How to Read and Write to a Hard Drive
How to Read and Write to a Hard
DriveWritten by Jack Dobiash
Updated May 8th, 1999
Since I've been getting several questions
regarding how to access the data on a Hard Drive, I figured I'd write up this
little starter. I don't plan on showing every detail, since each
programming language has a different way to implement it, but it should at least
point you in the right direction. Basically all these instructions
pertain to using the Software Interrupts that are built into DOS v5.0 and above,
including Windows '95 and '98 (Although I don't use any that require me to be
inside of the GUI environment). Note that this may not be (and probably
isn't) the only way to do this, but it is the method that I am using. If
you have a different operating system than something Microsoft based, while the
Interrupts may be different, the method to doing it is still probably
similar.
Int 13h Extensions
Right off the bat I have to talk about the Int
13h Extensions. Basically Interrupt 13h is the one that I use for all of
the disk accessing. The problem is that the original disk accessing
commands of Int 13h don't have the capability of reaching beyond the 8GB barrier
which some of you have probably heard of. The limit stems from the fact
that they only allow a maximum of 1024 cylinders, 256 heads, and 63 sectors per
track, which if you multiply those all together and then by 512 (number of bytes
in each sector), you get the 8GB (Actually it's a tad below 8GB, but pretty darn
close). Anyway, newer computers have BIOS's that have Int 13h Extensions
in them, which are new routines that can reach beyond the barrier. Older
computers which don't have this addition can have a Disk Overlay Manager (like
OnTrack or EZ-Drive) load up right at the start which will add in routines and
make the whole drive accessible. There were also other barriers too (the
504MB limit, the 2048MB limit) but those were for other reasons.
Basically the first thing you want to do is
check to see if the computer has the capability to use the Int 13h
Extensions. If it does, you can then just use those routines from that
point on, otherwise you'll need to use the older ones. The Interrupt to
check for the extensions is Int 13h, section
AH=41h. If the Carry Flag is NOT set, then the computer
has the extensions.
Note: Here is a href="http://www.teleport.com/~brainy/interrupts.htm">link to all of the
interrupts I'm going to talk about on this page, with information on what to
plug into the Registers to make them work. Since talking about the details
of how to use each interrupt would take to long, I'll just say which ones to use
and you can refer to the href="http://www.teleport.com/~brainy/interrupts.htm">Interrupts Page on how
to use them.
So from now on I'll make two references on how
to do things. One with the extensions, and one without.
Drive Parameters
First thing that needs to be done is to get the
parameters of the drive (How many Cylinders, Head, Sectors, Total # of Sectors,
etc).
Extensions Method :
Use Int 13h, section AH=48h.
Non-Extensions Method :
Use Int 13h, section AH=08h.
The Non-Extensions Method won't give you the
Total # of Sectors in the drive, so if you need to know you'll have to calculate
it. Also, the values you get from the Extensions Method are the TOTAL
amount of them, so if it says you have 128 heads, it means you have to use 127
as the Max Value, since Heads and Cylinders are 0 based values. The number
of sectors will be correct as is, since it is a 1 based value.
CHS <--> LBA Translations
If you can use Extensions, than basically you
won't need to use anything but LBA (Logical Block Addressing) Mode. In
this mode you can just reference the drive sector by sector, with 0 being the
MBR, and on up. No need to keep track of what Cylinder or Head or Sector
you are on that way. If you can't use the extensions, then you'll need
routines to convert an LBA value into a CHS value in order to use the reading
and writing routines. It's still a good idea to reference everything in
LBA, and to only use CHS when doing the actual reading and writing. For
example, if I want to access the 16000th sector through the 17000th sector of a
hard drive, instead of having to keep track of CHS values during the count, you
just use LBA, and send that number to the read/write routine, which at that
point the LBA gets turned into CHS to be used in the interrupt. It's must
easier than having to keep track of incrementing H when S is full and
incrementing C when H is full.
Anyway, onto the math. To Convert a CHS
value to a LBA value, you use this math formula:
LBA = (Sector - 1) + (Head * Total Sectors) + (Cylinder * (Total Heads +
1) * Total Sectors)
Total Sectors and Total Heads were values obtained from the Drive
Parameters.
To Convert an LBA value back to CHS, you use
these math formulas:
Sector = ((LBA Mod Total Sectors)
+1)
CylHead = (LBA Div Total
Sectors)
Head = (CylHead Mod (Total Heads +
1))
Cylinder = (CylHead Div (Total Heads + 1))
Basically what you are seeing here is how I did it in Pascal
without some of the syntax. MOD is a way to obtain the
remainder of the math, instead of the quotient. You'll need to figure
out what your languages equivalent syntax is. Also, CYLHEAD is just a
temporary variable I used to hold information which was then used to figure
out the # of Heads and Cylinders.
Reading and Writing to the Drive
Reading Routines
Extensions Method :
Use Int13h, section
AH=42h
Non-Extensions Method : Use Int13h, section
AH=02h
Writing Routines
Extensions Method :
Use Int13h, section
AH=43h
Non-Extensions Method : Use Int13h, section
AH=03h
Editing the Drive
I've also gotten some questions regarding how to directly
edit your drive. In response I've decided to release my own little href="http://www.teleport.com/~brainy/DDD14.ZIP">Disk Doctor program.
Basically it allows you to go through the sectors of your hard drive and make
byte by byte changes. Of course this is a USE AT YOUR OWN RISK
program, but I use it at work and it helps me to figure out what has happened to
a hard drive.
Well, that's about it! Check out the href="http://www.teleport.com/~brainy/interrupts.htm">Interrupts Page for a
listing of all the Interrupts shown in this document.
Home href="http://www.teleport.com/~brainy/rps.html">Reference Point Software href="http://www.teleport.com/~brainy/fat32.htm">FAT32 Structure Information
FAT16 Structure
Information Disk Access Information href="http://www.teleport.com/~brainy/bio.html">About Me href="http://www.teleport.com/~brainy/links.html">Links href="http://www.teleport.com/~brainy/dobiash.html">Dobiash?
