm - simple boot sector
-BITS 16
-ORG 0x7C00
-
-start:
-    mov ah, 0x0E
-    mov al, 'H'
-    int 0x10
-    mov al, 'i'
-    int 0x10
-
-    jmp $
-
-times 510 - ($ - $$) db 0
-dw 0xAA55
diff --git a/src/boot.s b/src/boot.s
new file mode 100644
index 0000000..9e240c4
--- /dev/null
+++ b/src/boot.s
@@ -0,0 +1,55 @@
+; boot.asm - simple boot sector
+[ORG 0x7C00]
+[BITS 16]
+jmp start
+
+%include "gdt.s"
+
+;Opting against using ORG 0x7C00 as I want to access memory literally for clarity. 
+;Remember to set a segment to 0x07C0 :)
+;ORG 0x7C00
+
+
+start:
+    ;Hello world! Let's get protected mode moving so we don't have to work in real mode the whole time.
+    cli
+    ;gimme dat
+    lgdt [gdtr]
+    ;Enable protected mode bit
+    mov eax, cr0
+    or eax, 1
+    mov cr0, eax
+    
+    ;32 bits!
+    jmp 0x08:prot_mode
+
+
+[BITS 32]
+prot_mode:
+    ;Hello 32 bit!
+    ;Some real setup now...
+    ;Set the segments
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+
+    ;Clear the screen
+    mov ax, 0x0720
+    mov edi, 0xB8000
+    mov ecx, 80*25
+    rep stosw
+
+
+done:
+    mov cx, 0x2220 
+    mov [0xB8000], cx
+
+    jmp $
+
+;Bootloader should be 512 bytes. So we are defining a padding of zeroes that would round out the size. 510 to account for the 0xAA55 after.
+times 510 - ($ - $$) db 0
+;Old BIOS expect the sector to end in 0xAA55. We are, in fact, targeting old hardware.
+dw 0xAA55
diff --git a/src/gdt.s b/src/gdt.s
new file mode 100644
index 0000000..4adbc26
--- /dev/null
+++ b/src/gdt.s
@@ -0,0 +1,26 @@
+;GDT code taken straight from Untitled Kernel. The Kernel mode ranges will do just fine.
+align 8
+gdt:
+    ;Null descriptor
+    dd 0x00000000
+    dd 0x00000000
+    ;Kernel code segment.
+    dw 0xFFFF
+    dw 0
+    db 0
+    db 0b10011010
+    db 0b11001111
+    db 0
+    ;Kernel data segment.
+    dw 0xFFFF
+    dw 0x0000
+    db 0x00
+    db 0b10010010
+    db 0b11001111
+    db 0x00
+gdt_end:
+gdtr:
+    ;size of the gdt
+    dw gdt_end - gdt - 1
+    ;location of the gdt
+    dd gdt