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memory design


Modern nonmechanical memory

Now we can proceed to studying specific types of digital storage devices. To start, I want to explore some of the technologies which do not require any moving parts. These are not necessarily the newest technologies, as one might suspect, although they will most likely replace moving-part technologies in the future.

A very simple type of electronic memory is the bistable multivibrator. Capable of storing a single bit of data, it is volatile (requiring power to maintain its memory) and very fast. The D-latch is probably the simplest implementation of a bistable multivibrator for memory usage, the D input serving as the data “write” input, the Q output serving as the “read” output, and the enable input serving as the read/write control line:

If we desire more than one bit’s worth of storage (and we probably do), we’ll have to have many latches arranged in some kind of an array where we can selectively address which one (or which set) we’re reading from or writing to. Using a pair of tristate buffers, we can connect both the data write input and the data read output to a common data bus line, and enable those buffers to either connect the Q output to the data line (READ), connect the D input to the data line (WRITE), or keep both buffers in the High-Z state to disconnect D and Q from the data line (unaddressed mode). One memory “cell” would look like this, internally:

When the address enable input is 0, both tristate buffers will be placed in high-Z mode, and the latch will be disconnected from the data input/output (bus) line. Only when the address enable input is active (1) will the latch be connected to the data bus. Every latch circuit, of course, will be enabled with a different “address enable” (AE) input line, which will come from a 1-of-n output decoder:

In the above circuit, 16 memory cells are individually addressed with a 4-bit binary code input into the decder. If a cell is not addressed, it will be disconnected from the 1-bit data bus by its internal tristate buffers: consequently, data cannot be either written or read through the bus to or from that cell. Only the cell circuit that is addressed by the 4-bit decoder input will be accessible through the data bus.

This simple memory circuit is random-access and volatile. Technically, it is known as a static RAM. Its total memory capacity is 16 bits. Since it contains 16 addresses and has a data bus that is 1 bit wide, it would be designated as a 16 x 1 bit static RAM circuit. As you can see, it takes an incredible number of gates (and multiple transistors per gate!) to construct a practical static RAM circuit. This makes the static RAM a relatively low-density device, with less capacity than most other types of RAM technology per unit IC chip space. Because each cell circuit consumes a certain amount of power, the overall power consumption for a large array of cells can be quite high. Early static RAM banks in personal computers consumed a fair amount of power and generated a lot of heat, too. CMOS IC technology has made it possible to lower the specific power consumption of static RAM circuits, but low storage density is still an issue.

To address this, engineers turned to the capacitor instead of the bistable multivibrator as a means of storing binary data. A tiny capacitor could serve as a memory cell, complete with a single MOSFET transistor for connecting it to the data bus for charging (writing a 1), discharging (writing a 0), or reading. Unfortunately, such tiny capacitors have very small capacitances, and their charge tends to “leak” away through any circuit impedances quite rapidly. To combat this tendency, engineers designed circuits internal to the RAM memory chip which would periodically read all cells and recharge (or “refresh”) the capacitors as needed. Although this added to the complexity of the circuit, it still required far less componentry than a RAM built of multivibrators. They called this type of memory circuit a dynamic RAM, because of its need of periodic refreshing.

Recent advances in IC chip manufacturing has led to the introduction of flash memory, which works on a capacitive storage principle like the dynamic RAM, but uses the insulated gate of a MOSFET as the capacitor itself.

Before the advent of transistors (especially the MOSFET), engineers had to implement digital circuitry with gates constructed from vacuum tubes. As you can imagine, the enormous comparative size and power consumption of a vacuum tube as compared to a transistor made memory circuits like static and dynamic RAM a practical impossibility. Other, rather ingenious, techniques to store digital data without the use of moving parts were developed.

Historical, nonmechanical memory technologies

Perhaps the most ingenious technique was that of the delay line. A delay line is any kind of device which delays the propagation of a pulse or wave signal. If you’ve ever heard a sound echo back and forth through a canyon or cave, you’ve experienced an audio delay line: the noise wave travels at the speed of sound, bouncing off of walls and reversing direction of travel. The delay line “stores” data on a very temporary basis if the signal is not strengthened periodically, but the very fact that it stores data at all is a phenomenon exploitable for memory technology.

