339 lines
No EOL
4.9 KiB
NASM
339 lines
No EOL
4.9 KiB
NASM
; DISK_ID EQU 0x80
|
|
|
|
KERNEL_START EQU 0x100000
|
|
ELF_START EQU 0x8000
|
|
|
|
NUM_SECTORS EQU 65
|
|
|
|
org 0x7C00
|
|
bits 16
|
|
|
|
jmp .start
|
|
|
|
[bits 16]
|
|
|
|
; Function: check_a20
|
|
;
|
|
; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
|
|
; 1 in ax if the a20 line is enabled (memory does not wrap around)
|
|
|
|
check_a20:
|
|
cli
|
|
|
|
xor ax, ax ; ax = 0
|
|
mov es, ax
|
|
|
|
not ax ; ax = 0xFFFF
|
|
mov ds, ax
|
|
|
|
mov di, 0x0500
|
|
mov si, 0x0510
|
|
|
|
mov al, byte [es:di]
|
|
mov bx, ax
|
|
|
|
mov al, byte [ds:si]
|
|
|
|
mov byte [es:di], 0x00
|
|
mov byte [ds:si], 0xFF
|
|
|
|
cmp byte [es:di], 0xFF
|
|
|
|
mov byte [ds:si], al
|
|
|
|
mov ax, bx
|
|
mov byte [es:di], al
|
|
|
|
mov ax, 0
|
|
je .check_a20__exit
|
|
|
|
mov ax, 1
|
|
|
|
.check_a20__exit:
|
|
ret
|
|
|
|
;;; Print string at bx
|
|
puts:
|
|
.puts_loop:
|
|
|
|
mov al, [bx] ; load the character
|
|
cmp al, 0 ; check for null byte
|
|
je .end_puts_loop
|
|
|
|
mov ah, 0x0e
|
|
int 0x10 ; print charachter
|
|
inc bx
|
|
jmp .puts_loop
|
|
.end_puts_loop:
|
|
ret
|
|
|
|
|
|
|
|
;;; Print bl as hex
|
|
puthex:
|
|
|
|
; Print high bits
|
|
mov cl, bl
|
|
shr cl, 4
|
|
|
|
cmp cl, 9
|
|
jg .puthex_high_alpha
|
|
|
|
; It's < 10, print as digit
|
|
mov al, cl
|
|
add al, 0x30
|
|
mov ah, 0x0e
|
|
int 0x10
|
|
jmp .puthex_low
|
|
|
|
.puthex_high_alpha:
|
|
; It's >= 10, print as letter
|
|
mov al, cl
|
|
add al, 0x57 ; 0x57 = (0x61 - 10)
|
|
mov ah, 0x0e
|
|
int 0x10
|
|
|
|
.puthex_low:
|
|
; Print low bits
|
|
mov cl, bl
|
|
and cl, 0b1111
|
|
|
|
cmp cl, 9
|
|
jg .puthex_low_alpha
|
|
|
|
; It's < 10, print as digit
|
|
mov al, cl
|
|
add al, 0x30
|
|
mov ah, 0x0e
|
|
int 0x10
|
|
jmp .puthex_end
|
|
|
|
.puthex_low_alpha:
|
|
; It's >= 10, print as letter
|
|
mov al, cl
|
|
add al, 0x57 ; 0x57 = (0x61 - 10)
|
|
mov ah, 0x0e
|
|
int 0x10
|
|
|
|
.puthex_end:
|
|
ret
|
|
|
|
|
|
|
|
.start:
|
|
|
|
; set up stack
|
|
mov ax, 0
|
|
mov ss, ax
|
|
mov sp, 0x7C00
|
|
mov ds, ax
|
|
mov es, ax
|
|
|
|
|
|
; disable vga cursor
|
|
mov ah, 0x01
|
|
mov ch, 0b00100000
|
|
mov cl, 0b00000000
|
|
int 0x10
|
|
|
|
mov bl, dl
|
|
call puthex
|
|
|
|
; reset disk system
|
|
mov ah, 0x00
|
|
; mov dl, DISK_ID
|
|
int 0x13
|
|
|
|
; print result on error
|
|
jnc .reset_disk_no_error
|
|
mov bl, ah
|
|
call puthex
|
|
.reset_disk_no_error:
|
|
|
|
mov cx, 2 ; start at sector 2 (skip bootloader)
|
|
mov bx, ELF_START ; write to ELF_START
|
|
.read_disk_loop:
|
|
|
|
; Read sector dl into memory
|
|
mov ah, 0x02
|
|
mov al, 0x01 ; number of sectors
|
|
mov ch, 0x00 ; cylinder number, low 8 bits
|
|
; mov cl, 0x01 ; cylinder number, high 2 bits + sector number (6 bits)
|
|
mov dh, 0x00 ; head number
|
|
; mov dl, DISK_ID ; drive number
|
|
int 0x13
|
|
|
|
; exit on error
|
|
jc .read_disk_error
|
|
|
|
cmp cx, NUM_SECTORS
|
|
jge .read_disk_end ; we have reached the sector limit, time to boot
|
|
|
|
inc cl ; next sector
|
|
