Note: Descriptions are shown in the official language in which they were submitted.
~0~73
BACKGROUND OF THE INVENTION
There do not appear to be any systems in the prior a~t
which provide for error correction for the content~ of random
access magnetic memories which have been subjected to a nuclear
radiation event, and other phenomena that produces an error in ~;
a word of memory. Random access memories which must retain vital
data, even after being subjected to nuclear radiation while writ-
ing or restoring data, can be rendered immune to the radiation
~ . ~
efects, by storing small quantities of data doubly or txiply -
redundantly in the memory. This enables a single word loss to be ~i~
remedied by reference to other error-free copies of the affected
~, word. However, practical limitations on memory sizes prohibits :
the entire contents of a typical memory from being stored in a
double or triple redundant manner.
Data loss may be obviated in a random access memory
without the need for redundant storage, in the case of the fixed
data, by the use of the plated wire type non-destructive read-out
memory. If suitable circumvent circuitry is provided to protect ~;
the unaddressed words stored in the plated wire memory, the memory
will be safe rom the effects of nuclear radiation insofar as the
fixed data is concerned. This is because of the non-destructive
.:
read-out characteristics of this type of memory. The plated wire
memory, therefore, may be used for the storage of fixed data,
since under normal operation, the fixed data will be read only
from the memory and, due to the non-destructive read-out properties
o the memory, any word being read during a nuclear event can be
reconstituted from the memory itself Howe~er, the plated wire
~3~
~ , .
3~ "F'
memory does present problems when used for variable data storaye,
since it is vulnerable inso~ar as a word being written into
memory is concerned.
, . :
; Thus, the plated wire memory is not immune to the
e~ffects of nuclear radiation which occurs when a particular
variable data word is being up-dated, since such a particular
data word being written into the plated wire memory during a
nuclear event could be lost. Therefore, without further protec- ~
tion, the plated wire memory is not suitable for the storage of ~ -
variable data.
,i~ . ~, .
Moreover, the non-destructive read-out plated wire
memory is more costly than the destructive read-out magnetic core
type, and significant cost problems arise when the non-destructive - ~
read-out type of memory is used for the fixed data storage. The ~-
option of using the less expensive destructive read-out memory
~0 for fixed data storage presents a problem, however, since lt is
susceptible to the effects of nuclear radiation on both the words ~-
being written into the memory or read from the memory. This is
because the destructive read-out characteristics of the magnetic
core memory creates the need for restoring data after each read
out operation. This makes this type of memory vulnerable to the
radiation effects when data is being read from the me~ory or writ-
ten into the memory~ Redundant storage of all the contents of
a destructive read-out magnetic core memory is equally imprac-
tical.
~ :~0
,
: ~ . , .: ~. ,
. .
The present invention provides various embodimen~s of `
an improved 5ys~em for rendering the destructive read ou~ mag- ~` !
netic core random access memory immune to the effects of nuclear
radiation, and the like, insofar as the 105s of fixed or variable
data words is concerned. The various embodiments to be described
are applicable to the fixed data portion of the memories in which
certain invariable, fixed, program data words are stored, and
which remain unchanged throughout the entire computer program;
and certain embodiments are also applicable to the variable data,
or scratch pad, portions of such memories in which variable data
words are stored that are subject to up-dating, or other changes, ¦~
from time to time. -
~'
The system of the invention overcomes the most diffi-
cult problem of error correction due to nuclear radiation, or
other interfering radiation, in the less expensive destructive
read-out core memory, and of achieving this result in a relatively
simple, economical and straightforward manner.
!~ .
; ~ .
The basic problem of rendering a random access magnetic ;
memory immune to the effects of nuclear radiation is similar for
both the core memory and the plated wire memory. The primary
similarity lies in the fact that during a write or restore cycle,
it is extremely difficult to control the currents in either memory
to the precision required to guarantee a correct write or restore
operation. The primary difference between the core and plated wire
memories lies in the fact that during the read cycle of a core
memory, a read/restore operation i8 required so that the core
-5-
~3~1~
member is also susceptible to the efects of nuclear radiation
during the read operation, where the plated wire memory i5 not,
.