Early computer delay lines used long tubes filled with liquid mercury, which was used as the physical medium through which sound waves traveled along the length of the tube. An electrical/sound transducer was mounted at each end, one to create sound waves from electrical impulses, and the other to generate electrical impulses from sound waves. A stream of serial binary data was sent to the transmitting transducer as a voltage signal. The sequence of sound waves would travel from left to right through the mercury in the tube and be received by the transdcer at the other end. The receiving transducer would receive the pulses in the same order as they were transmitted:

A feedback circuit connected to the receiving transducer would drive the transmitting transducer again, sending the same sequence of pulses through the tube as sound waves, storing the data as long as the feedback circuit continued to function. The delay line functioned like a first-in-first-out (FIFO) shift register, and external feedback turned that shift register behavior into a ring counter, cycling the bits around indefinitely.

The delay line concept suffered numerous limitations from the materials and technology that were then available. The EDVAC computer of the early 1950’s used 128 mercury-filled tubes, each one about 5 feet long and storing a maximum of 384 bits. Temperature changes would affect the speed of sound in the mercury, thus skewing the time delay in each tube and causing timing problems. Later designs replaced the liquid mercury medium with solid rods of glass, quartz, or special metal that delayed torsional (twisting) waves rather than longitudinal (lengthwise) waves, and operated at much higher frequencies.

One such delay line used a special nickel-iron-titanium wire (chosen for its good temperature stability) about 95 feet in length, coiled to reduce the overall package size. The total delay time from one end of the wire to the other was about 9.8 milliseconds, and the highest practical clock frequency was 1 MHz. This meant that approximately 9800 bits of data could be stored in the delay line wire at any given time. Given different means of delaying signals which wouldn’t be so susceptible to environmental variables (such as serial pulses of light within a long optical fiber), this approach might someday find re-application.

Another approach experimented with by early computer engineers was the use of a cathode ray tube (CRT), the type commonly used for oscilloscope, radar, and television viewscreens, to store binary data. Normally, the focused and directed electron beam in a CRT would be used to make bits of phosphor chemical on the inside of the tube glow, thus producing a viewable image on the screen. In this application, however, the desired result was the creation of an electric charge on the glass of the screen by the impact of the electron beam, which would then be detected by a metal grid placed directly in front of the CRT. Like the delay line, the so-called Williams Tube memory needed to be periodically refreshed with external circuitry to retain its data. Unlike the delay line mechanisms, it was virtually immune to the environmental factors of temperature and vibration. The IBM model 701 computer sported a Williams Tube memory with 4 Kilobyte capacity and a bad habit of “overcharging” bits on the tube screen with successive re-writes so that false “1” states might overflow to adjacent spots on the screen.

The next major advance in computer memory came when engineers turned to magnetic materials as a means of storing binary data. It was discovered that certain compounds of iron, namely “ferrite,” possessed hysteresis curves that were almost square:

Shown on a graph with the strength of the applied magnetic field on the horizontal axis (field intensity), and the actual magnetization (orientation of electron spins in the ferrite material) on the vertical axis (flux density), ferrite won’t become magnetized one direction until the applied field exceeds a critical threshold value. Once that critical value is exceeded, the electrons in the ferrite “snap” into magnetic alignment and the ferrite becomes magnetized. If the applied field is then turned off, the ferrite maintains full magnetism. To magnetize the ferrite in the other direction (polarity), the applied magnetic field must exceed the critical value in the opposite direction. Once that critical value is exceeded, the electrons in the ferrite “snap” into manetic alignment in the opposite direction. Once again, if the applied field is then turned off, the ferrite maintains full magnetism. To put it simply, the magnetization of a piece of ferrite is “bistable.”

Exploiting this strange property of ferrite, we can use this natural magnetic “latch” to store a binary bit of data. To set or reset this “latch,” we can use electric current through a wire or coil to generate the necessary magnetic field, which will then be applied to the ferrite. Jay Forrester of MIT applied this principle in inventing the magnetic “core” memory, which became the dominant computer memory technology during the 1970’s.

