GE PAC 4020 System Manual page 12

Memory Addressability
A single full operand instruction can address
16,384 words directly. In systems with more than
16K, direct addressing references the first 16K, in
which the Real-Time Operating System subroutines
and common data areas are generally found. In the
relative mode, the GE/PAC 4020 computer can
address any location within +8191 or -8192 locations
of the instruction, regardless of memory size.
By using one of its seven index locations as a
base address register, or by using its indirect load,
store or branch instructions (LDI, STI, LPR), the
computer can address any of its maximum 32,384
words of memory. The use of either mode for this
purpose is rare, primarily because most instructions
either reference locations within their own program
(seldom as large as the 16K range available) or
locations within the first 16K.
There are also available various interesting
combinations of indexing and direct, indirect, rela-
tive, and "immediate mode" addressing. In the
relative-indexed mode, the relative operation is
carried out before the index location's contents are
added.
Circular lists
A circular list consists of a set of consecutive
items contained within a fixed length block of mem-
ory. The first word of the block is a list control
word. The size of the list must be a power of two,
from one to eight.
Quasi-instructions append additional items to the
beginning (ABL) or end (AEL) of the list and remove
the beginning (RBL) or ending (REL) items. The
address of the beginning and ending items changes as
items are added and removed, as does the size of the
unused area.
An item, when appended, becomes the new first
or last item. The removal of an item forces the
adjacent item to become the new beginning or ending
item.
These lists are most useful for inter-program
communication, since the appending and the removing
programs can operate at different speeds and even at
different times. Each program must know only the
address of the control word. Each program must
also incorporate logic to handle the "full" or "empty"
conditions, depending on whether it is appending to
or removing from the list.
Items processed through a list must either not
require identification or must carry it with them; the
usual implicit identification of its contents by an
address does not pertain.
Control Word
(at addres f
r
t) s
0 lS
on of
ing item
r of
F = locati
beginn
N = numbe
items
in the
L =size o
f, e =full,
bits
currently :
list
REL woul
this item
RBL woul
this item
f list
empty
d remove
•
d remove
•
23 15 14 6 5 4 3 0
F
I
N
IrH
L
_a_
etc.
next-to-ending item
ending item
1 - - - - - - - - - -
--1
unused area
1 - - - - - - - - - -
--1
beginning item
second item
etc.
,,
Circular Lists
Counting and Time Keeping
.-
~
AEL would
append a new
ending item
here
ABL would
append a new
beginning
item here
Any location in the GE/PAC 4020 computer's
core memory can be used for counting external
pulses, for maintaining a real-time or elapsed-time
clock, or for program loop counting. The decrement
memory and test (DMT) instruction, when used as an
automatic program interrupt response command,
acts as a one-instruction subroutine to accomplish
this function.
When a pulse comes in, the DMT decrements a
count preset into a designated core location. When
the count passes from zero to minus one, the DMT
generates an "echo" interrupt that notifies the rele-
vant program to reset the count and take appropriate
action.
The total time taken away from the running
programs is only 4.8
us
per event counted, since
none of the AU's contents are disturbed, although
some additional time is required to store and reload
registers and to run the response program when the
count runs down and the "echo" is processed.
Another advantage of this approach is that counts
are not lost when the a-c power voltage or frequency
goes out of limits, since the core's contents are
protected against this contingency.
9