cannot address more than 1 mb

osmosys · **Joined:** Sun Oct 05, 2008 4:33 am **Posts:** 14

first of all to begin this is my code that enables A20.

Code:

/* to enable the A20 address line inorder to address more than 1MB of memory */
        call    empty_8042
        movb    $0xd1,%al
        out     %al,$0x64
        call    empty_8042
        movb    $0xdf,%al
        out     %al,$0x60
        call    empty_8042
   jmp   A20ENABLED


/* this function repeatedly checks the keyboard controller 8042 */
empty_8042:
   .word   0x00eb,0x00eb
        in      $0x64,%al
        test    $0x2,%al
        jnz     empty_8042
        ret

the qemu shows that A20 has been enabled.
now this is my GDT

Code:

gdtstructure:
   .2byte   0x27      /* limit of the gdt */
   .4byte   gdtstart   /* base address of gdt */

gdtstart:

   /* null descriptor to ensure that no segment registers get loaded with value 0 */
   /* causes "GENERAL PROTECTION FAULT" if we try to access this gdt entry */
   .4byte   0
   .4byte   0

   /* code segment for our bootloader */
   .2byte   0xFFFF, 0
   .byte   0, 0x9A, 0x4F, 0

   /* data segment for our bootloader */
   .2byte   0xFFFF, 0
   .byte   0, 0x92, 0x4F, 0   

   /* code segment for our kernel */
   .2byte   0xFFFF, 0000
   .byte   10, 0x9A, 0x4F, 0

   /* data segment for our kernel */
   .2byte   0xFFFF, 0
   .byte   10, 0x92, 0x4F, 0

but i cannot access the memory in the fifth entry of GDT.this what i was trying to do.i didn't enable paging.

Code:

   mov $0x20,%eax
     mov %ds,%ebx
    mov %eax,%ds
    movb $'A',0x8000
    movb 0x8000,%al
    movb %al,0xb8000
    movb $0x1e,0xb8001
mov %ebx,%ds

when i tried to print the character that i had written, i am not getting the result.

is the problem because A20 was not enabled properly.
should i need to do anything more

egos · **Joined:** Fri Nov 16, 2007 1:59 pm **Posts:** 612

The last two descriptors contain segment base 0xA0000. Therefore to write the character to the address 0xB8000 you must use the displacement 0x18000 (0xB8000=0xA0000+0x18000).

osmosys · **Joined:** Sun Oct 05, 2008 4:33 am **Posts:** 14

thanks friend...actually i meant base address as 0x100000...i completely forgot that "0x" that made it read as hexadecimal..sorry everyone..really a stupid one..

codemastersnake · **Joined:** Sun Nov 07, 2004 12:00 am **Posts:** 148

osmosys wrote:

thanks friend...actually i meant base address as 0x100000...i completely forgot that "0x" that made it read as hexadecimal..sorry everyone..really a stupid one..

no question is stupid untill it has an answer

Combuster · **Posted:** Mon Oct 27, 2008 4:26 am

Quote:

mov $0x20,%eax
mov %ds,%ebx

Where is this being executed? real mode, unreal mode, protected mode, long mode? what is the code size at that point?

PatrickV · **Posted:** Mon Oct 27, 2008 8:10 pm

It could be that you not have setout your protected mode right. You know 32 bit. That what could be the problem

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cannot address more than 1 mb

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