A grid of wires, electrically insulated from one another, crossed through the center of many ferrite rings, each of which being called a “core.” As DC current moved through any wire from the power supply to ground, a circular magnetic field was generated around that energized wire. The resistor values were set so that the amount of current at the regulated power supply voltage would produce slightly more than 1/2 the critical magnetic field strength needed to magnetize any one of the ferrite rings. Therefore, if column #4 wire was energized, all the cores on that column would be subjected to the magnetic field from that one wire, but it would not be strong enough to change the magnetization of any of those cores. However, if column #4 wire and row #5 wire were both energized, the core at that intersection of column #4 and row #5 would be subjected to a sum of those two magnetic fields: a magnitude strong enough to “set” or “reset” the magnetization of that core. In other words, each core was addressed by the intersection of row and column. The distinction between “set” and “reset” was the direction of the core’s magnetic polarity, and that bit value of data would be determined by the polarity of the voltages (with respect to ground) that the row and column wires would be energized with.

The following photograph shows a core memory board from a Data General brand, “Nova” model computer, circa late 1960’s or early 1970’s. It had a total storage capacity of 4 kbytes (that’s kilobytes, not megabytes!). A ball-point pen is shown for size comparison:

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The electronic components seen around the periphery of this board are used for “driving” the column and row wires with current, and also to read the status of a core. A close-up photograph reveals the ring-shaped cores, through which the matrix wires thread. Again, a ball-point pen is shown for size comparison:

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A core memory board of later design (circa 1971) is shown in the next photograph. Its cores are much smaller and more densely packed, giving more memory storage capacity than the former board (8 kbytes instead of 4 kbytes):

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And, another close-up of the cores:

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Writing data to core memory was easy enough, but reading that data was a bit of a trick. To facilitate this essential function, a “read” wire was threaded through all the cores in a memory matrix, one end of it being grounded and the other end connected to an amplifier circuit. A pulse of voltage would be generated on this “read” wire if the addressed core changed states (from 0 to 1, or 1 to 0). In other words, to read a core’s value, you had to write either a 1 or a 0 to that core and monitor the voltage induced on the read wire to see if the core changed. Obviously, if the core’s state was changed, you would have to re-set it back to its original state, or else the data would have been lost. This process is known as a destructive read, because data may be changed (destroyed) as it is read. Thus, refreshing is necessary with core memory, although not in every case (that is, in the case of the core’s state not changing when either a 1 or a 0 was written to t).

One major advantage of core memory over delay lines and Williams Tubes was nonvolatility. The ferrite cores maintained their magnetization indefinitely, with no power or refreshing required. It was also relatively easy to build, denser, and physically more rugged than any of its predecessors. Core memory was used from the 1960’s until the late 1970’s in many computer systems, including the computers used for the Apollo space program, CNC machine tool control computers, business (“mainframe”) computers, and industrial control systems. Despite the fact that core memory is long obsolete, the term “core” is still used sometimes with reference to a computer’s RAM memory.

All the while that delay lines, Williams Tube, and core memory technologies were being invented, the simple static RAM was being improved with smaller active component (vacuum tube or transistor) technology. Static RAM was never totally eclipsed by its competitors: even the old ENIAC computer of the 1950’s used vacuum tube ring-counter circuitry for data registers and computation. Eventually though, smaller and smaller scale IC chip manufacturing technology gave transistors the practical edge over other technologies, and core memory became a museum piece in the 1980’s.

One last attempt at a magnetic memory better than core was the bubble memory. Bubble memory took advantage of a peculiar phenomenon in a mineral called garnet, which, when arranged in a thin film and exposed to a constant magnetic field perpendicular to the film, supported tiny regions of oppositely-magnetized “bubbles” that could be nudged along the film by prodding with other external magnetic fields. “Tracks” could be laid on the garnet to focus the movement of the bubbles by depositing magnetic material on the surface of the film. A continuous track was formed on the garnet which gave the bubbles a long loop in which to travel, and motive force was applied to the bubbles with a pair of wire coils wrapped around the garnet and energized with a 2-phase voltage. Bubbles could be created or destroyed with a tiny coil of wire strategically placed in the bubbles’ path.