add bx, 512 ; add 512 to data start so we don't overwrite sectors
|
|
|
|
jmp .read_disk_loop
|
|
|
|
.read_disk_error:
|
|
|
|
mov bl, ah
|
|
call puthex
|
|
|
|
.read_disk_end:
|
|
|
|
call check_a20
|
|
cmp ax, 1
|
|
je .a20_enabled
|
|
|
|
mov ah, 0x24 ; set a20 gate
|
|
mov al, 0x01 ; turn it on
|
|
int 0x15
|
|
|
|
jnc .a20_enabled
|
|
|
|
mov bl, ah
|
|
call puthex
|
|
jmp .end
|
|
|
|
.a20_enabled:
|
|
|
|
; disable interrupts
|
|
cli
|
|
|
|
; reset ds
|
|
xor ax, ax
|
|
mov ds, ax
|
|
|
|
; load the GDT
|
|
lgdt [.gdt_desc]
|
|
|
|
; switch to protected mode
|
|
mov eax, cr0
|
|
or eax, 1
|
|
mov cr0, eax
|
|
|
|
; jump to 32 bit code
|
|
jmp 0x08:.clear_pipe
|
|
|
|
[BITS 32]
|
|
.clear_pipe:
|
|
|
|
; set Data Segment and Stack segment
|
|
mov ax, 10h
|
|
mov ds, ax
|
|
mov ss, ax
|
|
|
|
; set up stack
|
|
mov esp, 0x090000
|
|
|
|
; parse elf file at ELF_START to KERNEL_START
|
|
|
|
; verify ELF header
|
|
mov esi, ELF_START
|
|
cmp dword [esi], 464C457Fh ; ELF magic
|
|
jne .invalid_elf
|
|
cmp word [esi+4], 0101h ; lsb 32 bit, little endian
|
|
jne .invalid_elf
|
|
cmp word [esi+18], 03 ; x86 architecture
|
|
jne .invalid_elf
|
|
|
|
; read the entrypoint and store it
|
|
mov eax, dword [esi+0x18] ; program entry position
|
|
mov dword [.entrypoint], eax
|
|
|
|
mov cx, word [esi+0x2C] ; read phnum (number of program headers)
|
|
mov eax, dword [esi+0x1C] ; read phoff (offset of program header)
|
|
|
|
; ebx is now ELF_START, esi jumps to the start of the program header
|
|
mov ebx, esi
|
|
add esi, eax
|
|
|
|
; set up for loop
|
|
sub esi, 0x20
|
|
inc cx
|
|
|
|
.elf_ph_loop:
|
|
add esi, 0x20
|
|
dec cx
|
|
jz .invalid_elf ; there is no valid code block
|
|
cmp word [esi], 1 ; check if p_type is loadable
|
|
jne .elf_ph_loop
|
|
|
|
; add offset to ebx (ebx = pointer to code)
|
|
add ebx, dword [esi+0x04]
|
|
|
|
; store p_filesz (ecx = size of the segment)
|
|
mov ecx, dword [esi+0x10]
|
|
|
|
; calculate size of bss (edx = size of bss)
|
|
mov edx, dword [esi+0x14]
|
|
sub edx, ecx
|
|
|
|
; check if ecx is zero
|
|
or ecx, ecx
|
|
jz .invalid_elf
|
|
|
|
; set source
|
|
mov esi, ebx
|
|
|
|
; set destination
|
|
mov edi, KERNEL_START
|
|
|
|
; repeat ecx/4 times (because it moves 4 bytes at a time)
|
|
shr ecx, 2
|
|
|
|
; copy
|
|
repnz movsd
|
|
|
|
; jump to start of kernel
|
|
jmp [.entrypoint]
|
|
|
|
.end:
|
|
jmp .end
|
|
|
|
.invalid_elf:
|
|
mov byte [ds:0x0B8000], 'E'
|
|
mov byte [ds:0x0B8002], 'L'
|
|
mov byte [ds:0x0B8004], 'F'
|
|
jmp .end
|
|
|
|
; padding for bootloader - SFS superblock
|
|
times 404 - ($ - $$) db 0
|
|
|
|
; room for SFS superblock
|
|
times 42 db 0
|
|
|
|
; padding for SFS superblock - data
|
|
times 66 - (.file_end - .data) db 0
|
|
|
|
.data:
|
|
|
|
.gdt:
|
|
dd 0
|
|
dd 0
|
|
|
|
dw 0FFFFh
|
|
dw 0
|
|
db 0
|
|
db 10011010b
|
|
db 11001111b
|
|
db 0
|
|
|
|
dw 0FFFFh
|
|
dw 0
|
|
db 0
|
|
db 10010010b
|
|
db 11001111b
|
|
db 0
|
|
.gdt_end:
|
|
|
|
.gdt_desc:
|
|
dw .gdt_end - .gdt - 1
|
|
dd .gdt
|
|
|
|
.entrypoint: dd 0
|
|
|
|
; magic string
|
|
dw 0xAA55
|
|
|
|
.file_end: |