[TUHS] UNIX on S/370

Paul Winalski paul.winalski at gmail.com
Tue Nov 21 05:44:03 AEST 2017

On 11/20/17, Clem Cole <clemc at ccc.com> wrote:
> On Mon, Nov 20, 2017 at 11:03 AM, ron minnich <rminnich at gmail.com> wrote:
> IMO: The issue was less understanding how channels worked, as much as the
> biasing between the UNIX I/O model *vs.* the mainframe I/O model.   360 and
> CDC boxes for that matter, were a coprocessor that was programmed for the
> I/O.   Figuring out how to splice that into the UNIX world efficiently was
> not always easy.

The PDP-11 and VAX used memory-mapped I/O.  Devices presented
themselves to the CPU as special areas of main memory.  Instead of
retrieving or storing data, these memory locations performed device
operations when you read or wrote to them.

System/360/370 had a radically different I/O model.  I/O devices and
their control units were connected to specialized coprocessors called
I/O channels.  Devices were addressed via a number with three hex
digits, the first of which was the channel number.  The CPU had an
instruction called start I/O (SIO) that took as parameters a device
address and the address of a series of channel command words (CCWs)
that indicated to the channel the I/O operation to perform and its
parameters (such as the address of the I/O buffer in main memory).
The CPU was notified of I/O termination by three interrupts:
channel-end (the I/O channel reached the end of its channel program),
control-unit-end (the I/O control unit completed a command), and
device-end (the I/O device completed a command).

CCW programs were capable of primitive conditional branching and
looping.  It was possible to set up a channel program to perform
sequential multi-buffered reading or writing, using the device-end
interrupts to determine when each buffer was free for reuse.  If no
I/O errors occurred, it was possible to read or write a whole tape
file with a single SIO instruction.

-Paul W.

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