Control Data Cyber 170 State Hardware Reference Manual page 52

InputlOutput Unit
InputlOutput Unit
The input/output unit (IOU) performs the functions required to locate, select,
and initialize the external devices connected to the system. The IOU controls
the transfer of data between a selected device and CM. The IOU also performs
system maintenance functions.
The IOU contains the following functional areas.
Peripheral processor (PP).
I/O channels.
Real-time clock.
Two-port multiplexer.
Maintenance channel.
CM access.
Peripheral Processor
The basic IOU contains 20 PPs and 24 I/O channels.
Each PP is a logically
independent computer with its own memory. Each 5-PP group is organized into a
multiplexing system that allows the PPs to share common hardware for arithmetic,
logical, and I/O operations without losing independence. This multiplexing
system comprises five ranks of registers, which is termed a barrel. Each rank
contains information related to the instruction being executed by one PP.
Each PP can communicate with the CP by issuing a CYBER 170 exchange request to
a specific CYBER 170 exchange package associated with the issuing PP. In
addition, a PP can also communicate with the CP via CM read and write
operations. PPs can communicate with each other over the 110 channels and
through CM.
Each PP executes programs alone or with other PPs to control data transfers
between external devices and CM. These programs are comprised of instructions
from the IOU instruction set and respond to requests issued through CM by the
operating system. The programs translate generalized operating system requests
into c-ontrol functions for accessing the external devices and may also perform
device scheduling and optimization. The programs use PP memory as a buffer for
the data transfer between external devices and CM to isolate IOU data transfer
from variations in CM transfer rate.
An IOU upgrade is available which is an optional, concurrent input/output (CIO)
subsystem consisting of five or ten P P s . Optional intelligent standard
interface (ISI), intelligent peripheral interface (IPI), and CYBER 170 DMA
(direct memory access) I/O channel adapters can be installed in the CIO.