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Principles of Operation
Prime Proprietary
4.2.9.1 Process Exchange
A process is an executing procedure, a logical
continously
executing
sequence of code, such as a user program.
Processes may be halted for
indefinite
lengths
of time by interrupts, faults, and I/O transfers.
When a process is halted, a microcode function called process exchange
saves the state of the halted process, then schedules or
activates
a
new
one.
Because
process exchange is part of the microcode and not
the software, the entire process exchange can
take
as
little
as
6
microseconds.
Process
exchange
has
several
elements.
Each
process has its own
Process Control Block
(PCB)
which
provides
information
about
the
process:
e
Priority of this process
@
Instruction to be executed next
e
Register values
@
Timer
The Ready List is the priority scheduler and dispatcher.
The PCBs
of
all
processes
which are currently running or in queue to run, are on
the Ready List.
The Wait List keeps track of all processes which
can
be
started
only when some system condition is true.
For example, an
instruction may be on the Wait List until a DMX
transfer
or
another
instruction
is
completed.
Once
the
system condition is true, the
process is moved from the Wait List to the Ready List.
There are also four register sets.
Ome set is microcode scratch,
one
is
DMA
channels,
and
two
are
user
registers.
The User Current
Register Set (CRS) contains the
register
values
for
the
currently
running
or
most currently run process.
The Other Register Set (ORS)
contains the register values for some already run process.
Think
of
the
CRS
as
the
active
register
set
and
the ORS as the inactive
register set.
These process exchange elements work together as follows:
1.
The microcode activates the highest priority PCB on the Ready List
(Process A).
2.
Process A's values are stored in the CRS.
3.
When Process A is finished, the microcode stops the process.
4.
The microcode checks to see if any PCBs on the Wait List
may
now
be moved to the Ready List.
5.
Any PCBs that can be, are moved.
6.
Then the microcode activates the highest priority PCB on the Ready
List (Process B) and switches register sets.
7.
Values for Process B are stored in what was the ORS,
070~C
4-
12
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