LARGE-SCALE STORAGE AND MEMORY
4.2 MAGNETIC STORAGE
Magnetic storage takes two principal forms: one stores the individual bits in separate cores, as described in 2.1.5 and figure 3-33 of Chapter 2; the other stores each bit by magnetizing a separate, tiny spot of a mag-netic material coated on the surface of a plastic tape or a metal drum. In both forms of storage, the magnetic field that is left (remanent flux) after writing the in-formation indicates by its direction (polarity) whether a 1 or a 0 is stored.
The magnetic material coated on the surfaces of tapes or drums must, like the core materials, have a nearly rectangular hysteresis loop so that it will retain most of the flux impressed upon it after the magnetizing force has been removed. Thus, the magnetized portion of the material acts like a permanent magnet, the direc-tion of whose field can be reversed by applying a sec-ond magnetizing force of sufficient strength. This ex-ternal force is usually a temporary magnetic field about a coil through which a pulse of current is passed. The magnetic coating for tapes is normally one of several iron oxides, finely powdered and mixed with a binder or
MAGNETIZED SPOT
COIL
adhesive that is dried under controlled conditions to hold the oxide particles in a thin, even film or coat.
The drum surface is usually a plating of a metallic alloy, such as nickel-cobalt.
A coating of such a material on a surface that is relatively flat does not form a closed magnetic circuit for small fields (as the closed ring of a core does), so separate areas of the surface can be magnetized in oppo-site polarities without interfering with each other, as long as there is sufficient distance between them. If the applied magnetizing force is kept in a very small field, only a correspondingly small spot of the tape or drum coating is magnetized and more bits can be stored on a surface of given size. To magnetize different spots, either the coil providing the magnetizing force or the coated surface could be moved, but in practice the surface is always moved past the stationary coil at a constant speed, and writing and reading are done with the sur-face in motion. Long lengths of magnetic tape are wound on compact reels and pulled past the coils used for reading and writing (called magnetic heads). A magnetic drum revolves on its axis, passing its coated cylindrical surface under fixed heads.
The problem of holding the applied magnetizing force or field to a very small area is solved in the follow-ing manner. It is known that if a coil is wound on one side of a rectangular or ring-shaped core of magnetic material, as in figure 3-83, the core forms a closed magnetic circuit. When the coil is energized, the field set up finds it much easier to complete its circuit through the core material than through the surrounding air;
hence virtually all the lines of force remain in the core.
If the core is cut through at one point, forming a gap, the flux lines jump across the gap to complete their circuit.
MOVING MAGNETIC SURFACE
.. CORE
Figure 3-83. Magnetic Head
114
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PART 3 offers greater reluctance (magnetic resistance) than the core, the field tends to spread out, or expand in surface. When the short-duration pulse disappears, the motion of the surface pulls the spot away and then
When the small magnetized spots representing stored bits of information are passed again under the is nondestructive readout; i.e., the stored information remains on the tape or drum. saturation, then reducing the field strength gradually to zero. At saturation, the field overcomes all previously and writing. This mechanism must be capable of moving the tape at a rigidly controlled speed for writing or reading operations, which generally require careful tim-ing. It must also be capable of very fast starts and drive mechanism perform the desired operations when called for by the computer. A few manual controls are and reading, synchronizing, etc. One basic arrangement of a tape storage element appears in figure 3-84.
Magnetic tape is excellent for storing large amounts of information whenever rapid access is not required. If the information desired is at the opposite end of the tape from that under the heads, it is generally a matter of seconds before it is reached, during which time the average computer could perform thousands of opera-tions. For this reason, access is usually programmed instantaneously to start or stop commands.
When information recorded on the tape has to be
Magnetic Drums 4.2.1-4.2.2
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Arithmetic programs and data, programs for checking the operation of parts of the computer, lengthy mathe-matical tables, and other input information can be made available to the computer in this manner.
4.2.2 Magnetic Drums
Though tapes are valuable for storing large amounts of information, when it is essential to write and read information at frequent intervals and in ran-dom order, magnetic drums offer much faster access times, commonly ranging from 10 to 40 milliseconds.
