EDIT: My problem solved when I move everything after BITS 32 to disk_double_error but now it is still broken yesy it displays the message but then when i hit Y it just restarts/reboots the system What may be the reason for that ?
(sry for bad english) Hello I am trying to write a simple os just for fun I made my bootloader and I displayed some messages and read the disk but when it comes to switching to 32 bit protected mode all of the things I did doesnt work It normally displayed
`> `
`>succesfully loaded = if loaded succesfully > failed = if not `
`>Do you want to boot kernel mode Y/N then gets input`
`>Booting in kernel mode = if Y null = if no`
But after i added lgdt and other switches to 32 bit it just doesnt display anything at all so I thought that switching is somehow clearing the screen so i tried to print things in pm with print_string_pm function but still nothing I am using
nasm -fbin bootload.asm -o bootload.bin
to compile the bootloader
What may be wrong with my code please help ?
here is the code:
BITS 16
[ORG 0x7c00]
global _boot_start
_boot_start:
cli
mov bx,0x9000
mov bp,bx
mov sp,bx
sti
pusha
mov ax, 0x0000
mov ds, ax
mov es, ax
popa
mov [bootdev], dl
mov bx, MSG_START
call print_string
mov dl, [bootdev]
mov dh, 1
mov bx, 0x1000
call disk_load
mov bx, MSG_YESNO
call print_string
call get_yes_no
mov bx, MSG_BOOT_KERNEL
call print_string
lgdt [gdt_descriptor]
mov eax , cr0
or eax , 0x1
mov cr0 , eax
jmp CODE_SEG:pm
BITS 32
pm:
mov ax , DATA_SEG
mov ds , ax
mov ss , ax
mov es , ax
mov fs , ax
mov gs , ax
mov ebp , 0x90000
mov esp , ebp
call beg_pm
beg_pm:
mov ebx,MSG_BOOT_32
call print_string_pm
call KERNEL_OFFSET
jmp $
;******************************************************************
print_string:
.loop:
mov al,[bx]
cmp al,0
je return
push bx
mov ah,0Eh
int 10h
pop bx
inc bx
jmp .loop
return:
ret
;******************************************************************
get_yes_no:
pusha
.loop:
mov ah, 00h
int 16h
cmp ah, 15h
je .yes
cmp ah, 31h
je .no
jmp .loop
ret
.no:
mov bx, No
call print_string
mov ah, 00h
int 13h
jmp $
.yes:
mov bx, Yes
call print_string
jmp .done
.done:
popa
ret
;******************************************************************
disk_load:
push dx
mov ah, 02h
mov al, dh
mov ch, 0x00
mov dh, 0x00
mov cl, 0x02
int 0x13
pop dx
jc disk_error
cmp dh, al
jne disk_error_double
mov bx,MSG_LOAD_SUCC
call print_string
ret
disk_error:
mov bx,MSG_LOAD_FAIL
call print_string
jmp $
disk_error_double:
mov bx,MSG_LOAD_FAIL_C
call print_string
jmp $
;******************************************************************
print_string_pm :
pusha
mov edx , VIDEO_MEMORY ; Set edx to the start of vid mem.
.print_string_pm_loop :
mov al , [ ebx ] ; Store the char at EBX in AL
mov ah , WHITE_ON_BLACK ; Store the attributes in AH
cmp al , 0 ; if (al == 0), at end of string , so
je .print_string_pm_done ; jump to done
mov [edx], ax ; Store char and attributes at current
; character cell.
add ebx , 1 ; Increment EBX to the next char in string.
add edx , 2 ; Move to next character cell in vid mem.
jmp .print_string_pm_loop ; loop around to print the next char.
.print_string_pm_done :
popa
ret
; Global Descriptor table
; GDT
gdt_start :
gdt_null : ; the mandatory null descriptor
dd 0x0 ; 'dd ' means define double word ( i.e. 4 bytes )
dd 0x0
gdt_code :
dw 0xffff ; Limit ( bits 0 -15)
dw 0x0 ; Base ( bits 0 -15)
db 0x0 ; Base ( bits 16 -23)
db 10011010b ; 1st flags , type flags
db 11001111b ; 2nd flags , Limit ( bits 16 -19)
db 0x0 ; Base ( bits 24 -31)
gdt_data :
dw 0xffff ; Limit ( bits 0 -15)
dw 0x0 ; Base ( bits 0 -15)
db 0x0 ; Base ( bits 16 -23)
db 10010010b ; 1st flags , type flags
db 11001111b ; 2nd flags , Limit ( bits 16 -19)
db 0x0 ; Base ( bits 24 -31)
gdt_end :
gdt_descriptor :
dw gdt_end - gdt_start - 1
dd gdt_start
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start
MSG_START DB ">",13,10,0
MSG_LOAD_SUCC DB "> Succesfully loaded",13,10,0
MSG_LOAD_FAIL DB "> Failed to load Please try to restart the system",13,10,0
MSG_LOAD_FAIL_C DB "> Error while loading",13,10,0
MSG_YESNO DB "> Do you want to boot up in kernel mode Y/N :",0
MSG_BOOT_KERNEL DB 13,10,"> Booting in kernel mode",0
MSG_BOOT_32 DB "32 bit pm",13,10,0
Yes db "Y",0
No db "N",0
bootdev: db 0
KERNEL_OFFSET equ 0x1000
VIDEO_MEMORY equ 0xb8000
WHITE_ON_BLACK equ 0x0f
times 510-($-$$) db 0
dw 0xaa55
And here is my kernel code:
int kmain()
{
char* vm = 0xb8000;
*vm = 'X';
}
and my kernel entry
[BITS 32]
[extern _kmain]
call _kmain
jmp $
I use follwing lines to build my disk image:
nasm -fbin bootload.asm -o bootload.bin
gcc -ffreestanding -c kernel.c -o kernel.o
nasm kernel_entry.asm -f elf -o k_entry.o
ld -T NUL -o kernel.tmp -Ttext 0x1000 k_entry.o kernel.o
objcopy -O binary -j .text kernel.tmp kernel.bin
copy /b bootload.bin+kernel.bin os-image.bin
