diff options
| -rw-r--r-- | src/entry.asm | 125 |
1 files changed, 91 insertions, 34 deletions
diff --git a/src/entry.asm b/src/entry.asm index afe004c..2d40ad4 100644 --- a/src/entry.asm +++ b/src/entry.asm @@ -1,26 +1,68 @@ +; Melvin's awesome ext2 bootloader +; MIT License, Copyright (c) 2020 Marvin Borner + +; Definitions + +%define LOCATION 0x7c00 ; Bootloader location + +%define NEWLINE 0x0A ; Newline character (\n) +%define RETURN 0x0D ; Return character (\r) +%define NULL 0x00 ; NULL character (\0) + +%define VIDEO_INT 0x10 ; Video BIOS Interrupt +%define VIDEO_CLEAR 0x03 ; Clear screen command +%define VIDEO_OUT 0x0e ; Teletype output command + +%define DISK_INT 0x13 ; Disk BIOS Interrupt +%define DISK_EXT_CHECK 0x41 ; Disk extension check command +%define DISK_EXT_CHECK_SIG1 0x55aa ; First extension check signature +%define DISK_EXT_CHECK_SIG2 0xaa55 ; Second extension check signature +%define DISK_ZERO 0x80 ; First disk - TODO: Disk detection +%define DISK_READ 0x42 ; Disk extended read command + +%define EXT2_SB_SIZE 0x400 ; Superblock size +%define EXT2_SIG_OFFSET 0x38 ; Signature offset in superblock +%define EXT2_TABLE_OFFSET 0x08 ; Inode table offset after superblock +%define EXT2_NEW_TABLE 0x1000 ; New inode table location in memory +%define EXT2_INODE_SIZE 0x80 ; Single inode size +%define EXT2_ROOT_INODE 0x02 ; Root directory inode +%define EXT2_ROOT_DIR EXT2_NEW_TABLE + (EXT2_ROOT_INODE - 1) * EXT2_INODE_SIZE +%define EXT2_TYPE_OFFSET 0x00 ; Offset of filetype and rights +%define EXT2_COUNT_OFFSET 0x1c ; Offset of number of data blocks +%define EXT2_POINTER_OFFSET 0x28 ; Offset of first data pointer +%define EXT2_SIG 0xef53 ; Signature +%define EXT2_DIR 0x4000 ; Directory indicator +%define EXT2_REG 0x8000 ; Regular file indicator + +%define A20_GATE 0x92 ; Fast A20 gate +%define A20_ENABLED 0b10 ; Bit 1 defines whether A20 is enabled +%define A20_EXCLUDE_BIT 0xfe ; Bit 0 may be write-only, causing a crash + +; ENOUGH, let's go! + bits 16 -org 0x7c00 +org LOCATION global _start _start: ; Clear screen - mov ax, 0x003 - int 0x10 + mov ax, VIDEO_CLEAR + int VIDEO_INT ; Welcome user! mov si, hello_msg call print ; Check LBA support - mov ah, 0x41 - mov bx, 0x55AA - int 0x13 + mov ah, DISK_EXT_CHECK + mov bx, DISK_EXT_CHECK_SIG1 + int DISK_INT jc lba_error - cmp bx, 0xAA55 + cmp bx, DISK_EXT_CHECK_SIG2 jnz lba_error ; Check disk and move dl - and dl, 0x80 ; Use disk 0 + and dl, DISK_ZERO ; Use disk 0 jz disk_error mov [drive], dl @@ -33,22 +75,22 @@ _start: jmp stage_two print: - mov ah, 0x0E + mov ah, VIDEO_OUT xor bh, bh print_ch: lodsb test al, al jz print_end - int 0x10 + int VIDEO_INT jmp print_ch print_end: ret disk_read: mov si, packet ; Address of dap - mov ah, 0x42 ; Extended read + mov ah, DISK_READ ; Extended read mov dl, [drive] ; Drive