Format of IBM VM CMS files on tape?

[Chuck(G)]

Call me lazy, but if someone knows of a document to relieve me of having to search it out, I'd be grateful. I've got a 9 track tape file where the record structure starts out like this.

02 c3 d4 e2  c6 c4 c1 e3 c1 40 ...

which I take to be "/02 CMSFDATA" I'm assuming (given the age of the tape) that this may have been done under VM.
The remainder of the file appears to be 80 character blocked card images, followed by other /02 CMSFDATA sections.

Does this sound familiar to anyone?

I did find this post which seems to agree with what I see, but I'd like something authoritative: https://h390-vm.yahoogroups.narkive.com/DOlTatQG/getting-data-in-and-out-of-vm-370-using-tapes

 

[g4ugm]

There is no authoritative documentation on CMS tape dumps. could be a VMFPLC2 or TAPEDUMP tape. There is some code I adapted here:-

pctools

Folder

1drv.ms

let me know how it works

 

[Chuck(G)]

Thanks Dave, I think I've already figured it out. My issue was over-thinking it. Body of a file is 4101-byte blocks, starting with "\02CMSx" followed by blocked 80-character card images; last block is null-padded. The end-of-file is an 87 byte block starting with "\02CMSN", followed by what appears to be file metadata. Strange stuff.

 

[g4ugm]

Thanks Dave, I think I've already figured it out. My issue was over-thinking it. Body of a file is 4101-byte blocks, starting with "\02CMSx" followed by blocked 80-character card images; last block is null-padded. The end-of-file is an 87 byte block starting with "\02CMSN", followed by what appears to be file metadata. Strange stuff.

Then its a CMS TAPE format dump of a CMS file of card images. The last block is the File Status Table entry (think directory) that describes the record format. CMS files can have fixed or variable length records. Fixed length just contain data, and the record length is stored in the File Status Table and there are no record markers. If the file being dumped was variable length records there would be record headers on the front of each record containing the record length.

 

[Chuck(G)]

I think that summarizes it--and the layout of the final record is probably related to the FST structure described in this document:

https://bitsavers.org/pdf/ibm/370/VM_SP/Release_5_Dec86/LY24-5221-2_VM_SP_Release_5_Data_Areas_and_Control_Block_Logic_Volume_2_CMS_Dec1986.pdf

Or at least it's a close relative...