Because the information is stored on the surface of a cylinder revolving under fixed magnetic heads, the drum provides a form of cyclic storage (once written, a word comes back under the heads on every revolution).
As the drum rotates, the area in which a single fixed head can write or read is only a very narrow strip-called a track or channel-running around the circumference of the drum. Information can be stored in serial form simply by sending serial words to the single head while the drum revolves (translating O's and l's to current pulses of the proper polarities). The bits of each word are then stored as a sequence of mag-netized spots along the single channel running around the drum.
Another common storage method is parallel stor-age, shown in figure 3-85. To store a 5-bit word by this process, five heads are lined up side by side, each writing in a separate channel. The translated current pulses representing the bits of the word are sent in parallel form to the heads and the bits are written simultaneously. Now, the bits are stored as a row of magnetized spots in ad jacent channels. So, in this method, the registers are strips of drum surface run-ning toward the ends of the drum and including as many channels as there are bits in the computer word.
READ-WRITE CIRCUITRY
In the example of figure 3-85, a register stretches across five channels. The band of registers extending completely around the drum is called a field.
Typical drums used with the AN/FSQ-7, -8 meas-ure 10.7 inches in diameter and 12.5 inches long. Using 33-bit words, one of these drums holds six fields of 2,048 registers each, for a total storage capacity of 12,288 words.
Locating a given register or group of registers on the rotating drum to read or write information requires some means of keeping track of the drum position.
One common method uses a special timing channel in which is written either a series of l's or a regularly repeated combination of l's and O's. These bits are read by the timing channel head and used to synchro-nize the access circuitry with the drum rotation and to locate registers by a cycling count. A special combina-tion of l's and O's at one point on the track can be used as an index mark to tell the circuits that a new revolution of the drum is beginning.
In this method, each register in a field has its own address, and a given register is located by first selecting the proper field (by switching connections to the heads), then selecting the register by address. The method, sometimes called address selection, requires a circuit arrangement similar to that shown in figure 3-86.
It is entirely possible, of course, to write informa-tion on the drum with one set of heads and to read it with another set positioned over the same field. One reason for doing this may be to use the drum as a time buffer, or isolating device, between the fast-operating computer and much slower input or output devices.
With this method, writing is a matter of looking for a register in which to put new information and
CONTROL CIRCUITRY
MANUAL CONTROL
DATA
INSTRUCTIONS
J
COMPUTER TOFigure 3-84. Basic Tape Storage Arrangement
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Figs. 3-85 & 3-86
SINGLE
REGISTER TIMING
CHANNEL ADDITIONAL FIELDS
figure 3-85. Storage on Magnetic Drum
COMPUTER
READ-WRITE CIRCUITRY
ADDRESS CONTROL
FIELD SELECTION
TIMING
FIELD
TIMING CHANNEL
figure 3-86. Address Selection of Drum Registers
reading is concerned only with taking out information that has not been read before. Each set of circuitry, therefore, is interested in the status (state or condition) of each register in the field. Furthermore, each side is able to tell the other what it needs to know. This can be done by using two extra drum channels for con-trol, a write channel and a read channel (fig. 3-87).
When the writing side has information to store, it must locate one or more registers that are empty; that is, contain information that can be written over because is has already been read. To indicate the latter, the reading side inserts a 1 in the write channel each time its reads a register. Thus, when the writing side finds a 1 in the write channel, it is free to write fresh informa-tion into the corresponding register. As it does this, it inserts a 1 in the read channel, telling the reading side that this register now contains information that has not been read. A 0 in either channel tells the circuits
that the corresponding register should not be written in or read.
This process is called writing or reading by status.
Timing is necessary, as shown in figure 3-87, but in this case only to synchronize the access circuits to the drum. Writing and reading are not done continuously.
When input information is available, the writing side finds empty registers and writes it into them. When the computer wants more input information, the reading side locates registers containing it and reads it out to the computer. In output operations, of course, the com-puter side does the writing and the output side the reading.
