[TUHS] Old mainframe I/O speed (was: core)
peter at rulingia.com
Thu Jun 21 13:05:05 AEST 2018
On 2018-Jun-20 12:33:05 -0400, Paul Winalski <paul.winalski at gmail.com> wrote:
>All of the System/360 series except the model 25 used separate channel
>processors to perform I/O.
The S360 architecture defined separate main CPU and I/O channel processors
and the actual implementation varied between models. IBM stressed the
compatibility between models so it can be difficult to determine what the
actual implementation did in hardware vs microcode. At least the model 30
also emulated the channel processor using the main CPU.  confirms this
for the multiplexor channels and implies it for the selector channels.
The "CPU interference factors" in (p65) suggest the model 50 also
emulated the channel processors.
The idea of separate I/O processors was also used in the CDC6600.
>term "memory") completely independently from the CPU. The S/360 model
>25 was the last of the 360 series and was really a 16-bit minicomputer
>microprogrammed to execute the S/360 instruction set.
Note that most S360 machines were microcoded with the native ALU size
varying between 8 and 32 bits. The model 25 was also the only S360 with
writable microcode and there was a microcoded APL implementation for it so
it "natively" executed APL. I'm not sure if there were any other novel
microcode sets for it.
Going back to Greg's question of actual I/O performance: A model 50 could
support 3 selector channels, with a nominal rate of 800kBps each. Since
each selector channel could only perform a single I/O operation at a time, I
believe the actual rate was effectively limited to the fastest device on the
channel - which  indicates was 340kBps for a 7340-3 Hypertape at 3022bpi.
That implies a total of 1020kBps of I/O. The "CPU interference" indicates
that each byte transferred blocked the CPU for 0.95us, so 1020kBps of I/O
would also steal 97% of the CPU-storage bandwidth.
 "multiplexer" channels were used for low speed devices - card readers,
card punches, printers, serial communications.
 "selector" channels were used for high speed devices - tape, DASD
 Direct Access Storage Device - IBM speak for "disk"
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