16 bit real org 10000h lgdt

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lazer1

Joined: 24 Jan 2006
Posts: 185

lazer1 16 Feb 2006, 15:01

I want to switch to protected mode from 16 bit real code,
my 16 bit real code is in the second segment of memory
starting at 10000h, Cool

in this region of memory if you have say:

Code:

xyz      dw     3

then I think the CPU will fail if you do

Code:

            mov [xyz],cx           ; FAIL because xyz is > 0ffffh,
            mov [ds:xyz],cx       ; FAIL because xyz is > 0ffffh

(no problem if you are in the first 64K of memory,
it took at least a day to locate this bug!)

instead you have to do:

Code:

           mov ebx,xyz
           mov [ds:bx],cx  ; correct because offset is 16 bit,
   
           and ebx,0ffffh
           mov [ds:ebx],cx  ; also correct because ebx <= 0ffffh

I dont know if there is a better way around the problem,

so for lgdt I dont think I can do

Code:

            lgdt [some_label]

except if some_label is in the first 64K of memory,

anyway an attempt at lgdt is as follows:

Code:

        org 10000h

                ....
        mov ebx,gdt_pointer
        and ebx,0ffffh
        lgdt    [ds:ebx]
                ....

struc xdtr size,linear 
        {
        .size   dw  size + 0 ; no. bytes in table,
        .linear dd  linear + 0 ; linear address of byte 0 of DT,
        }

gdt_pointer  xdtr some_size,mytable

               ....
mytable:   .....

I dont know if this is correct, here is the question:

the docs seem to be saying that there are 3 versions of lgdt,
one with a 32 bit base (.linear above), 1 with a 24 bit base
and 1 with a 64 bit base,

the version I want is the 32 bit base, with the base actually
being in the first 1MB of memory, is the above fragment
correct?

Intel's docs (vol 3, ch 9, example code) use:

Code:

 
db 66h
lgdt  ....

they use "lgdt some_label" which on FASM would be
equivalent to "lgdt [some_label]", I cannot use that
because [some_label] is actually [ds:some_label]
and as its not a 16 bit offset (because of org 10000h)
the CPU will probably freeze up,

later I may need this from say protected mode,
with the GDT anywhere in the 4G space,

the docs say you need a 32 bit opsize, how do I
force that?
Rolling Eyes

16 Feb 2006, 15:01

Octavio

Joined: 21 Jun 2003
Posts: 366
Location: Spain

Octavio 17 Feb 2006, 09:37

Code:

push ds
push 1000h  ;10000h=1000:0000h
pop ds
lgdt pword [0]                ;pword for 32bits table at 1000:0000h
pop ds                           ;restore ds

Last edited by Octavio on 18 Feb 2006, 15:15; edited 1 time in total

17 Feb 2006, 09:37

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 17 Feb 2006, 10:17

With fasm LGDT needs PWORD operand.
But I actually forgot that in 16-bit variant it used the 5-byte memory location, not 6-byte. Perhaps I should add another size prefix? FBYTE?

17 Feb 2006, 10:17

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 17 Feb 2006, 12:52

altough i don't like, it seems to be unavoidable. But maybe you should mention it in documentation only in part with "lgdt", not in general operators to prevent confusing reader... most people have probably never heard of instructions taking five-byte arguments.

17 Feb 2006, 12:52

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 17 Feb 2006, 13:32

Well, there is other solution. I could make LGDT in 16-bit mode to make the 5-byte variant when no operand size is specified (just like it works now), and 6-byte variant with PWORD size defined. In 32-bit mode it would always be the 6-byte variant. This solution is not as versatile, but still is much better than the current one and it avoids introducing another strange operator.

17 Feb 2006, 13:32

lazer1

Joined: 24 Jan 2006
Posts: 185

lazer1 17 Feb 2006, 22:34

Tomasz Grysztar wrote:

With fasm LGDT needs PWORD operand.
But I actually forgot that in 16-bit variant it used the 5-byte memory location, not 6-byte. Perhaps I should add another size prefix? FBYTE?

for my example problem are you saying the following?

Code:

      org 10000h 

                .... 
        mov ebx,gdt_pointer 
        lgdt  pword   [ds:bx]

17 Feb 2006, 22:34

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 19 Feb 2006, 11:41

Rather something like:

Code:

mov bx,gdt_pointer and 0FFFh
mov ax,(gdt_pointer and 0FFFFF000h) shr 4
mov ds,ax
lgdt pword [ds:bx]

19 Feb 2006, 11:41

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