wrote a basic bootloader

2025-02-25 01:14:52 +00:00
commit 012b0454ed
36 changed files with 1667 additions and 0 deletions
@@ -0,0 +1,10 @@
+ENTRY(_start)
+OUTPUT_FORMAT(binary)
+SECTIONS
+{
+    . = 0x7C00;         /* Where BIOS loads us */
+    .text : {
+        *(.text)
+        . = 446;        /* Pad to 510 bytes */
+    }
+}
@@ -0,0 +1,92 @@
+section .text
+global _start
+
+[BITS 16]
+_start:
+    ; Set video mode (clear screen)
+    mov ah, 0x00    
+    mov al, 0x03    
+    int 0x10        
+
+    ; Print loading message
+    mov ax, 0xb800  
+    mov ds, ax      
+
+    mov dword [   0], "L O "
+    mov dword [   4], "A D "
+    mov dword [   8], "I N "
+    mov dword [  12], "G . "
+    mov dword [  16], ". . "
+
+    ; setup a stack
+    cli
+    mov ax, 0x9000
+    mov ss, ax
+    mov sp, 0x0000
+    mov bp, sp
+
+    mov bx, 0x0800 ; this is the first memory location we're interested in.
+    mov es, bx
+    xor bx, bx
+
+    mov cl, 0x02   ; this is the first sector we're interested in.
+
+    call load_sector
+
+    mov ax, [es:6]  ; load last two bytes (0x1FE = 510)
+    cmp ax, 0xAA55  ; check if it matches boot signature
+    jne error       ; if not equal, not a valid boot sector
+
+    ; jmp success
+
+    mov ax, [es:4]  ; load sector count
+
+next_sector:
+    dec ax             ; decrement sectors left to write
+    inc cl
+
+    cmp ax, 0          
+    je success
+
+    add bx, 0x200      ; Add 512 to offset
+
+    call load_sector
+    jmp next_sector
+
+load_sector:
+    pusha
+
+    ; setup registers for disk read
+    mov ah, 0x02        ; read sectors function
+    mov dl, 0x80        ; first hard disk
+    mov ch, 0x00        ; cylinder 0
+    mov dh, 0x00        ; head 0
+    mov al, 1           ; read 1 sector
+
+    ; do the read
+    int 0x13
+    jc error             ; if carry set, read failed
+
+    popa
+    ret
+
+hang:
+    jmp hang
+
+error:
+    ; error getting the magic number
+    mov ah, 0x00
+    mov al, 0x03
+    int 0x10
+
+    mov ax, 0xb800
+    mov ds, ax
+
+    mov dword [   0], "E R "
+    mov dword [   4], "R O "
+    mov dword [   8], "R . "
+    
+    jmp hang
+
+success:
+    jmp 0x0800:0x0016   ; jump to start of stage 2