🚀 go-pugleaf

Bill Fairchild <DASDBill2@aol.com> wrote:


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

(someone wrote)
>>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.

Did the magnetic ink character readers support it?


> 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.

Not so long ago some people were running TOS/360 on the Hercules
emulator.  It took fixing some bugs in Read Backwards to get
it to run.  Note also that the P/370 AWSTAPE software doesn't support
Read Backwards, even though the file format includes block length
trailers.


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

I believe it is an optional feature, the Move Inverse feature.

I don't know which processor do or do not support it.

-- glen