From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
From: andrey mirtchovski <andrey@lanl.gov>
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
Message-Id: <20020123234412.1ABFF19A02@mail.cse.psu.edu>
Subject: [9fans] calculating the gdt pointer in 32 bit (protected) mode
Date: Wed, 23 Jan 2002 16:43:46 -0700
Topicbox-Message-UUID: 42b6ec42-eaca-11e9-9e20-41e7f4b1d025

hello,

i'm trying to understand how the gdt pointer is calculated in
/sys/src/boot/pc/l.s ..  the exercise includes calculating this
pointer when already in 32 bit mode and i'm kind of lost at this
point...

here's the relevant code snippet:

/*
 * pointer to initial gdt
 */ TEXT tgdtptr(SB),$0

        WORD $(3*8)
        LONG $tgdt-KZERO(SB) /*
 * goto protected mode
 */ /* MOVL tgdtptr(SB),GDTR /**/
         BYTE $0x0f
         BYTE $0x01
         BYTE $0x16
         WORD $tgdtptr(SB)
        MOVL CR0,AX
        ORL $1,AX
        MOVL AX,CR0



using MOVL tgdtptr(SB),GDTR to calculate the gdt pointer when already
in protected mode crashes the machine, although not immediately...


thanx, andrey