Pmd2 SIR0

The SIR0 format is a pretty common wrapper file format with as primary function wrapping other file formats. It provides a pointer to the format's "entry point", along with a list of file offsets to the pointers which need to be translated to NDS memory when the file is loaded.

When a SIR0 file 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. The second pointer in the SIR header is also set to null when the file has been loaded and turned into a SIRO.

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 Pointer Offsets List	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 Pointer Offsets List	A pointer to the Pointer Offsets List 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 the pointers stored in the file. This includes 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 encoded offsets of the 2 pointers in the SIR0 header. Each encoded offset values are relative to the previous offsets.

Encoding

If a byte has its highest bit (1000 0000) set to 1, then we have to "append" the next byte. Here are the 3 possible cases, using example values:

0x80 0x81 0x82 0x75 => (0x80 & 0x7F) << 21 | (0x81 & 0x7F) << 14 | (0x82 & 0x7F) << 7 | 0x75
0x80 0x81 0x12      => (0x80 & 0x7F) << 14 | (0x81 & 0x7F) << 7  | 0x12
0x80 0x06           => (0x80 & 0x7F) << 7  |  0x06

Note that, since the offsets are stored as 32 bits integer, chaining more than 4 bytes is impossible. Also note that, we always get rid of the highest bit's value using the bitmask 0x7F, after checking if its toggled on!

If the byte's highest bit (1000 0000) is set to 0, we use the byte as is, still applying the 0x7F (0111 1111) bitmask.

An encoded byte with a value of 0 indicates the end of the list, but only when the byte that came before didn't have its highest bit set to 1 (1000 0000).

Also, each times we decode an offset using either of 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

Those are the offsets relative to the beginning of the file "/FONT/frame0.wte", where pointers are stored.

Some Code:
Here's a little C++11 code snippet, to decode the string of byte.

#include <vector>
#include <cstdint>
using namespace std;
//...

   std::vector<uint32_t> DecodeSIR0PtrOffsetList( const std::vector<uint8_t>  &ptroffsetslst )
   {
       vector<uint32_t> decodedptroffsets( ptroffsetslst.size() ); //worst case scenario
       auto     itcurbyte      = ptroffsetslst.begin();
       auto     itlastbyte     = ptroffsetslst.end();
       uint32_t offsetsum      = 0; //This is used to sum up all offsets and obtain the offset relative to the file, and not the last offset
       uint32_t buffer         = 0; //temp buffer to assemble longer offsets
       uint8_t  curbyte        = *itcurbyte;
       bool     LastHadBitFlag = false; //This contains whether the byte read on the previous turn of the loop had the bit flag indicating to append the next byte!
       decodedptroffsets.resize(0); //preserve alloc, and allow pushbacks
       
       while( itcurbyte != itlastbyte && ( LastHadBitFlag || (*itcurbyte) != 0 ) )
       {
           curbyte = *itcurbyte;
           //Ignore the first bit, using the 0x7F bitmask, as its reserved. And append or assign the next byte's value to the buffer.
           buffer |= curbyte & 0x7Fu; 
       
           if( (0x80u & curbyte) != 0 ) 
           {
               LastHadBitFlag = true;
               //If first bit is 1, bitshift left the current buffer, to append the next byte.
               buffer <<= 7u;
           }
           else
           {
               LastHadBitFlag = false;
               //If we don't need to append, add the value of the current buffer to the offset sum this far, 
               // and add that value to the output vector. Then clear the buffer.
               offsetsum += buffer;
               decodedptroffsets.push_back(offsetsum);
               buffer = 0;
           }
        
           ++itcurbyte;
       }
  
       //Avoid copying the vector by using std::move to explicitly call the move constructor
       return std::move(decodedptroffsets);
   }

Here's another one to encode the pointers offsets just as they would be at the end of the SIR0 file.

#include <vector>
#include <cstdint>
using namespace std;
//...
   
   void EncodeSIR0PtrOffsetList( const std::vector<uint32_t> &listoffsetptrs, std::vector<uint8_t> & out_encoded )
   {
       uint32_t offsetSoFar = 0; //used to add up the sum of all the offsets up to the current one
        
       for( const auto & anoffset : listoffsetptrs )
       {
           uint32_t offsetToEncode        = anoffset - offsetSoFar;
           bool     hasHigherNonZero      = false; //This tells the loop whether it needs to encode null bytes, if at least one higher byte was non-zero    
           offsetSoFar = anoffset; //set the value to the latest offset, so we can properly subtract it from the next offset.
            
           //Encode every bytes of the 4 bytes integer we have to
           for( int32_t i = 4; i > 0; --i )
           {
               uint8_t currentbyte = ( offsetToEncode >> (7 * (i - 1)) ) & 0x7Fu;
                
               if( i == 1 ) //the lowest byte to encode is special
               {
                   //If its the last byte to append, leave the highest bit to 0 !
                   out_encoded.push_back( currentbyte );
               }
               else if( currentbyte != 0 || hasHigherNonZero ) //if any bytes but the lowest one! If not null OR if we have encoded a higher non-null byte before!
               {
                   //Set the highest bit to 1, to signifie that the next byte must be appended
                   out_encoded.push_back( currentbyte | 0x80u ); 
                   hasHigherNonZero = true;
               }
           }
       }
       
       //Append the closing 0
       out_encoded.push_back(0);
   }