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                    The IPS File Format
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               A quick and easy file format
              description for one of the most
               widely used non-binary formats
                       in emulation.

                     Written by Neil_

                          --FAQ--

o Who will benefit from this document?

- The following people will benefit from this tech doc:
    o Programmers wishing to code an IPS patcher or
       add IPS patching ability to their programs.
    o Masochists who absolutely must know what they've
       been using all these years

o What is required knowledge for reading this document?

- Not much. It's a pretty straight forward topic.

o Why did you write this FAQ for a straight forward topic?

- I was wasting time waiting for my brother to get off
   the Internet on the downstairs computer so I could
   log on, and the amount of boredom overtook me.
   
                         --Intro--

 The IPS file format is probably the most widely used format
in   emulation.  It's  a  very  handy  tool  and  basically
revolutionized  the  way translation  and graphics hacks of
commercial ROM images were distributed. Up until the time of
it's  adoption  by ROM  Hackers, Hacks had been distributed
with  the entire ROM image zipped up in a ZIP archive. This
proved to be a major waste of space as most hacks changed no
more  than 5%  of the entire ROM image. That meant that the
Hacker ended up distributing a file that contained 95% more
data  than  it  needed  to.  Why, in  a  world  of  limited
bandwidth,  make  users download  more than they absolutely
have  to? Here's where the IPS file format comes in. An IPS
file  contains only  a list of the bytes that were changed,
and  what  they were  changed to. For  the first time, hack
archives that had been 3 megs suddenly became 22k. Suddenly
ROM  hack  and translation  fans could save themselves from
downloading  3 megs  every time a new version came out, and
just download another 30k or so patch. How was this amazing
feat of size reduction accomplished?

                  --The IPS File Format--


The first 5 bytes of the IPS format are the letters
'PATCH' in ASCII format. This allows a program reading
the file to verify that it is indeed an IPS patch.

The next three bytes are the place in the destination file
(the file to be patched) where a change is to be made.

The two bytes following that are the length of the amount
of data to be changed.

The next string of bytes are the replacement bytes, which
go on for how ever many bytes were specified in the previous
two bytes.

If the two bytes specifying the length of the patch are
zeroes, then there is a section of code being replaced
by a single character, say FF. In this case, the next byte
gives the number of bytes to be replaced, and the byte after
that is the data that will be filled in that area.

Once the end of the patching length given is reached, you'll
either reach the end of the patch file or another two bytes
giving you the location of the next place in the destination
file to be changed. In that case, you go through these steps
again.

                          --End--

That should just about cover it all... If I missed
something, just drop me a line.

-Neil_

Neil@parodius.comver it all... If I missed
something, just drop me a line.

-Neil_

Neil@parodius.com