🚀 go-pugleaf

In a message dated 10/10/2005 8:42:19 A.M. Central Daylight Time,
roland.johansson@seb.se writes:

>Old channel connected tape stations could read tapes backwards and  moved
byte >after byte to memory. The result was a block with bytes in  reversed
order. You used >MVCIN to restore the block.
So can new channel connected tape stations.  The read backwards CCW  command
code has been supported since day one of S/360.  It's in the  PrincOps.  Any
device which uses linear motion to move from one byte to  the next on its
storage media is a candidate for read backwards.  If the  device's control unit
reference manual describes the command, then it works  for that device type.
Tapes, card readers, magnetic ink character  readers, paper tape readers, etc.
The CCW data address points to the  last byte of the buffer.  The bytes are
read off the storage media by the  control unit and placed one at a time and in
successively lower  addresses in central storage by the channel.

This was used a lot in TOS/360 (Tape Operating System) when reading
backwards was often faster than reading forwards.  E.g., if you have  to read a load
module from a load library and the tape is positioned somewhere  in that
library, then you check the known position with the module name you  need to read
in.  If the needed module compares high, you use read  forward.  If it compares
low, you use read backwards.  This is much  faster than backing the tape up to
the beginning of the load library  and then reading forward.

The MVCIN instruction is also still supported.  It is documented in  the
z/Architecture PrincOps (SA22-7832-03).

Bill Fairchild