~ .
The system of the invention will be discussed in con-
junction with a random access core in which each memory element
is selected by an X-Y switchiny matrix, and by an applied inhibit
current (I)~ Such a memory is well known to the art and is
described, for example, at page 185, of Volume 4, McGraw Hill
Encyclopedia of Science and Technology (1960 Edition).
'`~
The protective system of the invention includes circum- ; ;
vent circuitry which responds to the detection o nuclear radia-
tion event to isolate magnetic core random access memory from
the effects thereof. The circumvent circuitry enables unaddressed
memory locations to be protected. However, it is extremely diffi-
cult by circumvent circuitry to control the required currents in
the memory should exposure to ~he nuclear radiation occur during
an actual write or read/restore operation. This means that the
word being read from the core memory, or written into the memory,
during the exposure may be lost. As described, the plated wire
memory operates on a non-destructive read-out basis which safe-
guards a stored word from being destroyed during a read cycle,
even if it is being read when a nuclear event occurs. In the case
of the magnetic core memory, however, the word desired in each
read operation must be restored in a subsequent write operation,
; so that the affected word may be lost during the presence of
nuclear radiation. Thus, additional means must be provided to
reconstruct words being accessed during the nuclear event. In
...
)73il4
, . .
i the case of the plated wire memory, the additional means is re-
quired only to reconstruct a word actually being written into the
memory during the nuclear event. In the case of the core memo~y,
however, the corrective measures must be taken with reepect to
words being read or written during the nuclear event. i~ ;
', ^ , . ' ':~ ~
The system of the inventionj as explained briefly above,
in one of its embodiments provides an error correction word for
each block of the fixed data words~ and this error correction word
serves to permit the destructive read-out core memory to meet all
:;
the performance and operational requirements of a radiation immune
system with respect to the fixed data program storage. A second
embodiment of the invention applies the error correction word con-
cept to the variable data storage situation; and a third embodi- ;
ment applies another error correction technique to the variable
data storage.
' , ... .
~' . '~ ":'.
~ `
BRIEF DESCRIPTION OE' THE DRAWINGS
.'
FIGURE 1 is a circuit diagram of circuitry for protect-
ing the inhibit portion of a random access memory system from the
effects of nuclear radiation;
FIGURE 2 is a circuit diagram of shunt circuitry for
;~ protecting the X-Y selection circuit of a random access memory
from the effects of nuclear radiation;
_7_
. -:
.
31~
~ FIGUl~ 3 is a diagr~m illu9 trating blocks o~ fixed ~ata
- words stored at known addresses in a memoryf and also illustrat-
ing a corresponding error correction word which is a1~Q ~tored in
a memory in which each bit is an "exclusive or sum" bit for the
corresponding bits of the various words in the block; ,~
FIGURE 4 is a table setting forth an example of word
recovery by a first embodiment of the invent:ion, using the error
correction word of FIGURE l;
FIGURE 5 is a functional block diagram of a first embodi-
ment of the invention, which may be used for word recovery with
respect to fixed data words; .
FIGURE 6 is a functional block diagram of a second
embodiment o the invention, in which the error correction word
is continuously up-dated for use in reconstituting variable data
words affected during a nuclear event;
'.'
FIGURES 7, 8 and 9 are flow diagrams of various routines
which may be instituted to carry out the word recovery action by
the system of FIGU Æ 6;
FIGURE lO is a block diagram of a further embodiment o
the invention, as applied to a plated wire random access memory
system for reconstituting a variable data word which may be
affected during a nuclear radiation event; and
:~ FIGURE ll is a block diagram'of a further embodlment of
the invention for reconstituting a variable data wordf as applied .
to a random access core memory,
-8
.
