• Keyboard Printers: available with or without a paper
tape reader and paper tape punch
• Card Readers: read cards punched in binary or EBCDIC
card code, 200 to 1500 cpm
• Card Punches: binary or EBCDIC card codes, 300 cpm
• Line Printers: fully buffered, with 132 print positions
and carriage control, 600 or 1000 Ipm
• Graph Plotter: for two-axis plotting of data under
digital control, 300 increments/second
• Display Equipment: oscilloscope display units, light
guns, and character and vector generators
• Data Communications Equipment: a complete line of
character- and message-oriented equipment
REAL-TIME AND MULTIUSAGE FEATURES
Real-time applications are characterized by a need for hard-
ware that provides quick response to an external environment,
sufficient speed to keep up with the real-time process itself,
and input/output flexibility to handle a wide variety of types
of data at varying speeds.
Multiusage applications, as implemented in SIGMA 2, are
defined as the combining of real-time and background pro-
cessing techniques into one system. The most difficult gen-
eral computing problem is the real-time application with its
requirements for extreme speed and capacity. Because the
SIGMA 2 system has been designed on a real-time base, it
is well qualified for the mixture of applications in a multi-
usage envi ronment. Many of its hardware featu res that
prove valuable for real-time applications are equally useful
in background processing, but in different ways.
The major features that make SIGMA 2 uniquely suitable
for both real-time and multiusage appl ications are described
in the following paragraphs.
Input/Output Facilities. Three distinct SIGMA 2 input/
output systems offer flexibil ity and capacity to meet the
needs of both real-time and general-purpose users: the
byte-oriented, the direct-to-CPU, and the direct-to-
memory I/O systems.
In the byte-oriented I/O system, each automatic I/O chan-
nel has its own high-speed registers and operates indepen-
dently without requiring attention from the program once it
has been started. Data is transferred one byte (8 bits) at
a time. For high-speed peripherals, bytes are assembled
into words in the I/O section and only one memory refer-
ence is made for two bytes. For slow-speed peripherals,
one reference is made for every byte, with a partial write
operation performed by the memory. All I/O channels
may operate concurrently and parity checking is performed
automatically.
The optional direct-to-CPU input/output system uses only
a single instruction to transfer a full 16-bit data word to
and from the A register. The same instruction that trans-
fers data also provides a 16-bit control field for external
control and selection, and accepts status information
returned from the external device to permit rapid sensing of
an external condition. The direct I/O system is generally
used for short bursts of asynchronous data transfers to avoid
tying up an automatic channel. Direct I/O is also useful
when data is to be accepted at medium to high speeds and
each input must be examined immediately when received.
The optional direct-to-memory input/output system provides
an additional memory bus to each of four external memories.
It is used for very high speed I/O transfers to and from
external devices or other processors. Transfers proceed at
full memory speed on a word-oriented basis, with overlap-
ping of multiple I/O and compute occurring automatically
when multiple memory modules are available.
Priority Interrupt System. In a multiusage environment,
many elements are operating asynchronously with respect to
each other. Thus, having a true priority interrupt system,
as the SIGMA 2 does, is especially important. With it the
computer system can respond quickly (and in proper order)
to the many demands made upon it, without the high over-
head cost of complicated programming, lengthy execution
time, and extensive storage allocations. Programs that deal
with interrupt signals from special equipment must sometimes
be checked out before the equipment is actually available.
To simulate special equipment, any external SIGMA 2 inter-
rupt level can be triggered by the CPU itself through execu-
tion of a single instruction.
Context Switching. When responding to a new set of
interrupt-initiated circumstances, a computer system must
preserve the current operating environment while it sets up
the new environment. In SIGMA 2, relevant information
about the current environment is retained as a 32-bit pro-
gram status doubleword (PSD). When an interrupt occurs,
the current PSD is automatically stored at an arbitrary loca-
tion in memory and the interrupt-servicing routine begins,
following the location into which the PSD is stored. At the
end of the interrupt-servicing routine, the PSD is restored
and the interrupt level cleared.
Protection System. Both real-time and background programs
can be run concurrently in a SIGMA 2 system because the
real-time program can be protected against alteration. The
optional protection feature guarantees that protected areas
of memory cannot -be written into by a program residing in
unprotected memory. The protection feature also prevents
the execution of unprotected instructions that could change
the I/O system or the protection system. The protection
pattern can be changed very rapidly.
Real-Time Clocks. In real-time systems, timing informa-
tion must be provided to cause certain operations to occur
at specific instants. Other timing information is also neces-
sary, such as elapsed time after a given event, or the cur-
rent time of day. SIGMA 2 provides up to four real-time
clocks, with varying degrees of resolution, to meet these
needs. These clocks also facilitate handling of separate
time bases and relative time priorities. Three of the clock
counters can be driven from commercial o, c , line frequency
(60 or 50 Hz), from 2- or 8-Hz oscillators, or from an ex-
ternal input; the first (operational) counter is driven by a
SOO-Hz source.
Real-Time and Multiusage Features 3
