Twistor is a form of computer memory formed by wrapping magnetic tape
around a current-carrying wire. Operationally, twistor was very
similar to core memory. Twistor could also be used to make ROM
memories, including a re-programmable form known as piggyback twistor.
Both forms were able to be manufactured using automated processes,
which was expected to lead to much lower production costs than
1 Core memory
1.1 Construction 1.2 Data retrieval 1.3 Manufacturing
2.1 Operation 2.2 Permanent magnet twistor 2.3 Piggyback twistor
3 Applications 4 References 5 External links
Core memory Main article: Magnetic core memory Construction
Diagram of a 4×4 plane of magnetic core memory in an X/Y line coincident-current setup. X and Y are drive lines, S is sense, Z is inhibit. Arrows indicate the direction of current for writing.
In core memory, small ring-shaped magnets - the cores - are threaded
by two crossed wires, X and Y, to make a matrix known as a plane. When
one X and one Y wire are powered, a magnetic field is generated at a
45-degree angle to the wires. The core magnets sit on the wires at a
45-degree angle, so the single core wrapped around the crossing point
of the powered X and Y wires will be affected by the induced field.
The materials used for the core magnets were specially chosen to have
a very "square" magnetic hysteresis pattern. This meant that fields
just below a certain threshold will do nothing, but those just above
this threshold will cause the core to be affected by that magnetic
field. The square pattern and sharp flipping states ensures that a
single core can be addressed within a grid; nearby cores will see a
slightly different field, and not be affected.
The basic operation in a core memory is writing. This is accomplished
by powering a selected X and Y wire both to the current level that
will, by itself, create ½ the critical magnetic field. This will
cause the field at the crossing point to be greater than the core's
saturation point, and the core will pick up the external field. Ones
and zeros are represented by the direction of the field, which can be
set simply by changing the direction of the current flow in one of the
In core memory, a third wire - the sense/inhibit line - is needed to
write or read a bit. Reading uses the process of writing; the X and Y
lines are powered in the same fashion that they would be to write a
"0" to the selected core. If that core held a "1" at that time, a
short pulse of electricity is induced into the sense/inhibit line. If
no pulse is seen, the core held a "0". This process is destructive; if
the core did hold a "1", that pattern is destroyed during the read,
and has to be re-set in a subsequent operation.
The sense/inhibit line is shared by all of the cores in a particular
plane, meaning that only one bit can be read (or written) at once.
Core planes were typically stacked in order to store one bit of a word
per plane, and a word could be read or written in a single operation
by working all of the planes at once.
Between reads or writes the data was stored magnetically. This means
that core is a non-volatile memory.
Manufacturing core was a major issue. The X and Y wires had to be
threaded through the cores in a weave pattern, and the sense/inhibit
line passed through every core in a plane. In spite of considerable
effort, no one successfully automated the production of core,[citation
needed] which remained a manual task into the 1970s. To increase
memory density one had to use smaller cores, which greatly increased
the difficulty of wiring them onto the lines. Although the density of
core increased many times over its operational lifetime, the per-bit
cost of core remained steady.
Twistor was similar in concept to core memory, but replaced the
circular magnets with magnetic tape to store the patterns. The tape
was wrapped around one set of the wires, the equivalent of the X line,
in such a way that it formed a 45-degree helix. The Y wires were
replaced by solenoids wrapping a number of twistor wires. Selection of
a particular bit was the same as in core, with one X and Y line being
powered, generating a field at 45 degrees. The magnetic tape was
specifically selected to only allow magnetization along the length of
the tape, so only a single point of the twistor would have the right
direction of field to become magnetized.
The original twistor system used permalloy tape wrapped around a 3 mil
copper wire. For any given length of wire, the tape was wound up over
only the first half. The copper wire was then bent at the point where
the tape ended, and ran back alongside the portion with the tape,
forming a return conductor. This meant all the connections were at one
end. Several such twistor lines were laid side-by-side and then
laminated into a PET film plastic sheet, with the twistors and their
return wires about 1/10th of an inch apart. A typical tape might have
five twistor wires and their returns, so the sheet was just over an
inch wide. The solenoid was similarly constructed, consisting of a
number of 0.15 inch wide copper tapes laminated into a plastic tape of
the same basic dimensions as the twistor. Unlike a traditional
solenoid with many turns of wire around an open core, this system was
essentially nothing more than single wires in a sheet of plastic.
