From 86c0c87425f2a482d5f21a1cb6878806650760d6 Mon Sep 17 00:00:00 2001
From: Nocturn9x <hackhab@gmail.com>
Date: Tue, 15 Nov 2022 15:47:33 +0100
Subject: [PATCH] Initial work on the bootloader

---
 .gitignore                |  4 ++++
 README.md                 |  9 +++++++-
 src/bootloader/mbr.s      | 41 ++++++++++++++++++++++++++++++++++++
 src/bootloader/util/tty.s | 44 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 97 insertions(+), 1 deletion(-)
 create mode 100644 src/bootloader/mbr.s
 create mode 100644 src/bootloader/util/tty.s

diff --git a/.gitignore b/.gitignore
index 65d6b54..9227b93 100644
--- a/.gitignore
+++ b/.gitignore
@@ -65,3 +65,7 @@ compile_commands.json
 CTestTestfile.cmake
 _deps
 
+# TSOS stuff
+
+*.bin
+dist/
\ No newline at end of file
diff --git a/README.md b/README.md
index befd716..dd5e9cb 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,10 @@
 # TSOS
 
-The Simple Operating System
\ No newline at end of file
+A toy project to implement:
+- A BIOS-compatible bootloader written in x86 Assembly
+- A 32-bit kernel and associated operating system written in C
+- Drivers (keyboard, VGA, etc.)
+- Dynamic memory management
+- A shell
+- Multitasking
+- More?
\ No newline at end of file
diff --git a/src/bootloader/mbr.s b/src/bootloader/mbr.s
new file mode 100644
index 0000000..8db254b
--- /dev/null
+++ b/src/bootloader/mbr.s
@@ -0,0 +1,41 @@
+; Definition of the MBR (Master Boot Record). This is basically our bootloader and
+; is located in the first 512 bytes of the drive we're booting from. From here, we
+; do some basic setup and then call into the kernel
+
+[org 0x7c00]    ; Address where the code expects to be loaded in. The BIOS always loads us here
+[bits 16]       ; All x86 CPUs start in 16 bit (aka "real") mode, so we tell nasm to emit 16-bit code
+
+; We print a simple startup message using the ISRs
+; from the BIOS
+mov si, startup_msg
+call print
+
+
+; Now we setup the stack by setting the
+; base pointer to address 0x8000. The 
+; address itself doesn't matter as long 
+; as it's far enough away from memory already
+; in use by the BIOS
+mov bp, 0x8000
+mov sp, bp        ; The stack starts out empty, so sp == bp
+
+
+jmp $ ; Keeps jumping at the current address (loops forever)
+
+; These two lines never execute (for now), but they might once 
+; we jump into the kernel, so we disable interrupts and halt the
+; CPU, just in case
+cli 
+hlt
+
+; Now we include our "function definitions" (after the halting, so
+; they're never executed unless explicitly called)
+%include "src/bootloader/util/tty.s"
+
+; Here we define our variables: They need to be defined after the
+; halting because otherwise they will be executed as code!
+startup_msg: db "TSOS - Bootloader: Starting up", 0xA, 0xD, 0x0
+
+; padding and magic number
+times 510 - ($-$$) db 0
+dw 0xaa55
\ No newline at end of file
diff --git a/src/bootloader/util/tty.s b/src/bootloader/util/tty.s
new file mode 100644
index 0000000..ce0ca79
--- /dev/null
+++ b/src/bootloader/util/tty.s
@@ -0,0 +1,44 @@
+; Some utilities to deal with the TTY using the BIOS
+; during real mode
+
+next_line:
+    ; Points the TTY cursor to the next
+    ; line
+    pusha
+    ; Source: http://www.techhelpmanual.com/118-int_10h_03h__query_cursor_position_and_size.html
+    mov ah, 0x3   ; Get cursor position
+    mov bh, 0x0   ; Page 0
+    int 0x10
+    ; Go to the next row
+    ; Source: http://www.techhelpmanual.com/117-int_10h_02h__set_cursor_position.html
+    mov ah, 0x2  ; Set cursor position
+    mov bh, 0x0  ; Page 0
+    xor dl, dl   ; Goes to column 0 (i.e. start of the line)
+    inc dh       ; Goes to the next row
+    int 0x10
+    popa
+    ret
+
+
+print:
+    ; Prints a null-terminated string whose address
+    ; is located in the si register
+    pusha
+    mov ah, 0xe  ; Set the screen in TTY mode
+    print_loop:
+        mov al, [si]
+        int 0x10           ; Writes the content of al to the screen
+        inc si
+        cmp byte [si], 0x0 ; If we got to the null byte, we're done
+        jne print_loop   ; Otherwise, we run this again
+    popa
+    ret
+
+
+println:
+    ; Prints a null-terminated string whose address
+    ; is located in the si register and sets the TTY
+    ; cursor to the next line
+    call print
+    call next_line
+    ret
\ No newline at end of file