Real-Time Features - Xerox 560 Reference Manual

(also, by a single instruction). The current program status
words, which contain the entire description of the current
user's environment and mode of operation, may be stored
anywhere in memory, and new program status words may be
loaded, all with a single instruction.
Multiple Register Blocks. The availability of four blocks
of 16 general-purpose registers improves response time by
reducing the need to store and load register blocks. A
distinct block may be assigned for different functions as
needed; the program status words automatically select the
applicable register block.
User Protection. The slave mode feature restricts each user
to his own set of instructions while reserving to the operat-
ing system certain "privileged" (master mode) instructions
that could destroy another user's program if used incor-
rectly. Also, a memory access - protection feature pre-
vents a user from accessi ng any storage areas other than
those assigned to him. It permits him to access certain areas
for reading only, such as those containing publ ic subrou-
tines, while preventing him from reading, writing, or ac-
cessing instructions in areas set aside for other users.
Storage Management. Main memory is expandable to 256K
(K
=
1024) words. To make efficient use of available mem-
ory, the memory map hardware permits storing a user's pro-
gram in fragments as sma II as a page of 512 words, wherever
space is avai lable; yet all fragments appear as a single,
contiguously addressable block of storage at execution time.
The memory map also automatically handles dynamic pro-
gram relocation so that the program appears to be stored in
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tually be stored in a different set of locations each time it
is brought into memory. The memory map provides the
abi I ity to locate any 128K-word virtual program in the basic
processor's logical addressing space. Thus, the system can
always address a virtual memory of 128K words regardless
of physical memory size.
Input/Output Capability. Time-sharing input/output re-
quirements are handled by the same general-purpose input/
output capabi I i ti es descri bed under
II
Genera I-Purpose
Features".
Nonstop Operation. A "watchdog" timer assures that the
system continues to operate even in case of halts or delays
due to fai lure of special I/O devices. Multiple real-time
clocks with varying resolutions permit independent time
bases for flexible allocation of time slices to each user.
Reliability, Maintainability, Availability. Since time-
sharing systems have many on-line users needing immediate
system response, "downtime" defeats time sharing's primary
purpose. Pool i ng of resources a long wi th fl exi bl e recon-
figuration control ensures a high level of continuous avail-
ability. Configuration controls are provided to switch the
load from one unit to another in the event of a failure with
no loss of functional capability, only capacity. In addi-
tion, a nonworking subset of the total system may be
logically isolated (partitioned) so that maintenance may
proceed on the subset while the remainder of the system
conti nues to operate.
To minimize the effect of transient errors, automatic retry
of fa i led instructions is performed.
REAL-TIME FEATURES
Real-time applications are characterized by a need for:
(1) hardware that provides quick response to an external
environment; (2) speed that is sufficient to keep up with
the real-time process itself; (3) input/output flexibility to
handle a wide variety of data types at different speeds;
and (4) reliabi lity features to minimize irreplaceable lost
time.
Multilevel, Priority Interrupt System. The real-time-
ori ented system provi des rapi d response to external interrupt
levels. Each interrupt is automatically identified and res-
ponded to according to its priority. For further flexibi lity,
each level can be individually disarmed (to discontinue in-
put acceptance) and disabled (to defer responses). Use of
the disarm/disable feature makes programmed dynamic re-
assignment of priorities quick and easy, even while a real-
ti me process is in progress.
Programs involving interrupts from specially designed equip-
ment often require checkout before the equipment is actually
avai lable. To permit simulating this special equipment, any
external interrupt level can be "triggered" by the basic
processor through execution of a single instruction. This
capability is also useful in establishing a modified hierarchy
of responses. For example, in responding to a high-priority
interrupt, after the urgent processing is completed, it may
be desirable to assign a lower priority to the remaining por-
tion so that the interrupt routine is free to respond to other
critical stimul i.
The interrupt routine can accomplish this
by triggering a lower-priority level, which processes the
remaining data only after other interrupts have been handled.
READ DIRECT and WRITE DIRECT instructions (described in
Chapter 3) allow the program to completely interrogate,
preserve, and a I ter the conditi on of the interrupt system at
any time and to restore that system at a later time.
Nonstop Operation. When connected to special devices
(on a ready/resume basis), the basic processor may be ex-
cessively delayed if the specific device does not respond
quickly. As in the time-sharing environment, the built-in
watchdog timer assures that the basic processor cannot be
delayed for an excessive length of time.
Real-Time Clocks. Many real-time functions must be timed
to occur at specific instants. Other timing information isalso
needed - for example, elapsed time since a given event, or
the current time of day. The computer system can contain
up to four real-time clocks with varying degrees of resolu-
ti on to meet these needs. These clocks a I so a II ow easy hand-
ling of separate time bases and relative time priorities.
Real-Time Features 5