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Index > OS Construction > [ask] How to access memory more than 1 MB in pmode

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egos



Joined: 10 Feb 2009
Posts: 144
egos
Code:
  mov eax,FLAT_DATA_SEGMENT_SELECTOR
  mov ds,ax
  mov al,[1fffffffh]
    

Very Happy
Post 27 May 2011, 18:31
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christiandy



Joined: 03 Mar 2011
Posts: 25
Location: 101
christiandy
neville wrote:
christiandy wrote:
Can you give me an example like how to access memory 1fffffff ?
Code:
push 0
pop ds
mov al,[1fffffffh]    

Its that easy if you set up unreal mode or flat real mode first. You don't need pmode.

So the ds value refer to base address on GDT right?
if we can jus mov al,[1fffffffh] it's like real mode right?
Post 28 May 2011, 05:54
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
Quote:
So the ds value refer to base address on GDT right?

Yep. DS value is index to descriptor in GDT. So for example if GDT has three descriptors (each is 8 bytes), then index to first is 0, index to second is 8, and index to third is 16. Now if first descriptor in GDT has base 0, then preceding code would access linear address 1fffffffh. If descriptor had base address 10000000h, then ds:[0fffffffh] would access same linear address. etc. However these days, base address 0 is always used except for some special cases.

Do you have intel manuals? Make sure to read them. They are bit harder to grasp at first, but one most definitively has to learn working with them, if you plan to go into protected mode.
Post 28 May 2011, 08:13
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Dex4u



Joined: 08 Feb 2005
Posts: 1601
Location: web
Dex4u
vid wrote:
Quote:
So the ds value refer to base address on GDT right?

Yep. DS value is index to descriptor in GDT. So for example if GDT has three descriptors (each is 8 bytes), then index to first is 0, index to second is 8, and index to third is 16. Now if first descriptor in GDT has base 0, then preceding code would access linear address 1fffffffh. If descriptor had base address 10000000h, then ds:[0fffffffh] would access same linear address. etc. However these days, base address 0 is always used except for some special cases.

Do you have intel manuals? Make sure to read them. They are bit harder to grasp at first, but one most definitively has to learn working with them, if you plan to go into protected mode.


Remeber the base is usually zero, but the index is not, as you leave the first one zero.
So your first index is 8h
Code:
gdt:        dw    0x0000, 0x0000, 0x0000, 0x0000
 ; left zero, not used
sys_data:   dw    0xFFFF, 0x0000, 0x9200, 0x00CF
 ; 8h
sys_code:   dw    0xFFFF, 0x0000, 0x9800, 0x00CF
 ;10h decimal (16)  
gdt_end:
    
Post 28 May 2011, 16:22
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egos



Joined: 10 Feb 2009
Posts: 144
egos
Or typically 0x9A00 (readable/executable).

_________________
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Post 28 May 2011, 22:37
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christiandy



Joined: 03 Mar 2011
Posts: 25
Location: 101
christiandy
Thanks all, for replying very helping.
now I run on protected mode but i can't try it on DOS BOX. anw can someone explain me why multi tasking only run on protected mode?
Post 29 May 2011, 09:54
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bitshifter



Joined: 04 Dec 2007
Posts: 764
Location: Massachusetts, USA
bitshifter
Multitasking can be done in 2 ways.

1) With hardware, using built-in functionality.
2) With software, using time-slice scheduling.

In the near future we will have true mutitasking,
where each cpu core could have its own task.
Post 29 May 2011, 22:18
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Dex4u



Joined: 08 Feb 2005
Posts: 1601
Location: web
Dex4u
bitshifter wrote:

In the near future we will have true mutitasking,
where each cpu core could have its own task.

I have been tell people this for years, when they go on about multi-tasking.
I tell them your just sharing a bit of the cpu, that was done when processor's were very expensive.
Wait until every task is given a core, then we will be multi-tasking.

And thats the day my OS will be multi-tasking.
Post 29 May 2011, 22:57
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christiandy



Joined: 03 Mar 2011
Posts: 25
Location: 101
christiandy
sorry I mean time slicing multitasking ( round robin algorithm cmiiw) why does protected mode can provide software multitasking
Post 30 May 2011, 08:56
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bitshifter



Joined: 04 Dec 2007
Posts: 764
Location: Massachusetts, USA
bitshifter
Basically software and hardware multitasking work the same way.
Starting with 16bit x80286 intel provides instructions to help do it.

So since were talking about 32bit pmode MT lets try some tutorials...
These were the ones i learned from, they are written in NASM but
with little change can be ported to FASM.
In this set there are examples of hardware and software MT under 32bit pmode.
http://geezer.osdevbrasil.net/os/pm.zip

If you get BOOTPROG you can use it as bootloader for this series.
It works very nicely and saves a bunch of time having to write one.
This is intended for FAT12 floppy or FAT16 hard disks only.
http://alexfru.chat.ru/programming/bootprog.zip

I can help make the two work under FASM if you cant do it yourself.
Feel free to drop me a line...
Post 30 May 2011, 17:40
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christiandy



Joined: 03 Mar 2011
Posts: 25
Location: 101
christiandy
Dear bitshifter,
thanks for helping.
Can you please give me an example of multitasking like.
running cli and update clock every 60 detik?
Post 31 May 2011, 04:50
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