The presence of a bubble represented a binary “1” and the absence of a bubble represented a binary “0.” Data could be read and written in this chain of moving magnetic bubbles as they passed by the tiny coil of wire, much the same as the read/write “head” in a cassette tape player, reading the magnetization of the tape as it moves. Like core memory, bubble memory was nonvolatile: a permanent magnet supplied the necessary background field needed to support the bubbles when the power was turned off. Unlike core memory, however, bubble memory had phenomenal storage density: millions of bits could be stored on a chip of garnet only a couple of square inches in size. What killed bubble memory as a viable alternative to static and dynamic RAM was its slow, sequential data access. Being nothing more than an incredibly long serial shift register (ring counter), access to any particular portion of data in the serial string could be quite slow compared to other memory technologies.

An electrostatic equivalent of the bubble memory is the Charge-Coupled Device (CCD) memory, an adaptation of the CCD devices used in digital photography. Like bubble memory, the bits are serially shifted along channels on the substrate material by clock pulses. Unlike bubble memory, the electrostatic charges decay and must be refreshed. CCD memory is therefore volatile, with high storage density and sequential access. Interesting, isn’t it? The old Williams Tube memory was adapted from CRT viewing technology, and CCD memory from video recording technology.

Read-only memory

Read-only memory (ROM) is similar in design to static or dynamic RAM circuits, except that the “latching” mechanism is made for one-time (or limited) operation. The simplest type of ROM is that which uses tiny “fuses” which can be selectively blown or left alone o represent the two binary states. Obviously, once one of the little fuses is blown, it cannot be made whole again, so the writing of such ROM circuits is one-time only. Because it can be written (programmed) once, these circuits are sometimes referred to as PROMs (Programmable Read-Only Memory).

However, not all writing methods are as permanent as blown fuses. If a transistor latch can be made which is resettable only with significant effort, a memory device that’s something of a cross between a RAM and a ROM can be built. Such a device is given a rather oxymoronic name: the EPROM (Erasable Programmable Read-Only Memory). EPROMs come in two basic varieties: Electrically-erasable (EEPROM) and Ultraviolet-erasable (UV/EPROM). Both types of EPROMs use capacitive charge MOSFET devices to latch on or off. UV/EPROMs are “cleared” by long-term exposure to ultraviolet light. They are easy to identify: they have a transparent glass window which exposes the silicon chip material to light. Once programmed, you must cover that glass window with tape to prevent ambient light from degrading the data over time. EPROMs are often programmed using higher signal voltages than what is used during “read-only” mode.

Memory with moving parts: “Drives”

The earliest forms of digital data storage involving moving parts was that of the punched paper card. Joseph Marie Jacquard invented a weaving loom in 1780 which automatically followed weaving instructions set by carefully placed holes in paper cards. This same technology was adapted to electronic computers in the 1950’s, with the cards being read mechanically (metal-to-metal contact through the holes), pneumatically (air blown through the holes, the presence of a hole sensed by air nozzle backpressure), or optically (light shining through the holes).

An improvement over paper cards is the paper tape, still used in some industrial environments (notably the CNC machine tool industry), where data storage and speed demands are low and ruggedness is highly valued. Instead of wood-fiber paper, mylar material is often used, with optical reading of the tape being the most popular method.

Magnetic tape (very similar to audio or video cassette tape) was the next logical improvement in storage media. It is still widely used today, as a means to store “backup” data for archiving and emergency restoration for other, faster methods of data storage. Like paper tape, magnetic tape is sequential access, rather than random access. In early home computer systems, regular audio cassette tape was used to store data in modulated form, the binary 1’s and 0’s represented by different frequencies (similar to FSK data communication). Access speed was terribly slow (if you were reading ASCII text from the tape, you could almost keep up with the pace of the letters appearing on the computer’s screen!), but it was cheap and fairly reliable.

