Reading/Writing Spectrum+3 discs

It is possible to transfer files between an Amstrad CPC and the Spectrum+3 because both systems use 3" floppy discs and a C/PM compatible filesystem.

The "Expanded Disc Parameter Block" (XDPB), which contains the parameters describing a CP/M based disc format, must be setup with the parameters defining the Spectrum+3 disc format.

The Amstrad will then be able to list the directory and read/write the files.

The Amstrad CPC has a XDPB structure per drive. Therefore XDPB of the drive that will be used to read the Spectrum discs must be initialised with the appropiate parameters.

To transfer files to the Amstrad:

To transfer files to the Spectrum+3:

Spectrum +3 disc file header

The +3 disc file header has the following structure:
08"PLUS3DOS" identification text
81&1a marker byte (note 1)
91Issue number
101Version number
114Length of the file in bytes (note 2)
15..22+3 Basic header data
23..126Reserved (note 3)
127Checksum (note 4))


  1. This byte is used by C/PM to mark the end of a file. It is called the "soft end of file". The "hard end of file" describes the length of the file in complete sectors. There may be additional bytes (which are ignored), between the position marked by the "soft end of file" and the position marked by the "hard end of file".
  2. The length of the file is a 32 bit number.
  3. These bytes are reserved for a future use. Set these bytes to 0 when creating files with this header
  4. The checksum is generated from the sum of the bytes 0..126. This value is the low 8 bits (bit 7..0) of the calculated value.