ISMUTL.LIT supports an option for compressed data records (which are very similar to variable length data records). Such records are made up of two parts: a fixed length, uncompressed part, and a variable length, compressed part. The fixed length part must be the first part of the record, and it must be long enough to accommodate all keys. From an application statement, the both parts are combined into a normal fixed length record map, and read/write operations behave as if you were dealing with fixed length records. Internally, the fixed length first part of the record is stored in the DAT file, and the remainder of the record is compressed and stored in the IDX file. (The IDX file was used because it already has solid and efficient infrastructure for dealing with variable-length chunks of data.)

The compression algorithm is a simple run length encoding scheme which condenses contiguous runs of ASCII 0 (null), 32 (space) or 48 ("0") bytes. (By filling the unused trailing bytes of the record with one of these characters, you get the effect of a variable length record.)

To specify this type of data record, at the record size prompt in ISMUTL, enter the total record size (as normal) followed by a slash and the size of the fixed length portion of the record. For example, if you have a 512 byte fixed record, with a 7680 byte variable length message area on the end, you would specify it as: 8192/512

As with the compressed key options, this cryptic format was chosen to preserve compatibility with existing command files that execute ISMUTL.

There is not yet a corresponding way to specify compressed records using ALLOCATE'INDEXED.