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 +1,118 @@
+[BITS 16]
+[ORG 0x7C00]
+
+    ; Initialize segment registers
+    xor ax, ax
+    mov ds, ax
+    mov es, ax
+    mov ss, ax
+    mov sp, 0x7C00
+
+    ; Reset disk system
+    mov ah, 0
+    int 0x13
+    
+    ; Load FAT32 BPB
+    mov si, bpb_oem_name
+    mov di, 0x7E00
+    mov cx, bpb_size
+    rep movsb
+
+    ; Read FAT32 root directory
+    mov ax, [bpb_reserved_sectors]
+    mov [dapack.lba_start], ax
+    
+    mov ah, 0x42
+    mov si, dapack
+    int 0x13
+    jc error
+
+    ; Find kernel file
+    mov si, kernel_name
+    mov di, 0x8000
+    mov cx, 11      ; FAT32 8.3 filename length
+    
+find_kernel:
+    push cx
+    push si
+    push di
+    rep cmpsb
+    pop di
+    pop si
+    pop cx
+    je found_kernel
+    
+    add di, 32      ; Move to next directory entry
+    cmp di, 0x8200  ; Check if we've reached the end of the sector
+    jl find_kernel
+    
+    ; Kernel not found
+    mov si, err_no_kernel
+    call print
+    jmp $
+
+found_kernel:
+    ; Get cluster number
+    mov ax, [di + 20]  ; High word
+    shl eax, 16
+    mov ax, [di + 26]  ; Low word
+    
+    ; Load kernel
+    mov [cluster], eax
+    call load_cluster
+    
+    ; Jump to kernel
+    jmp 0x8000
+
+error:
+    mov si, err_disk
+    call print
+    jmp $
+
+print:
+    lodsb
+    or al, al
+    jz .done
+    mov ah, 0x0E
+    int 0x10
+    jmp print
+.done:
+    ret
+
+load_cluster:
+    ; Convert cluster to LBA
+    sub eax, 2
+    mul dword [sectors_per_cluster]
+    add eax, [first_data_sector]
+    
+    mov [dapack.lba_start], eax
+    mov ah, 0x42
+    mov si, dapack
+    int 0x13
+    ret
+
+align 4
+dapack:
+    db 0x10   ; Size of packet
+    db 0      ; Reserved
+    dw 1      ; Number of sectors
+    dw 0x8000 ; Offset
+    dw 0      ; Segment
+    dq 0      ; LBA
+
+cluster:      dd 0
+sectors_per_cluster: dd 8
+first_data_sector:  dd 0
+
+kernel_name:  db "KERNEL  BIN"
+err_disk:     db "Disk error", 0
+err_no_kernel: db "No kernel", 0
+
+bpb_oem_name:         times 8 db 0
+bpb_bytes_per_sector: dw 512
+bpb_sectors_per_cluster: db 8
+bpb_reserved_sectors: dw 32
+bpb_size: equ $ - bpb_oem_name
+
+times 510-($-$$) db 0
+dw 0xAA55