~ 3~
-- DETAILED DESCRIPTION OF THE ILLVSTRATED EMBODIMENTS
In order to prevent the unaddressed contents o a random
access memory from being altered during a nuclear radiation event~ ;
it is necessary to insure that excessive currents will not flow
in any of the selection lines of the memory. In general, this
means that the sum of all the currents (X, Y, I) through a par-
ticular memory element must be held until less than the maximum ~-
allowable "half select" current. Circumvent shunt circuits are
employed to divert the drive currents from the individual elements
; of the memory on all three axes, this being achie~ed by the cir~
cuits of FIGURES 1 and 2. These circuits prevent the disturbance
of all unaddressed memory elements during a radiation event. In
addition, the signal from the radiation detec~or Dl (FIGURE 2) is
used to turn ofE all activa circuits immediately after the nuclear
radiation pulse to prevent burn-out of the associated circuits and
circuit elements. The radiatlon detector Dl may be of any known
type which responds to the presence of nuclear radiation, or the
like, above a predetermined threshold to produce an output signal.
" !
The circuit of FIGURE 1 is incorporated into the drive
circuit fox the inhibit current I which is controlled to flow
through the memory stack to ground, during normal operation of
the memory system. The circuit include~ a usual grounded emitter
NPN transistor Q2 which responds to an inhibit control pulse I
applied to its base to become conductive and complete the base
circuit of a PNP transistor Ql. This causes transistor Q1 to be-
come conductive and to draw the inhibit current through the memorv
element~ in the memory ~tack Ml. The collector of the transistor
1~3~
-~ Q2 is connected to the base of transistor Ql through a resistor `
R2, the collector of the transistor Ql being connected to the
inhibit lead of the memory stack Ml, and its emitter is connected
to the positive terminal of a 12.5 volt c~ource. ~ resistor Rl
is connected to the base of transistor Q] and to the positive
terminal of ~he 12.5 volt source. The j~mction of the resistors :
Rl and R2 is connected to a diode CRl. A fast turn-off pulse is
applied to the diode to terminate the inhibit current flow through
the memory Ml at the end of the inhibit control pulse. ;~
~ .
During a nuclear event, transient leakage currents ipl,
ip2 and ip3 flow in the transistor Ql. The resistances of resis-
tors Rl and R2 are reduced to relatively low values to prevent
the transistor Ql from becoming conductive during a nuclear evenk,
and thereby to the current ipl through the memory during the nuc-
lear radiation pulse from reaching an appreciabla value.
In the case of the circuit of FIGURE 2, the circuitrv
of a pair of transistors Q3 and Q4 provides current shunts around
the X-Y switches of the memory selection network. In the event
o~ a nuclear radiation pulse, the resulting signals from the
detector D1 renders the transistors Q3 and Q4 conductive, so as
to establish shunts around the X-Y switching circuits, and there-
~; by to prevent the selection curxents in the memory Ml from rising
above a predetermined half-current threshold. The techniques of
`~ FIGURE 1 or 2 can be applied to either the X Y or inhibit currents.
--10--
-lV73~
Error correction in the case of the word being accessed
in the fixed data storage portion of the memory stack Ml, as car
ried out by the first embodimenk of the invention, is suCh tha~
the loss of a single data word from a known location in the fixed
data portion of the memory can be reconstructed.
Each block of fixed data words in the fix~d data portion
of the memory stack Ml, such as the block shown in FIGURE 3, has
a corresponding error correction word. In the correction code
illustrated here each bit of the error correction word is an
"exclusive or sum" bit for the corresponding column of bits of
the fixed data words in the corresponding block. As pointed out
previously two conditions are necessary in order for the word
recovery system o this embodiment to achieve its intended purpose.
These are that the error must be limited to a sin~le fixed data
word, and that the address of the affected fixed data word must
be known.
2G
; An example of word recovery in the fixed data portion
o~ the random access memory by means of the word recovery system
of the first embodiment is shown in the block diagram o~ FIGURE 4,
in which the block comprises four word~, as shown in the "Memory
Word Address" column, and in which the four words are located in
the fixed data storage portion of the memory at addresses aO~ al,
a2 and a3; and in which the address of the error correction word
PO is at any pred~termined memory location (S.D.). The block of
data shown in the column "Original Content", and the particular
block illustra ed in FIGURE 4 consists of four data word~ of four
- 1 1
~ 7311fl~ 1-
.,-", .
