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Prime Proprietary
Principles of Operation
The following are three classes of faults defined by the CPU:
@
Restricted Instruction
e
Unimplemented Instruction
@
[Illegal Instruction
These three
classes
are
described
in
detail
in
the
following
subsections,
4.2.10.2.1 Restricted Instruction
A
restricted
instruction is an op code which is not part of a user's
access rights.
For example,
to
execute
instructions
to
halt
the
machine,
change
the CPU's operating mode, or shut down a disk drive,
the user must have RINGO
access
rights.
If
an
executing
program
contains
an
instruction
outside
of its access rights, a restricted
instruction fault results.
Restricted instructions are only found
in
systems with virtual memory.
4.2.10.2.2 Unimplemented Instruction
Unimplemented
instructions
are op codes defined, but not implemented
in the CPU.
These instructions are often hardware features on
higher
processors, implemented in software on lower processors (UII package).
For
example,
multiplication
and
division
were
unimplemented
instructions on the P100 and 200.
An unimplemented instruction causes
a fault to fetch the routine from software.
4,.2.10.2.3 Illegal Instruction
An op code that is not implemented on any Prime processor is called an
illegal instruction.
Attempts to execute illegal
instructions
cause
faults.
4.2.10.3 Checks
Checks
are
not
caused
by
executing
software.
Instead
they are
processor execution errors.
The microcode check
handler
stores'
the
current
value
of the PB, KEYS and MODALS registers, then updates the
diagnostic status word,
Checks discussed in this chapter are;
@
Machine Checks
@
Power Fail
@®
Memory Parity
e
I/O Parity
4
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15
070-C
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