Stern pinball machines based on Spike 2 have their software updates packaged as .spk files. These are downloaded over the air by Internet-connected machines or may be manually installed by the owner by copying them to a flash drive that is then inserted into a USB slot on the MPU's motherboard.
This tool can parse and validate the contents of these files. It supports both single file and split update formats.
The format consists of chunks with 8 byte headers like so:
struct Header {
magic: [u8; 8],
size: u32,
}The magic value indicates the type of the chunk (SPKS, SPK0, SIDX,
STRS, FINF, FEND, SDAT). The size indicates the number of bytes
contained within the chunk.
Chunks can contain other chunks.
SPKS is the top-level chunk. Its header contains the number of SPK0 chunks
contained within. Each SPK0 chunk corresponds to a partition on the system
that will be updated.
SPK0 chunks serve as containers for SIDX, STRS, FINF and SDAT chunks
and do not contain any additional data themselves.
SIDX provides metadata, including the partition type to updated, the package
name, and its version number.
STRS contains null-separated strings for all of the file names contained
within the package.
After STRS are zero or more FINF chunks containing information aboout the
files contained within the update. These are terminated by a FEND chunk.
FINF chunks contain the file name (represented as an offset into STRS), the
file size, the offset of the file data within SDAT, an HMAC-SHA1 of the data,
and an MD5 of the data.
SDAT chunks contain the file data. The data is indexed by FINF.
Since updates are installed on machines by being copied onto USB drives, the file size limitations of FAT32 drives are a concern. Updates that may come close to those limits are split into multiple smaller files.
Rather than simply splitting the .spk file into multiple files, they are first encapsulated in a SquashFS file system. These are then split.
The Spike 2 system software uses affuse to present the chunks of the SquashFS
file as a single logical file that is in then mounted via SquashFS.
Extract files to disk. The data is right there, but care needs to be taken when creating the files on disk. It's easier to get the file data from the SD card images that Stern makes available.