- bits each, followed by the error correction word in which each bit
is an even parity o the corresponding column of bit~ in the block~
': '
,'
Should a nuclear radiation pulse occur during the access~
I ing of the word stored at address a , it may be assumed that the
particular word is lost, as shown in the l'Altered Content" column.
It should be noted tha~ in the example under con~ideration, only
one word is affected by the radiation pulse, and that the address
of the affected word is known. Following the radiation pulsel the
affected word is accessed, and it is loaded into memory with all `~
zeros, as shown in the "Reconditioned Content" column. The ori
ginal word which was affected by the radiation pulse may now be
reconstructed by forming a computed error correction word, as an
"exclusive or sum" word of the entire block o data, including
the affected word (which is now zeros), plus the original error
correction word. This computed error correction word is a recon- ¦
struction of the word which was affected by the radiation pulse,
as shown in the "Corrected Content" column. Actually, the com- !
puted error correction word is the "exclusive or sum" of the block.
,
" . ,.,
There are several word recovery error correction sys-
tems which may be implemented to achieve the desired results of
~ the invention, that is, the recovery of a potentially lost word
-; when a radiation pul~e occurs during a write cycle or during a
read/restore cycle of a random access memory. Word recovery
with respect to the fixed data words may be carried out by the
error correction system represented by the first eMbodiment o
the inven~ionO In the first embodiment, all the ixed data memory
~12-
' ' 10~3~
j,
words are partitioned into bloc]cs in the memory stack, with each
block having a corresponding error correction word stQred at a
convenient address in the memory.
'''.
The re~uirement for rendering the fixed data in the
. . .
memory immune to radiation, in addition to the circumvention cir~
cuits of FIGURES 1 and 2, in accordance with the first embodiment,
:" ,
is ~he provision of a word recovery system which may be represen-
ted by the functional block diagram of FIGURE 5. The memory cir-
cumvention circuitry of FIGURES 1 and 2 assures ~hat only a single
memory location can be affected in the presence of a radiation
pulse, while the system of FIGURE 5 assures the recovery of the
potentially lost ixed data word whlch ~as being accessed when ~;
the radiation pulse occurred.
';
.~.
As pointed out above, a nuclear radiation pulse occurring
during a write ox a read/restore operation of a random access
memory can result in the loss of the word being accessed. All
that is required to recover the word by the system of FIGURE S i~ ~ `
that the address of the affected word be known, and that the
error correction word be available of the block in which the
affect0d word is located. ~he error correction word is normally
stored in an unussd portion of memory and is not accessed until
after a nuclear event, hence it will never be altered during an
event.
,
13
.
,~ ... . . .... . . .. .. . .
~ Y3~
The address o the aEfected word is held in a hardened
address register 10 in the system of FIGURE 5. Recovery of the
affected word then occurs in the normal radiation recovery rou~ine
of the computer, by loading zeros from a source 15 into the memory
location designated by the address in register 10. In this way,
the affected word is replaced by zeros. T~e computed error cor-
rection word is then formed in a hardened register 17, by feeding
all the words in the block containing the affected word, including
the affected word itself (which is now zeros), plus the original
error correction word (which has been stored in the memory at a
normally unused address), through an "exclusive or" summing net-
work l9 to the hardened register 17. The word formed in the har-
dened register 17 is a reconstruction of the affected word, and
it is introduced into the memory Ml at the memory location of the
affected word and replaces the zeros.
: :~
During the read/restore operation of a magnetic core
memory associated with the system of FIGURE 5, each address of
the successively accessed words is introduced from the computer
bus to the various modules of the memory Ml to a series of latch
; circuits represented by the block 12. The latch circuits 12 are
coupled to the hardened register 10 so that the address of each
fixed data word being accessed during a read/restore operation
is held in the regi$ter 10, while the corresponding data word,
applied to the data bus interface circuitry 14 during the opera-
tion, is processed. Then, should a radiation pulse occur during
the processing of any such fixed data word, its address is pre-
served in the hardened register 10, so that the above-described
word reconstruction operation can be caxried out~ The hardened
-14-
.'
