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 Post subject: [Solved] FAT12: bug in the wiki or in my head ?
PostPosted: Mon Oct 02, 2017 12:19 pm 
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Hi.
trying to understand how to read the clusters from the FAT table.
According to the wiki:
Code:

unsigned short table_value = *(unsigned short*)&FAT_table[ent_offset];

if(active_cluster & 0x0001)
   table_value = table_value >> 4;
else
   table_value = table_value & 0x0FFF;


is enough for getting the value, but it contradicts the description I found here:
http://elm-chan.org/docs/fat_e.html#fat_access


What's the verdict ?

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Last edited by stdcall on Tue Oct 03, 2017 3:12 pm, edited 1 time in total.

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 Post subject: Re: FAT12: bug in the wiki or in my head ?
PostPosted: Mon Oct 02, 2017 12:30 pm 
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I don't see any contradiction. Both appear to be different ways of writing the same thing (ignoring the fact that the wiki code is undefined behavior due to unaligned pointer access).


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 Post subject: Re: FAT12: bug in the wiki or in my head ?
PostPosted: Mon Oct 02, 2017 12:40 pm 
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Octocontrabass wrote:
I don't see any contradiction. Both appear to be different ways of writing the same thing (ignoring the fact that the wiki code is undefined behavior due to unaligned pointer access).


How can it be the same thing. the wiki only reads one entry from the fat table, and the site code reads two entries.

Code:
Load a value of FAT12 entry:
    ReadSector(SecBuff, ThisFATSecNum);
    if (N & 1) {    /* Odd entry */
        ThisEntryVal = (SecBuff[ThisFATEntOffset] >> 4)
                     | ((uint16)SecBuff[ThisFATEntOffset + 1] << 4);
    } else {        /* Even entry */
        ThisEntryVal = SecBuff[ThisFATEntOffset]
                     | ((uint16)(SecBuff[ThisFATEntOffset + 1] & 0x0F) << 8);
    }

_________________
“Meaningless! Meaningless!”
says the Teacher.
“Utterly meaningless!
Everything is meaningless.” - Ecclesiastes 1, 2

Educational Purpose Operating System - EPOS


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 Post subject: Re: FAT12: bug in the wiki or in my head ?
PostPosted: Mon Oct 02, 2017 12:43 pm 
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The wiki code reads a single 16-bit integer. The code on the page you linked reads two 8-bit integers.


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 Post subject: Re: FAT12: bug in the wiki or in my head ?
PostPosted: Mon Oct 02, 2017 12:46 pm 
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Facepalm.
Stop the Internet ! can we delete this thread ??? :)

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“Meaningless! Meaningless!”
says the Teacher.
“Utterly meaningless!
Everything is meaningless.” - Ecclesiastes 1, 2

Educational Purpose Operating System - EPOS


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