Quick Release Sector Transfer

The QRST disc image format was used by Compaq to distribute disk images of diagnostic software. The file QRST.EXE or QRST5.EXE would be supplied with the disc images to write them to a floppy drive.

Identification
QRST files start with the 4-character ASCII string. The first character of the file extension is an underscore, followed by a 2-digit hexadecimal (version <5) or decimal (version 5) volume number.

File header
The file begins with a 796-byte header:

Version 5 extends the header with an additional 25 bytes:

Version <5 data
In versions below 5, the header will be followed by the data tracks. These can be in one of three forms:

Compressed track
The compressed data stream consists of alternating literal runs (a byte giving the length of the run, followed by that number of bytes data) and compressed runs (two bytes; first gives number of repeats, second gives byte to repeat).

Version 5 data
In version 5, the data is an entire disk image compressed with PKWARE Data Compression Library's Implode. (This is not the same as PKZIP's Implode.)

Checksum
In versions below 5, the checksum is the sum of all bytes on the disc, each byte multiplied by (1 + its offset on the disc). So for a 360k disc it would be (1 * first byte of first sector) + (2 * second byte of first sector) + ... + (368640 * last byte of last sector).

In version 5, the CRC-32 covers the compressed data.

Sample files

 * Compaq Portable III setup and diagnostics