BIOS Code space, memory map, fs info, and exec format info 
Author Message
 BIOS Code space, memory map, fs info, and exec format info

Does anyone know of a good source for info on any of these things:
1. FAT AND VFAT filesystems...
2. theory of Log-based file systems
3. executable file format info, for exe and com files
4. an explanation of what code is in use, after a reset on a pc
        (i know the bios loads in somewhere, im just not sure where,
        basically, if i were writing a loader or an os, what
        areas should i watch out for...)

In real mode a processor can only address how much memory? 1 mb?
arent we talking about 20 bit pointers here?

Thanks for any help!
Andy



Tue, 18 Jul 2000 03:00:00 GMT  
 BIOS Code space, memory map, fs info, and exec format info

Quote:

> Does anyone know of a good source for info on any of these things:
> 1. FAT AND VFAT filesystems...

any good dos internals book will tell about FAT.  What is VFAT?

Quote:
> 2. theory of Log-based file systems

What is that?

Quote:
> 3. executable file format info, for exe and com files

probably can find something on ftp://x2ftp.oulu.fi

Quote:
> 4. an explanation of what code is in use, after a reset on a pc
>         (i know the bios loads in somewhere, im just not sure where,
>         basically, if i were writing a loader or an os, what
>         areas should i watch out for...)

All of memory is availbe at boot time except the real-mode
interrupttable (address 0-400h) and a bios work area( 400h-500h).  If
you never
plan to use the bios you can overrite the bios work area.  Needless to
say
this is getting more and more impractical.

Quote:
> In real mode a processor can only address how much memory? 1 mb?
> arent we talking about 20 bit pointers here?

yep.  But the stupid architecture we are saddled with breaks
memorypointers into two 16-bit portions, for a total of 32 bits.  These
are
called segment and offset.  A physical address is calculated by adding
the offset to 16 times the segment.  Because of this there is overlap,
two adjacent segments start addresses share a bunch of bytes.

David

Quote:
> Thanks for any help!
> Andy



Tue, 18 Jul 2000 03:00:00 GMT  
 BIOS Code space, memory map, fs info, and exec format info

Quote:

> > Does anyone know of a good source for info on any of these things:
> > 1. FAT AND VFAT filesystems...

> any good dos internals book will tell about FAT.  What is VFAT?

VFAT is the Win95 filesystem, extended FAT really, with long file name
support.  I was just wondering how it differed from fat or what it
added.

Quote:
> > 2. theory of Log-based file systems

> What is that?

Im not sure, BUT i THINK it is similar to cache algorithms of
recently used stuff being held in memory...but i REALLY dotn know.
Thats why i was asking :)

Quote:

> > 3. executable file format info, for exe and com files

> probably can find something on ftp://x2ftp.oulu.fi

> > 4. an explanation of what code is in use, after a reset on a pc
> >         (i know the bios loads in somewhere, im just not sure where,
> >         basically, if i were writing a loader or an os, what
> >         areas should i watch out for...)

> All of memory is availbe at boot time except the real-mode
> interrupttable (address 0-400h) and a bios work area( 400h-500h).  If
> you never
> plan to use the bios you can overrite the bios work area.  Needless to
> say
> this is getting more and more impractical.

I know!  Unless you can get a driver for you filesystem within 512
bytes, right? haha

andy



Tue, 18 Jul 2000 03:00:00 GMT  
 BIOS Code space, memory map, fs info, and exec format info

Quote:

> Does anyone know of a good source for info on any of these things:
> 1. FAT AND VFAT filesystems...
> 2. theory of Log-based file systems
> 3. executable file format info, for exe and com files
> 4. an explanation of what code is in use, after a reset on a pc
>         (i know the bios loads in somewhere, im just not sure where,
>         basically, if i were writing a loader or an os, what
>         areas should i watch out for...)

> In real mode a processor can only address how much memory? 1 mb?
> arent we talking about 20 bit pointers here?

> Thanks for any help!
> Andy

As for EXE and COM formats, goto http://wotsit.simsware.com/ . For plain
FAT stuff you can look at the FreeDOS source codes, at
http://sunsite.unc.edu/pub/micro/pc-stuff/freedos/

Rather that 20 bit pointers Real Mode uses 32 bit pointers. But its a
lot more complex than that!! Intel processers, in real mode that is, use
_segmented_ memory. To acess a memory address you use two 16bit values a
segment and an offset. The segment must be loaded into a segment
regester CS, DS, ES or SS it is then _multiplied_ by 16 and the offset
added to that. So the segment can point to any address that is a
multiple of 16 and the offset used to address the 64K of addresses after
that.

If you find anything on VFAT _please_ point me to it!

What do you need this info for anyway?

__________________________________________________________

 ___________
|\
| \
|  \
    \ The Qube



Mon, 24 Jul 2000 03:00:00 GMT  
 BIOS Code space, memory map, fs info, and exec format info

Quote:


> > Does anyone know of a good source for info on any of these things:
> > 1. FAT AND VFAT filesystems...
> > 2. theory of Log-based file systems
> > 3. executable file format info, for exe and com files
> > 4. an explanation of what code is in use, after a reset on a pc
> >         (i know the bios loads in somewhere, im just not sure where,
> >         basically, if i were writing a loader or an os, what
> >         areas should i watch out for...)

> > In real mode a processor can only address how much memory? 1 mb?
> > arent we talking about 20 bit pointers here?

> > Thanks for any help!
> > Andy

> As for EXE and COM formats, goto http://wotsit.simsware.com/ . For plain
> FAT stuff you can look at the FreeDOS source codes, at
> http://sunsite.unc.edu/pub/micro/pc-stuff/freedos/

> Rather that 20 bit pointers Real Mode uses 32 bit pointers. But its a
> lot more complex than that!! Intel processers, in real mode that is, use
> _segmented_ memory. To acess a memory address you use two 16bit values a
> segment and an offset. The segment must be loaded into a segment
> regester CS, DS, ES or SS it is then _multiplied_ by 16 and the offset
> added to that. So the segment can point to any address that is a
> multiple of 16 and the offset used to address the 64K of addresses after
> that.

> If you find anything on VFAT _please_ point me to it!

> What do you need this info for anyway?

> __________________________________________________________

>  ___________
> |\
> | \
> |  \
>     \ The Qube

Thanks for the info!
What I'm trying to do is have a bootloader grab a file off
a vfat filesystem...
What I'm confused about is this:
If I have a bootloader that needs to access a file on a filesystem...
and i dont always know WHERE EXACTLY it is...
how can i find it?
thats why i wanted the vfat / fat info...
i assume i will have to read from the fat and locate the file...
I did find something on vfat...but i cant locate it right now...
i will try and send it to you later..i think i left it at home...
Thanks again!

andy



Tue, 25 Jul 2000 03:00:00 GMT  
 
 [ 5 post ] 

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