Architecture
of
the
Model
22
processor
1.2.2 The Arithmetic and Control
Unit
(ACU)
The Arithmetic and Control Unit (ACU) carries
out
three important functions
that
are fundamental
to the running
of
the machine:
To allocate processing time to each
IOC
and its associated partition in turn. Each IOC in
turn
is
selected'by the ACU and monopolises the unit for (nominally)
40
milliseconds. See
the Branch instruction, section 3.2.3
2 To
extract
each instruction and its associated data from core store; operate on
the
data and
store the results
3 To transfer data character by character between core store and the FAC or one
of
the IOCs
1.2.3 The File
Access
Channel (FAC)
The File Access Channel (FAC) provides an interface between the ACU and magnetic tape and
disc storage. Depending on the high-speed devices provided in the system, the F
AC
includes a
Magnetic Tape Controller (handling up
to
four tape drives) and/or a Disc Controller (handling up
to
16
logical disc devices). Unlike the IOCs, the F
AC
is
shared. The devices attached
to
the F
AC
are directly accessible
to
all partitions. Thus, several programs may share the same disc
or
magnetic tape files.
Data
is
transferred between core store and disc units in groups
of
100 characters, and between
core store and magnetic tape in groups
of
up
to
10,000 characters.
Note
that
(unlike the IOCs) the FAC retains control
of
the processor until a data transfer
is
complete. It
is
therefore possible for a lengthy tape transfer (for example reconstituting a disc
from a tape security copy)
to
occupy the processor for a considerable time.
1.2.4 The I
nput/Output
Controllers (I
DCs)
The
Input/Output
Controllers (IOCs) provide communication between the
ACU
and
the
various
peripheral devices.
It
is
the IOCs
that
perform the conversion between the ASCII seven-bit
character
pattern
and the System Ten internal six-bit pattern (see section 2.4.1). Each IOC can
address any location in its own partition or above 299 Common.
The following IOCs are available:
IOC
type
Multi-terminal
laC
(MTIOC II)
Multi Device
laC
(MDIOC II)
Synchronous Communications
Adaptor
(SeA)
Asynchronous Communications
Adaptor (ACA)
Asynchronous Terminal
Adaptor (ATA)
Digital clock
1.3
How
the
ACU
operates
User device
Workstation(typebar or Visual Display Unit (VDU), Line
Printer
Point
of
sale
(PaS)
terminal, Job Information terminal, Data
Collection terminal
Remote computer, visual display unit (VDU) or
other
synchronous device
Remote asynchronous devices including computers
Devices from Other Equipment Manufacturers (OEM) using
asynchronous communications methods
Provides time
of
day information for user
The
ACU
performs all major functions by means
of
hardware logic controlled
by
hardware
function codes which are similar
to,
but
at a lower level than, instruction function codes. These
hardware codes control the manipulation
of
information and its transfer between components
of
the processor and
to
and from peripherals.
Even though the general course
of
action
of
the ACU
is
set by the instruction function code, the
ACU must step through and branch between hardware functions many times to perform the
various checks and actions necessary
to
complete one instruction. The sequence
of
steps and
functions
is
different for each instruction, and during execution the sequence
is
constantly being
modified depending on certain conditions encountered, for example a data error or a busy or
inactive peripheral.
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