From 50353bafdd2a17aa02b5fa449acebe67309e448e Mon Sep 17 00:00:00 2001
From: pantonshire <tom@tomandtally.co.uk>
Date: Sun, 23 Jun 2024 21:07:00 +0100
Subject: [PATCH] little boot sector that writes hi to vga

---
 boot.s   | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 justfile |  6 ++++++
 2 files changed, 53 insertions(+)
 create mode 100644 boot.s
 create mode 100644 justfile

diff --git a/boot.s b/boot.s
new file mode 100644
index 0000000..09e789c
--- /dev/null
+++ b/boot.s
@@ -0,0 +1,47 @@
+; BIOS puts our boot sector at 0000:7c00
+org 0x7c00  
+; We're (probably) in real mode
+bits 16 
+
+  ; Disable interrupts, fuck knows what an interrupt would do right now
+  cli
+
+  xor ax, ax
+
+  mov ds, ax
+  mov es, ax
+
+  ; Put the stack base at the address of our program text (stack will grow high->low away from it).
+  ; We're not doing anything with a stack right now but who knows we might want to later
+  mov ss, ax
+  mov sp, 0x7c00
+
+  ; Segment for VGA (0xb800 * 16 = 0xb8000)
+  mov ax, 0xb800
+  mov fs, ax
+
+  mov ah, 0x00
+  mov al, 0x03
+  int 0x10
+
+  ; Hi!
+  mov word fs:[0x0000], 0xc048
+  mov word fs:[0x0002], 0xc069
+  mov word fs:[0x0004], 0xc021
+
+  hlt
+
+  ; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
+  ; e.g. in a fixed memory location
+
+  ; Make sure we fit in a single 512-byte sector
+  %if ($ - $$) > 510
+  %error "exceeded boot sector size"
+  %endif
+
+  ; Pad to the boot signature offset
+  times 510 - ($ - $$) db 0
+  ; Boot signature
+  db 0x55
+  db 0xaa
+
diff --git a/justfile b/justfile
new file mode 100644
index 0000000..55b1ba0
--- /dev/null
+++ b/justfile
@@ -0,0 +1,6 @@
+run:
+  qemu-system-x86_64 -drive format=raw,file=boot.bin
+
+build:
+  nasm -f bin -o boot.bin boot.s
+