[cctalk] Re: RSX11M tape copy

On Wed, Apr 16, 2025 at 8:01 PM Peter Ekstrom via cctalk < cctalk(a)classiccmp.org&gt; wrote: SIMH tape format inserts 4 bytes before and after each tape record. The 4 bytes hold a 32-bit record length, and that length is placed both before and after the record to allow a tape to be read backwards. If the record is an odd number of bytes it is padded to be an even number by adding a zero byte. The length should indicate the 'true' (odd) record length in such cases. File marks on the tape are indicated with a single 4-byte marker of all zero bytes. The practice of indicating end of tape with 2 consecutive file marks would be represented by having 8 zero bytes. Because a tape can contain arbitrary-sized records, there is no simple way to convert a SIMH tape file to a 'real' tape image short of writing it to a real tape. That is, a 'tape image' would need to have some way to encode the record lengths and presence of file marks. But if you happen to know that your tape contains all the same-size records or that the program reading the tape won't care what the record size is, and you don't need to preserve file marks, you can strip out all those markers. There are tools out there to 'enblock' and 'deblock' tape images, for example the TUHS archive has one but it's buried here: https://www.tuhs.org/Archive/Distributions/Research/Bug_Fixes/V6enb/ That folder contains mostly stuff for working with Unix 6th Edition; you can safely ignore everything besides the enblock/deblock utilities. -ken