3~4
. ,. ~ .
registers 10 and 17 can be formed of any suitable semiconductor,
magnetic or other device which will not be altered by the ma~
mum radiation. : .
The system shown in FIGURE 5, and the discussion up to
this point in the specification, presumes accessing fixed field
data from the fixed data portion of the memory. A system for
implementing word recovery for the variable data words is shown ;.
as a second embodiment of the invention in the functional block
: diagram of FIGURE 6. The system of FIGURE 6 includes the hardened
address register 10 of FIGURE 5, as well as the address latch ::-
circuits 12, the data bus interface 14, and the additional blocks
15, 17 and 19. In addition, the system of FIGURE 6 includes a
data input register 20, a data output register 22, "exclusive or"
.ogic 26.
The v~riable data word reconstruction system, as repre-
sented by the functional hlock diagram of FIGURE 6, is interposed
between the computer data and address buses and the memory Ml,
and it permits the retention in hardened address register 10 of
the memory address of the word being accessed, this address being
retained for the duration o the accessing cycle. The memory
circumvention circuitrv described above assures that only a.single
memoxy location can be affected by a radiation pulse, and the
hardened address register 10 assures retention of the address of
the affected data word. The input and output data r~gisters 20 ~:
and 22, in conjunction with the "exclusive or" logic 26 and Delta
correction word register 24, provide the capability to up-date
"~ , , - . - :
~ 3~4
' . .
continuously the hardened computed error correction register 17
for the variable data words. The system of ~IGURE 6 operates in
the s~me manner as the system of FIGURE 5, after a nuclear radia-
tion event to use the up-dated error correction word to recon-
struct the afected word.
It should be noted that it is necessary to change the
error correction word in two steps because it is affected by the
removal of one data word and its replacement by another in the
data block.
The hardenecl computed error correction word register 17
in the system of FIGURE 6 is a double register which is alternately
up-dated so that one register is not up-dated until the other has
been set so that radiation occurring during the up-date cycle will `
not destroy the correction word.
`
`~ 20
The radiation recovery operation may be controlled by
a sub-routine of the computer as a part o the normal radiation
recovery operation of the computer. The recovery action to be
taken is dependent upon whether a read/restore or clear/write cycle -
is in process when the radiation event occurs, and on the portion
of the cycle in which the radiation event occurred. The final
mechanication for the variable data does not affect the recovery
operations for the fixed data portion of the memory. Action taken
for the variable data portion of the memory depends upon the
~0~73i~
!
existence of a proper up-dated error correction word from the :.
hardened register 17 of the system of FIGURE 6.
.;
Actual memory cycle execution in conjunction with the
system of FIGURE 6 is best illustrated in the flow diagram of .:.'
FIGURE 7 for a read/restore memory cycle, and in the flow diagram
of FIGURE 8 for a clear/write memory cycle. The recovery operation
is depicted in the flow diagram of FIGURE 9. The flow diagrams
show the full approach to the mechanization of the error recovery
technique by the system of FIGURE 6. ::
Variable data word recovery is possible with respect l `
to the random access memories without the production and use of
error correction words~ such as was the case in the systems of
FIGURES 5 and 6. FIGURE 10 illustrates a system for protecting :
a plated wire random access memory during write operations, and :
FIGURE 11 shows a system for protecting a random access magnetic
core type of system during eitheF write or read/restore operations. .
.
,
In the case o the plated wire random access memory,
for example, and as shown in FIGURE 10, critical variable data
that must be retained in the event of a nuclear radiation event .
is normally partitioned, for example, into blocks within the .
memory, the blocks being designated here as A, B, C, D and E. :
A block buffer 100 is provided in the system. If the A block,
for example, is to be up-dated, the contents of this block are
loaded into the buffer 100, under the con~rol of appropriate
; 17 ..
