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