Tape suffered the disadvantage of being sequential access. To address this weak point, magnetic storage “drives” with disk- or drum-shaped media were built. An electric motor provided constant-speed motion. A movable read/write coil (also known as a “head”) was provided which could be positioned via servo-motors to various locations on the height of the drum or the radius of the disk, giving access that is almost random (you might still have to wait for the drum or disk to rotate to the proper position once the read/write coil has reached the right location).

The disk shape lent itself best to portable media, and thus the floppy disk was born. Floppy disks (so-called because the magnetic media is thin and flexible) were originally made in 8-inch diameter formats. Later, the 5-1/4 inch variety was introduced, which was made practical by advances in media particle density. All things being equal, a larger disk has more space upon which to write data. However, storage density can be improved by making the little grains of iron-oxide material on the disk substrate smaller. Today, the3-1/2 inch floppy disk is the preeminent format, with a capacity of 1.44 Mbytes (2.88 Mbytes on SCSI drives). Other portable drive formats are becoming popular, with IoMega’s 100 Mbyte “ZIP” and 1 Gbyte “JAZ” disks appearing as original equipment on some personal computers.

Still, floppy drives have the disadvantage of being exposed to harsh environments, being constantly removed from the drive mechanism which reads, writes, and spins the media. The first disks were enclosed units, sealed from all dust and other particulate matter, and were definitely not portable. Keeping the media in an enclosed environment allowed engineers to avoid dust altogether, as well as spurious magnetic fields. This, in turn, allowed for much closer spacing between the head and the magnetic material, resulting in a much tighter-focused magnetic field to write data to the magnetic material.

The following photograph shows a hard disk drive “platter” of approximately 30 Mbytes storage capacity. A ball-point pen has been set near the bottom of the platter for size reference:

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Modern disk drives use multiple platters made of hard material (hence the name, “hard drive”) with multiple read/write heads for every platter. The gap between head and platter is much smaller than the diameter of a human hair. If the hermetically-sealed environment inside a hard disk drive is contaminated with outside air, the hard drive will be rendered useless. Dust will lodge between the heads and the platters, causing damage to the surface of the media.

Here is a hard drive with four platters, although the angle of the shot only allows viewing of the top platter. This unit is complete with drive motor, read/write heads, and associated electronics. It has a storage capacity of 340 Mbytes, and is about the same length as the ball-point pen shown in the previous photograph:

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While it is inevitable that non-moving-part technology will replace mechanical drives in the future, current state-of-the-art electromechanical drives continue to rival “solid-state” nonvolatile memory devices in storage density, and at a lower cost. In 1998, a 250 Mbyte hard drive was announced that was approximately the size of a quarter (smaller than the metal platter hub in the center of the last hard disk photograph)! In any case, storage density and reliability will undoubtedly continue to improve.

An incentive for digital data storage technology advancement was the advent of digitally encoded music. A joint venture between Sony and Phillips resulted in the release of the “compact audio disc” (CD) to the public in the late 1980’s. This technology is a read-only type, the media being a transparent plastic disc backed by a thin film of aluminum. Binary bits are encoded as pits in the plastic which vary the path length of a low-power laser beam. Data is read by the low-power laser (the beam of which can be focused more precisely than normal light) reflecting off the aluminum to a photocell receiver.

The advantages of CDs over magnetic tape are legion. Being digital, the information is highly resistant to corruption. Being non-contact in operation, there is no wear incurred through playing. Being optical, they are immune to magnetic fields (which can easily corrupt data on magnetic tape or disks). It is possible to purchase CD “burner” drives which contain the high-power laser necessary to write to a blank disc.

Following on the heels of the music industry, the video entertainment industry has leveraged the technology of optical storage with the introduction of the Digital Video Disc, or DVD. Using a similar-sized plastic disc as the music CD, a DVD employs closer spacing of pits to achieve much greater storage density. This increased density allows feature-length movies to be encoded on DVD media, complete with trivia information about the movie, director’s notes, and so on.

Much effort is being directed toward the development of a practical read/write optical disc (CD-W). Success has been found in using chemical substances whose color may be changed through exposure to bright laser light, then “read” by lower-intensity light. These optical discs are immediately identified by their characteristically colored surfaces, as opposed to the silver-colored underside of a standard CD.

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