~0~3
logic circuitry 102, so that the data may be redundan~ly stored
in the buffer 100 and in the memory block A.
Now, should a nuclear event occu:r while block A is being
up-dated, the original contents of block A are retrievable from
the bufer 100. In other respects, the system operates in a
manner similar to the previously described systems, and may use
the circumvent circuitry of FIGURES 1 and 2. Since during the
particular operation under consideration, :information is being
written only into the memory block A, the other memory blocks are
in the read/only mode and are protected by the circumvent cir-
cuitry. It is clear that the other blocks can be up-dated in a
similar manner, with each up-dating operation being preceded by
the loading of the contents of the par~icu.lar block in the buffer
100. ,'
: '
',:'
In the case of the random access magnetic core memory,
it is necessary to add an additional level of independent redun-
dant data storage, as shown in FIGU~E 11, in order to render the
memory immune from the effects of a pulse of nuclear radiation.
As shown in FIGURE 11, the memory may be divided into five primary
memory blocks A-E, each of which is wired to a corresponding
"write only" "shadow" memory block A~E. For up-dating purposes/
the contents of the particular primary memory block beiny up-dated
are loaded into the buffer 106, as in the system of FIGURE 10; and
simultaneously the contents of the corresponding shadow block are
loaded into the buffer 108. The particular block and its cor-
responding shadow block are then up-dated simultaneously. Then,
~18-
. '~ ' '
. ! . , ,' ,;, , .~ ., . . ~ .:, '., ; r
lOq311~ . ' -
as in the previously descxibed system, should a nuclear event
occur during the up-dating of any block, the original contents ;~
of the block are still retrievable from the shadow.
During normal read-out operatians, data is read from
the primary blocks only, and the shadow blocks are not accessed.
Then, should a nuclear event occur during a read/restore cycle,
the data is always retrievable by activating the corresponding
shadow block after the event has terminated. The shadow or
redundant memory block then performs all the above-descr:ibed
functions. - -
,:''
The shadow memory is controlled to write only during
normal operation of the computer. When a read-out is requested
at an address in this portion of the memory, only the primary
,'~' ! '
block cycles and replies. It is therefore evident that the only
data that may be corrupted during a nuclear event is that being
written in~o both primary and shadowj or that being read from, -
the primary blocks. The shadow memory blocks are never read
during a read cycle and, therefore, always retain their memo~
contents~ If a nuclear event occuxs, however, the output of the
radiation detector is used to change the mode relationship of
the two memory blocks of the affected word, so that the primary
block becomes a write-only memory and the shadow block becomes
the read/write memory.
"~
~ .
.
,.. . ~ . .. . ~ . . . .
~3~
,'~.
In the manner described abovs, therefore, by adding
current shun~ing circuitry, such as shown in FIGU~S 1 and 2,
to the random access'destructive read-out or random access memory,
it is possible to restrict any memory loss to a known addr~ss being
accessed at the time of the radiation event. The accessed word in
; the fixed memory or program storage can be reconstituted by the
use of block parity words, as described in conjunction with the
system of FIGURE 5. Likewise, the accessed word in the variable
data portion of the memory can be reconstituted by the various
systems described in FIGURES 6, 10 and 11. ~
',
The invention provides, therefore, an error correction
system which permits random access magnetic core memories to achieve
immunity to the ef~ects of nuclear radiation. The system of the
invention protects the unaddressed words in the memory from the
effects of such radiation, and it permits potentially affected
data words to be reconstructed.
,;
~
It will be appreciated that although various embodi- ;
ments of the invention have been shown and described, further
modi~ications may be made. For example, the systems described
herein may be adapted, .in whole or in part, to many different types
of memory systems, such as, plated wire, drum, and the like, pro-
viding the basic memory cell is radiation hard when not being
accessed. It is intended in the following claims to cover all
such modifications which come within the true spirit and scope
of the invention,
::
:~ :
-20-
