Flat Real Mode GDT question

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adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 03 Jul 2005, 18:57

Hi all,

I have a quetion concerning FRM. What is the purpouse of the GDT, and what should it look like for FRM? I found this example from another thread, but I'm having trouble understanding what it means, and it fails to compile under FASM (1.62):

Code:

StartOf_GDT:
FlatRM_GDTR:
dw (EndOf_GDT - StartOf_GDT - 1)
dd (StartOf_GDT)
dw 0
;32-bit data selector descriptor
db 0xFF
db 0xFF
db 0x00
db 0x00
db 0x00
db 10010010b
db ((1100b << 4)|(0x0F))
db 0x00
EndOf_GDT:

Thanks,
Anthony

03 Jul 2005, 18:57

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 03 Jul 2005, 21:00

look for some "protected mode description" or "protected mode introduction" or something like that.

03 Jul 2005, 21:00

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 04 Jul 2005, 09:49

The source you've quoted doesn't compile with fasm, because it uses NASM's operators << and |, for fasm you should use "shl" and "or" operators.

As for the GDT, it is needed to set up the segment descriptors with 4 GB limit (in this thread you've got more info on FRM: http://board.flatassembler.net/topic.php?t=135 but I guess you've already found it).

Last edited by Tomasz Grysztar on 10 Oct 2005, 21:04; edited 1 time in total

04 Jul 2005, 09:49

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 04 Jul 2005, 13:09

Ok, 2 more questions:

1. How can I test that FRM is enabled correctly?
2. Do I need to enable the A20 gate to access the full 4GB flat address space?

04 Jul 2005, 13:09

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 04 Jul 2005, 13:14

1. If you try to access offset above 0FFFFh when no FRM is enabled, you will get exception (interrupt) 13. As you can read in the thread I linked to, I used this to enable FRM only when it's not enabled, since I put the FRM initialization routine inside the interrupt 13 handler.

2. Yes, otherwise the addresses above 1 MB will "wrap" and you will access the memory in first megabyte anyway.

04 Jul 2005, 13:14

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 04 Jul 2005, 13:38

One more question that I just thought of:

Do I still address memory via the segment:offset pair (ex, can I still do a "JMP 0x2000:0x0000", or do I not use segments, since it is "flat" memory space?

04 Jul 2005, 13:38

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 04 Jul 2005, 13:46

You still use segments, since it's still the real mode. What makes it FRM is that you can use 32-bit offset within each segment.

04 Jul 2005, 13:46

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 04 Jul 2005, 13:56

Thanks for all of your help! I think I'm OK for now...

04 Jul 2005, 13:56

Chewy509

Joined: 19 Jun 2003
Posts: 297
Location: Bris-vegas, Australia

Chewy509 04 Jul 2005, 22:40

adefeo wrote:

Thanks for all of your help! I think I'm OK for now...

Hi Adefeo,

Take a look at my last years 512byte OS enrty, it ran in FRM, so you could use it as a working model. It should answer all your questions.

Darran.

PS. http://chewy509.atspace.com/256.html

04 Jul 2005, 22:40

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