To build the complete memory system, a sheet of the solenoid was laid
out flat, say along the X direction, and then a sheet of the twistor
was laid on top at right angles to it along the Y axis. The solenoid
tape was then folded over, so that it wrapped the twistor sheet,
producing a series of U-shaped solenoids. Now another layer of the
solenoid tape is laid over the first, the twistor tape folded over so
it now runs along the negative Y axis across the top of the new
solenoid tape, and then the solenoid tape is folded over to form a
second set of loops. This process continues until the twistor strip is
"used up", forming a compact cube of memory. Along one side of the
memory, connected to each of the solenoid loops, was a series of small
cores used solely for switching (their original purpose, development
as a memory came later).
The main reason for Bell's development of twistor is that the process
could be highly automated. Although the folding process that completed
the twistor might be carried out by hand, the layup and laminating of
the sheets was easily handled by machine. Improved versions of twistor
also wrapped the section of bare copper initially used solely for the
return path, thereby doubling density without any changes to the
Writing to twistor was effectively identical to core; a particular bit
was selected by powering one of the twistor wires and one of the
solenoid loops to ½ of the required power, such that the required
field strength was created only at the intersection of the two.
Reading used a different process. Unlike core, twistor did not have a
sense/inhibit line. Instead, it used a larger current in the solenoid,
large enough to flip all of the bits in that loop, and then used the
twistor wires as the read line.
Twistor was thus read and written one plane at a time, rather than in
core, where only one bit per plane could be used at once.
Permanent magnet twistor
Twistor could be modified to produce a ROM that could be easily
re-programmed. To do this, one-half of each solenoid loop was replaced
with an aluminum card into which tiny vicalloy bar magnets were
embedded. As the solenoids have to be complete circuits in order for
current to flow through them, they were still inserted as folded
sheets, but in this case the loop was inserted between the folds of
twistor instead of around them. This allowed the single sheet to act
as one half of a solenoid loop for two folds of the twistor, above and
below. To complete the loop, the card of magnets was placed on the
other side of the twistor tape.
Reads were performed by powering the solenoid to a point about half of
that needed to produce a write. This field was "reflected" by the
aluminum sheet, closing the loop, magnetically. The resulting field
was greater than the write strength, causing the permaloy state to
flip. If the bit was beside an unmagnetized bar magnet in the card,
the field was not opposed and the flip caused a current pulse in the
twistor wire, reading a "1". However, by magnetizing the bar at that
bit, the bar magnet opposed the field being created by the solenoid
current, causing it to be below the write strength, and preventing the
flip. This read a "0".
The permanent magnet twistor (PMT) was re-programmed by removing the
plates and placing them over a custom writer.
Vicalloy was used
because it required much more power to re-magnetize than the permaloy
tape, so that the system would never come close to re-setting the
permanent magnets while in use in the memory system. The writer system
used much larger currents that overcame this resistance.
The PMT that was used in the 1ESS system used modules with 128 cards
with 2880 magnets on each. This produced a module with 8096 44-bit
words. The complete store used 16 modules for a total of 131,000
words, or 731,500 bytes.
Another form of twistor ROM replaced the permanent magnet cards with a
second magnetic tape wrapped around the first on the twistor lines, in
a "piggyback" configuration. This tape was coated with coballoy
instead of permaloy, which is much "harder" magnetically, requiring
about twice the field in order to flip. To make the system even
harder, the coballoy tape was about two and a half times thicker than
the permaloy one, so the resulting field strength was five times. The
external current required to flip the state of the coballoy tape was
about 15 times that of the normal operational current.
Read operations in the piggyback are identical to the permanent magnet
version. Writes were slightly more complex, due to the fact that
piggyback twistors all featured the magnetic tape along the entire
length of the X wire. This meant that any one solenoid was wrapping
both the bit that is being written as well as the one on the section
of return wire. To set the one both and not the other, the solenoid
was first powered in one direction and then the other, while the
current in the twistor line remained constant. This created two
magnetic fields in turn, one aligned with the first section of wire
and then the second. All reads and writes were carried out on paired
bits in this fashion.
Twistor was used in a number of applications. Much of the development
funding was supplied by the US Air Force, as twistor was to be used as
the main memory in the
LIM-49 Nike Zeus
"Telephone Memory Devices",
Stress insensitive permalloys for memory application Memory Units[dead link] - a general discussion of computer memory systems written in the late 1960s, which includes a discussion of twistor. EETimes - Misunderstood Milestones
v t e
Ferrite core (1949)
Hard disk (1956)
Stripe card (1956)
Thin film (1962)