You can use the standard z/OS® access methods to read tapes that were created on the IBM® OS/400® operating system.

One incompatibility to consider is variable-length blocks that are shorter than 18 bytes. In the unlikely event that the maximum block size for the data set is less than 18 bytes, z/OS writes the block normally although these very short blocks are very inefficient. These very short blocks require a great deal of overhead processing for a small amount of data.

If the maximum block size for the data set is more than 18 bytes but a block is shorter than 18 bytes, then the access method pads those blocks to 18 bytes. This means the system adds bytes at the end of the block but does not increase the block length value in the block descriptor word (BDW). If the record format (RECFM) is V, then the access method adds bytes of X'00' to the end of the block. If the record format is D, then the access method adds bytes of X'5B' to the end of the block. X'5B' is called the ASCII circumflex.

On OS/400, the system pads a short format-V block with a byte of X'80' and then bytes of X'00'. The z/OS access method treats this as an invalid record and issues system ABEND 002.

When the z/OS access method reads variable spanned records, the access method ignores the rest of the block when it finds a segment that begins with X'80'. Therefore when you read non-spanned format-V records from OS/400, when the block might be shorter than 18 bytes, you might be able to take advantage of this. You can code RECFM=VS or RECFM=VBS on the DD statement even if the records are not spanned. This will cause the access method to ignore the rest of this block when a record begins with X'80'.