Status and address are two of the principal meth-ods of reading and writing on drums, using them either as the main memory or as auxiliary storage. There are a great many possible variations in the details, using either method.
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Magnetic Cores 4.2.3
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:3
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INPUT DEVICE
WRITE CIRCUITRY
STATUS CIRCUITRY
READ CIRCUITRY
STATUS CIRCUITRY
COMPUTER
Figure 3-87. Writing and Reading by Status 4.2.3 Magnetic Cores
Ferrite cores are probably the best present-day de-vices for use in comparatively large, fast-access memory elements. No other device offers faster access time and the core registers can all be reached with equal speed and ease.
The theory of operation of cores has been covered in Chapter 1, including the description of core regis-ters. These discussions were concerned primarily with tape cores, but all the principles apply equally well to the ferrite cores used in the memory element. The two types differ physically in that the ferrite core is a ceramic-like material, rather than a metal, and can be made very small for memory use. The required coil can be replaced by a single wire threaded through the open center of the core.
When a number of core registers are grouped to-gether, as must be done in the memory element, the problem of reaching the individual cores of a given register to insert or remove a word becomes a little more difficult than in a single register. The principle emphasized in Chapter 1 is used to solve this-the prin-ciple that a magnetizing force of H is more than enough to be certain of switching a core, while a force of H/2 definitely will not switch it. These forces must be closely controlled, which can be done by controlling the cur-rents used to set up the fields. If a current, I, sets up a field of force H about a conductor, then a current of 1/2 will set up a field of H/2.
Using this principle, four registers grouped side by side in a 2-dimensional array (sometimes called a memory plane) are shown in figure 3-88. This is called a 4-by-4 array because there are four cores on a side.
Yz
\..~--+T- ADDRESS I
Xz ADDRESS 2
.... ...,::r-.--- ADDRESS 3
ADDRESS 4
Figure 3-88. Core Memory Plane
Each horizontal row of cores is a separate, 4-bit regis-ter. The leads labeled X and Yare the wires that make up the core windings. The X windings of all cores in a single register are in series, while the Y windings of the cores in each vertical column are series-connected. Each X line, therefore, selects a particular register, and each Y line selects a certain bit position in all registers.
To write the word 0001 in the register at address 3, for example, a current pulse of 1/2 is placed on the Xa line to select the desired register (all registers are
as-118 UNCLASSIFIED
PART 3 anyone piece of information might be needed a number of times in the course of a program. The solution is to write each word back into the register from which it came immediately after reading it out. At the same time, of course, it is also sent to the arithmetic element or wherever it is needed. The necessity for rewriting each time a word is read out slows the access time since the memory circuits cannot handle another operation until rewriting is completed. Nevertheless, the total access time using ferrite cores is usually 10 microseconds or processed in whatever manner is necessary, another de-mand pulse is sent to the ring counter and the word in the second register is read out. The memory plane, used in this fashion, can accept words at the fast rate dictated by the computer and give them to other
cir-119
Magnetic Cores sequence of events with minor variations, somewhat simplifying the access and control circuitry.
For a reading operation, the desired register is re-ceive new information unless the programmer knows that the word in it is no longer needed, or it is empty.) During each memory cycle, therefore, the selected regis-ter is read and then written into, whether the opera-tion called for is reading or writing.
The single memory plane under discussion thus far is not large enough to provide the amount of storage space needed in a big computer, although-depending upon word. length - a large number of registers can be reg-isters, as previously described, each address must specify the number of the X line and the number of the plane.
This method, however, is likely to lead to complicated wiring within the plane itself. The alternative method is the one shown in figure 3-89. Here, each bit position received from the operation-address register in the con-trol element must be decoded or translated into a pair of one-hot signals, one to select the X line, the other the Y line. This decoding is commonly done in a matrix of some sort. If a new word is to be written into
stor-120 UNCLASSIFIED
PART 3 an-other memory operation, completing the memory cycle.
In this manner the ferrite core memory array han-dles the storage requirements of the computer quickly and efficiently.