Multiuse Features; Multiprocessor Features; Multiprocessor Interlock - Xerox 550 Reference Manual

Rapid Context Switching. When responding to a new set of
interrupt-initiated circumstances, a computer system must
preserve the current operating environment, for continuance
later, while setting up the new environment. This changing
of environments must be done quickly, with a minimum of
II
overhead" time costs. Anyone of the four blocks of
generar-purpose arithmetic registers can, if desired, be as-
signed to a specific environment. All relevant information
about the current environment (instruction address, current
general register block, memory-protection key, etc.) is
kept in the program status words. A single instruction
stores the current program status words anywhere in memory
and loads new ones from memory to establish a new en-
vironment, which includes information identifying a new
block of general-purpose registers. Thus, the system's
operating environment can be preserved and changed com-
pletely through the execution of a single instruction.
Memory Protection. Both foreground (real-time) and back-
ground can run concurrently in the system because a fore-
ground program is protected against destruction by an un-
checked background program. Under operating system
control, the memory access-protection feature prevents
accessing memory for specified combinations of reading,
writing, and instruction acquisition.
Variable Precision Arithmetic. Much of the data encoun-
tered in real-time systems are 16 bits or less. To process
this data efficiently, both halfword and fullword arithmetic
operations are provided. For extended precision, double-
word arithmetic operations are also included.
Direct Input/Output. For handling asynchronous I/O, a
32-bi t word can be transferred di rectly between any genera 1-
purpose register and external devices.
Reliability, Maintainability, Availability. The capabil-
ities described in the section, .. Time-Sharing Features"
apply equally to the real-time environment.
MULTIUSE FEATURES
As implemented in this system, IImultiuse" combines two or
more application areas. The real-time application is the
most difficult general computing task because of its severe
requirements. Similarly, another difficult multiuse task is
a time-sharing application that includes one or more real-
time processes. Because the system is designed on a real-
time base, it is qualified for a mixture of applications in a
multiuse environment. Many hardware features that prove
valuable for certain application areas are equally useful in
others, although in different ways. This multiple capa-
bi I ity makes the system particularly effective in multi-
use applications.
The major multiuse features are described in the follow-
i ng paragraphs.
6 Muitiuse Features/Multiprocessor Features
Priority Interrupt System. In a multiuse environment, many
elements operate simulatneously and asynchronously. Thus,
an efficient priority interrupt system is essential. It allows
the computer system to respond quickly, and in proper or-
der, to the many demands made on it, with attendant im-
provements in resource efficiency.
Quick Response. The many features that combine to pro-
duce a quick-response system (multiple register blocks,
rapid context saving, multiple push-pull operations) benefit
a II users because more of the system's resources are readily
available at any instant.
Memory Protecti on. The memory protecti on features protect
each user from every other user and guarantee the integrity
of programs essential to critical real-time applications.
Input/Output. Because of the wide range of capacities and
speeds, the I/O system simultaneously satisfies the needs of
many different appl ication areas economically, both in
terms of equipment and programming.
Instruction Set. The comprehensive instruction set provides
the computational and data-handling capabilities required
for widely differing application areas; therefore, each user's
program length and running time is minimized, and the
throughput is maximized.
MULTIPROCESSOR FEATURES
System design readi Iy permits expansion to shared memory
in a multiprocessor system. The system can contain a com-
bination of functional clusters, each of which in turn may
contain multiple processors. The total number of clusters
is restricted to the maximum port limitation of six. All pro-
cessors ina system may share common memory.
The following paragraphs describe the major multiprocessor
features of the system.
MULTIPROCESSOR INTERLOCK
In a multiprocessor system, the basic processors often need
exclusive control of a system resource. This resource may
be a region of memory, a particular peripheral device, or,
in some cases, a specific software process. There isa special
instruction to provide this required multiprocessor interlock.
This special instruction, LOAD AND SET, unconditionally
sets a Ill" bit inthe sign position of the referenced memory
location during the restore cycle of the memory operation.
If this bit had been previously set by another processor, the
interlock is said to be "set" and the testing program pro-
ceeds to another task. On the other hand, if the sign bit
of the tested location is a zero, the resource is allocated
to the testing processor, and simultaneously the interlock
is set for any other processor.