🚀 go-pugleaf

Not only did the tape drives provide a read backwards command but also,
if I recall correctly, the old 1419 MICR reader read data in backwards
ordinarily as I recall helping someone fix some code in this area. MVCIN
was not available at the time so the reordering of the data was done
with TR.

-----Original Message-----
From: IBM Mainframe Assembler List
[mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of glen
herrmannsfeldt
Sent: Monday, October 10, 2005 8:30 AM
To: ASSEMBLER-LIST@LISTSERV.UGA.EDU
Subject: Re: SV: Re: Shifting memory to the right

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.

Read Backwards puts the bytes in memory starting from the end of
the buffer, so they end up in the original order, though for short
blocks they will be at the end of the buffer instead of the beginning.

Someone else wrote:

> MVCIN (a) is no longer supported and (b) moved data in reverse order,
so
> C'ABCDE' would be moved as C'EDCBA'.  Very useful if you put a card in
the
> reader backwards, I'm sure, but I for one could never think of a real
use
> for it.

Rumor is that it was an RPQ for a company in Israel to correct for
languages written right to left on the page.  Sometime later it
was added as an instruction set optional feature, supported on many
later S/370's.  I am not sure which machines after that did or did
not support it.

-- glen