Unable to jmp kernel in PMODE
Posted: Sat Aug 27, 2011 4:47 am
Hello,
Yet another newbie
, I have written code for bootloader stage1 and stage2. stage1 loads the stage2 successfully. And kernel binary is also ready.
the code for stage2 :
I executed the code on my ACER ASPIRE5738 laptop. the stage1 works fine, it loads the second stage. But in second stage
if I assemble code with
jmp $ ( see jmp to kernel part)
instruction (just to check the code for activating pmode works or no without jmp to kernel) the stage2 code also works fine.
It prints all the message to screen and goes for jmp $ instruction. But if I change jmp $ to jmp 0x08:0x01000
the loading kernel part fails and it shows "Unable to load kernel" message on the screen.
I am not able to understand how come changing jmp instruction causes not to load kernel??. weird isn't it??
please help me!!!!
Yet another newbie
, I have written code for bootloader stage1 and stage2. stage1 loads the stage2 successfully. And kernel binary is also ready. the code for stage2 :
Code: Select all
[BITS 16]
[ORG 0x500]
main:
popf
popa
mov si, a20msg
call PutStr
;; enable the A20 gate
jmp Enable_A20Gate
LoadKernel:
mov si, done
call PutStr
;; load kernel
mov si, ldmsg
call PutStr
xor ah, ah
mov si, DAP
mov dl, 0x80
mov ah, 0x42 ;extended read
int 13H ;bios interrupt 13H
jc error
mov si, done
call PutStr
jmp $
mov si, pmmsg
call PutStr
cli
push ds
mov ax,0x0800
mov ss,ax
mov sp,0x1000
;; load gdt table
lgdt [gdtinfo]
;; enable protected mode
mov eax, cr0 ; switch to pmode by
or al,1 ; set pmode bit
mov cr0, eax
mov bx, 0x08 ; select descriptor 1
mov ds, bx ; 8h = 1000b
pop ds
;; jmp $
;; jmp to kernel in memory
jmp 0x08:0x01000
PutStr: ; Procedure label/start
;; Set up the registers for the interrupt call
mov ah,0x0E ; The function to display a chacter (teletype)
mov bh,0x00 ; Page number
mov bl,0x07 ; Normal text attribute
.nextchar ; Internal label (needed to loop round for the next character)
lodsb ; I think of this as LOaD String Block
or al,al ; Sets the zero flag if al = 0
jz .return ; If the zero flag has been set go to the end of the procedure.
;; Zero flag gets set when an instruction returns 0 as the answer.
int 0x10 ; Run the BIOS video interrupt
jmp .nextchar ; Loop back round tothe top
.return ; Label at the end to jump to when complete
ret ; Return to main program
new_line:
mov ah, 0EH
mov al,13
int 10H
mov ah, 0EH
mov al,10
int 10H
mov dx, 0000H
ret
clearscreen:
;; clear the screen
mov ah, 06
mov al, 00
mov bh, 07
mov ch, 00
mov cl, 00
mov dh, 24
mov dl, 79
int 10H
mov ah, 0x02
mov bh, 0x07
mov dh, 0x00
mov dl, 0x00
int 10H
ret
Empty_KeyboardBuffer:
xor al, al
in al, 0x64
test al, 0x02
jnz Empty_KeyboardBuffer
ret
Enable_A20Gate:
cli
call Empty_KeyboardBuffer
mov al, 0xD1
out 0x64, al
call Empty_KeyboardBuffer
mov al, 0xDF
out 0x60, al
call Empty_KeyboardBuffer
sti
jmp LoadKernel
error:
call new_line
mov si, errmsg
call PutStr
jmp $
DAP:
size:
db 0x10 ;Size of the disk address packet
res:
db 0 ;reserved byte
num_of_sec:
dw 24 ;number of sectors to read
offset:
dw 0x1000 ;read into the address specified by segment:offset
segment:
dw 0
LBA:
dq 2
;; readchar:
;; mov ah, 01
;; int 21H
number db "PUCSD>",0
errmsg db "Unable to load kernel",10,0
a20msg db "Activating A20Gate",10,0
ldmsg db "Loading Kernel",10,0
pmmsg db "Activating Protected Mode",10,0
startmsg db "Starting...",10,0
done db "Done :)",10,0
gdtinfo:
dw gdt_end - gdt - 1 ;last byte in table
dd gdt ;start of table
gdt dd 0,0 ; entry 0 is always unused
flatdesc db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0
gdt_end:
I executed the code on my ACER ASPIRE5738 laptop. the stage1 works fine, it loads the second stage. But in second stage
if I assemble code with
jmp $ ( see jmp to kernel part)
instruction (just to check the code for activating pmode works or no without jmp to kernel) the stage2 code also works fine.
It prints all the message to screen and goes for jmp $ instruction. But if I change jmp $ to jmp 0x08:0x01000
the loading kernel part fails and it shows "Unable to load kernel" message on the screen.
I am not able to understand how come changing jmp instruction causes not to load kernel??. weird isn't it??
please help me!!!!