70's computers

[Jon Elson]

On 10/24/2018 07:36 AM, Paul Koning via cctalk wrote:
>
> IBM channels are (from the programmer point of view at 
> least) merely hardwired controllers
Well, no.  They actually can do a lot more.  They can do 
branching and simple arithmetic.  We had a program to deal 
with damaged/deteriorating tapes.  You could operate sense 
switches on the tape control unit to tell it when to give up 
on retries on a bad block and go to the next one.  This 
allowed the operator to copy all the recoverable blocks from 
a bad tape.  Once started by the CPU, this program ran 
entirely in the channel.

The original scheme for disks was that the control unit + 
channel would be able to scan a range of disk tracks to find 
a record in which a string of bytes matched a pattern.  This 
turned out to not work so well on larger systems as it could 
tie up not only the control unit but the whole channel for 
the duration of the search. In 1962-3 or so, when the 360 
was designed, IBM had no idea how heavily the systems were 
going to depend on the disk drives.

Jon