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flat assembler > Windows > question about files

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Ali.A



Joined: 08 Jan 2018
Posts: 139
i want to read specific bytes from specific offsets in a file.

i know for sure i can use "GetFileSize" to get how large is the file and then "ReadFile" and pass the file size to load the full file in my program.
and read as many bytes as i want and from offsets i want.

but im not looking for this way, i know files in my SSD have pointers.
when i want to open a .txt file the OS will search for the pointer to this file and load from SSD.

the question is how to get the file pointer, im thinking if its possible to get the file pointer and do like this:

Code:
mov eax,[FilePointer]   ; file pointer
mov eax,[eax+5]   ; move 4 bytes after offset 5
mov [SomeData],eax   ; store the data    

i know im missing some stuff, and there are some gaps that must be filled with knowledge!

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Post 15 Jul 2018, 00:50
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DimonSoft



Joined: 03 Mar 2010
Posts: 454
Location: Belarus
You seem to be missing knowledge about file systems. SSDs are external devices from CPU’s perspective and they don’t share addressing space with RAM (unless you specifically setup some kind of DMA-like mechanism (if any SSD in the whole world would ever supports it) or pretend you have such a mechanism by emulating it with memory-mapped files). Anyway, you have a 32-bit pointer (in your example) which limits you to 4 GB. I doubt your SSD is so small.
Post 15 Jul 2018, 07:58
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7008
Location: Kraków, Poland
DimonSoft wrote:
unless you specifically setup some kind of DMA-like mechanism (if any SSD in the whole world would ever supports it)
Nowadays this is a DAX mechanism (also supported by Windows), and while it looks like a memory mapped file, there is no intermediary, the addresses are mapped directly to the device I/O. Still, this won't work with just any SSD, the drive must be specifically set up for DAX.
Post 15 Jul 2018, 08:53
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Ali.A



Joined: 08 Jan 2018
Posts: 139
okay, then how would the OS load a file when there is no pointer points to the current location of clusters / sector that have the file and file data.
Post 15 Jul 2018, 09:14
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16149
Location: Hyperborea
External devices read and write data to and from RAM through the I/O mechanisms. A normal user mode process doesn't have any privilege to directly do I/O operations.
Post 15 Jul 2018, 09:27
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Ali.A



Joined: 08 Jan 2018
Posts: 139
okay thanks guys, now i really hate user mode ):
Post 15 Jul 2018, 23:58
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16149
Location: Hyperborea
Elevate yourself to SYSTEM privilege and then you can do anything you want.
Post 16 Jul 2018, 00:25
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Ali.A



Joined: 08 Jan 2018
Posts: 139
true, but im missing knowledge rev.
there are lot of things i dont know about, i even dont know boot process .. what load firsts and next.

compiling a device driver using fasm not a hard task just PE native and you are done, but what about functions that a .sys must have. (i know most device drivers communicate with ntdll, hal.dll, ntoskrnl.exe)
but does these modules have everything i need? (and what are the abilities i can perform) and how would my .sys communicate with my .exe or .dll? because .dll and .exe runs in usermode using usermode thread, and .sys runs in kernelmode so calling a function in my .sys from my .dll or .exe will result access violation.

excuse me tho, no one taught anything about computers, hardware, software. (no school no institute nor university)

everything i know is from reading documents on internet. (it took over 15 years to understand)

sorry im not talking it personal, im just trying to say every single information i know is from internet .. and not everything i need is available in this network.
Post 16 Jul 2018, 05:01
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