number - int 0x13 + int DISK_INT jc disk_error ret @@ -63,12 +105,12 @@ lba_error: jmp $ ; Variables -hello_msg db "Welcome! Loading Melvix...", 0x0A, 0x0D, 0x00 -disk_error_msg db "Disk error!", 0x0a, 0x0d, 0x00 -lba_error_msg db "LBA error!", 0x0a, 0x0d, 0x00 -stage_two_msg db "Stage2 loaded", 0x0a, 0x0d, 0x00 -disk_success_msg db "Disk is valid", 0x0a, 0x0d, 0x00 -inode_table_msg db "Found inode table", 0x0a, 0x0d, 0x00 +hello_msg db "Welcome! Loading Melvix...", NEWLINE, RETURN, NULL +disk_error_msg db "Disk error!", NEWLINE, RETURN, NULL +lba_error_msg db "LBA error!", NEWLINE, RETURN, NULL +stage_two_msg db "Stage2 loaded", NEWLINE, RETURN, NULL +disk_success_msg db "Disk is valid", NEWLINE, RETURN, NULL +inode_table_msg db "Found inode table", NEWLINE, RETURN, NULL drive db 0 ; Data @@ -93,20 +135,20 @@ stage_two: call print ; yay! ; Verify signature - mov ax, [superblock +56] - cmp ax, 0xEF53 + mov ax, [superblock + EXT2_SIG_OFFSET] + cmp ax, EXT2_SIG jne disk_error mov si, disk_success_msg call print - ; load inode table - mov ax, [superblock + 1024 + 8] ; Inode table + ; Load inode table + mov ax, [superblock + EXT2_SB_SIZE + EXT2_TABLE_OFFSET] ; Inode table mov cx, 2 mul cx ; ax = cx * ax mov [lba], ax ; Sector mov ax, 2 mov [count], ax ; Read 1024 bytes - mov bx, 0x1000 ; Copy data to 0x1000 + mov bx, EXT2_NEW_TABLE ; Copy data to 0x1000 mov [dest], bx call disk_read mov si, inode_table_msg @@ -115,13 +157,23 @@ stage_two: ; Load root dir xor bx, bx mov ax, bx - mov bx, 0x1080 ; First block ((2 - 1) * 128 + dest) - mov ax, word [bx + 0] ; Filetype - and ax, 0x4000 ; Check if dir - cmp ax, 0x4000 + mov bx, EXT2_ROOT_DIR ; First block + mov ax, word [bx + EXT2_TYPE_OFFSET] ; Filetype + and ax, EXT2_DIR ; Check if directory + cmp ax, EXT2_DIR jne disk_error - mov cx, [bx + 28] ; Number of sectors for inode - lea di, [bx + 40] ; Address of first block pointer + mov cx, [bx + EXT2_COUNT_OFFSET] ; Number of sectors for inode + lea di, [bx + EXT2_POINTER_OFFSET] ; Address of first block pointer + jmp $ + + ; Find kernel + xor ax, ax + mov dx, ax + mov ax, [di] + mov dx, 2 + mul dx + mov [lba], ax + mov [dest], bx mov bx, 0x5000 mov [dest + 2], bx @@ -143,9 +195,9 @@ stage_three_load: call disk_read - add bx, 0x400 ; 1kb increase + add bx, 1024 ; 1kb increase add di, 0x4 ; Move to next block pointer - sub cx, 0x2 ; Read 2 blocks + sub cx, 2 ; Read 2 blocks jnz stage_three_load ret @@ -156,9 +208,14 @@ protected_mode_enter: cli ; Turn off interrupts ; TODO: Check A20 support? - in al, 0x92 ; Enable A20 - or al, 2 - out 0x92, al + ; TODO: 0x92 method may not work on every device + in al, A20_GATE + test al, A20_ENABLED + jnz .a20_enabled + or al, A20_ENABLED + and al, A20_EXCLUDE_BIT + out A20_GATE, al + .a20_enabled: ; Clear registers xor ax, ax |