Pmd2 SIR0

The SIR0 format is a pretty common wrapper file format. Its also fairly simple. When it gets loaded into memory, its magic number turns from SIR0 to SIRO, and all the pointers in the entire file are modified to be offset relative to the NDS's memory.

File Structure

Overview

Offset	Length	Endianness	Type	Name	Description
0x00	16			Header	The SIR0 header
After Header	Varies			Content Data	The data wrapped by the SIR0.
After Content Data	Varies			Content Padding	Some 0xAA padding bytes inserted to align the next section on 16 bytes. May be omitted completely if not required.
After Content Padding	Varies			Pointer Offsets List	A list containing the offsets to every pointers in the entire file.
After End of File Data Block	Varies			End of File Padding	Some 0xAA padding bytes to make the file end on a size divisible by 16 bytes with no leftovers.

Header

SIR0 header. (Total length 16 bytes)

Offset	Length	Endianness	Type	Name	Description
0x00	4	big		Magic Number	The 4 ASCII characters for "SIR0" (0x53 0x49 0x52 0x30)
0x04	4	little	uint32	Pointer to Content's Header	A pointer to the header of the data the SIR0 contains. If there are no headers, it points to the first byte after the SIR0 header.
0x08	4	little	uint32	Pointer to End of File Data	A pointer to a block of data located after the contained data.
0x0C	4			Null	4 bytes of zeros.

Pointer Offsets List

This list is what makes the SIR0 container what it is. Its a "compressed" list of all the offsets of all pointers in the files. Both the SIR0 structure, and the contained structure. The game use this list to change the value of each pointers in the file after it has been loaded in memory, so they're relative to NDS memory. The list will always begin with 04 04, as those are the offsets of the 2 pointers in the SIR0 header.

A byte with a value of 0 indicates the end of the list.

If a byte has its highest bit (1000 0000) set to 1, then we have to "chain" the next byte. Here are the possible cases, using example values:

0x80 0x81 0x82 0x75 => (0x80 & 0x7F) << 23 | (0x81 & 0x7F) << 15 | (0x82 & 0x7F) << 7 | 0x75
0x80 0x81 0x12      => (0x80 & 0x7F) << 15 | (0x81 & 0x7F) << 7  | 0x12
0x80 0x06           => (0x80 & 0x7F) << 7  |  0x06

Note that, since the size are stored as 32 bits integer, chaining more than 4 bytes is impossible. Also not that, we get rid of the highest bit's value using the bitmask 0x7F!

If the byte's highest bit (1000 0000) is set to 0, we use the byte as is, still applying the 0x7F bitmask.

Also, each times we compute an offset using the above, we add its value to the sum of all the previous offsets to get the actual offset. This is why the list starts with 04 04, and not 04 08 for example. Because they're added to each others as we process them.

Example:

04 04 92 0C 14 00 AA AA AA AA AA AA AA AA AA AA

This list comes from the "/FONT/frame0.wte" file. We ignore all 0xAA bytes, as they are padding.

Here are the calculations:

4
4 + 4
4 + 4 + ( (0x92 & 0x7F) << 7) | 0xC 
4 + 4 + ( (0x92 & 0x7F) << 7) | 0xC + 0x14

And the results:

0x4
0x8
0x914
0x928

Some Code: Here's a little snippet(untested as of now), to decode the string of byte. Its mainly meant to help understand the process, not to be the most efficient, or sensible algorithm.

std::vector<uint32_t> Handle_SIR0_PointerList( std::deque<uint8_t> pointerlistbytes  )
{
   vector<uint32_t> result;
   uint32_t buffer = 0;    //temp buffer to assemble longer offsets
   uint8_t lastbyteread = pointerlistbytes.front();
   pointerlistbytes.pop_front();
   
   //The first 2 values are guaranteed to be 0x04 and 0x04, so no worries about checking if they're stored on multiple bytes!
   while( lastbyteread != 0 && !pointerlistbytes.empty() )
   {
       buffer |= lastbyteread & 0x7F;
       
       if( 0x80 & lastbyteread != 0 )
       {
           buffer <<= 7;
       }
       else
       {
           result.push_back(buffer);
           buffer = 0;
       }
       
       lastbyteread = pointerlistbytes.front();
       pointerlistbytes.pop_front();
   }
   
   return std::move( result );
}