The
Memories Papers |
Magnetic
Storages |
Magnetic
Disks |
|
Optical
discs |
||
|
Return
to the synopsis |
To
contact the author |
Low
of page |
Created it, 06/09/09
Update it, 06/09/26
N° Visitors
As we saw in the history of theory 1, the concept of memory is an old and general concept.
Just as the books (i.e. the writing) which is a collective memory of the man, the first memories of even industrial technical use are already old. They are the punched paper tapes of the barrel organ or the Jacquard loom. An electronic rocker or a switch is also elementary memories as we will see it.
1. - MEMORIES
1. 1. - DEFINITIONS
Memory : One calls memory, any system allowing to store an information and to have which it thereafter: for example a bit, a byte (eight bits) or more generally a word of n bits or Byte.
Capacity : One calls capacity of a memory the number of bits which it can preserve : one generally expresses it in bytes or kilooctets (1024 bytes) or in megabytes (1024 kilooctets).
One distinguishes the or not formatted theoretical capacity of formatted capacity. The latter is the capacity really usable after deduction of bits of service necessary in certain technologies.
Access time : (Access time). One calls access time time necessary to go to read or write information in memory.
Cycle time : Time separating two successive operations from reading or writing.
Mass memory : It is characterized by its great capacity and generally by a high access time.
Main memory of calculation : It is generally characterized by its more limited capacity and a very short access time. The contents of the mass memories and the main memories will be the subject of a study of the course microprocessor.
Volatile memory : A memory is known as bird when it loses its information in the absence of power supply.
Address : In a memory, the data (succession of 1 bits 1 or 0) are stored in fictitious boxes whose sequence number is the address of the box considered. One calls the operation of research related to the use of the address : addressing.
Memory with access series : To read them, it is necessary to read information in the order where they were written.
Random access memory : The access to information made there not in a predetermined order.
In data processing, many systems to memorize information were successively used.
There are two groups of memories : electronic memories using a mechanical drive and memories.
Among the memories using a mechanical drive, one can distinguish several kinds from them : here are the principal types.
1. 2. - THE MEMORIES PAPERS
1. 2. 1. - METAL, BUT MYLAR, PAPER PUNCHED TAPES ALSO (FIGURE 1).
The punched tape is provided in its medium with a sprocket channel. The code corresponding to a character is registered transversely thanks to perforations which represent the bits with 1 or to 0. The number of perforations (generally bit with 1) is even or odd, to this end a channel is reserved for the bit of parity or disparity.
There are many codes developed by each manufacturer. This system which was very widespread on the electromechanical invoicing machines is still used on certain machines with numerical control (milling machines, turns, etc…).
It is slow, its access is series : i.e. it is necessary to read all the band in the order, before reaching the required data. It is gradually replaced by the diskettes.
1. 2. 2. - PUNCH CARDS: 80 COLUMNS TYPE I.B.M. (FIGURE 2)
The codes are carried out by means of rectangular perforations according to a principle similar to that of the punched tape, by an electromechanical puncher or burning ; the reading is carried out by means of feelers, of brushes, by detection of dielectric variations, or optical character reading.
This system is in the process of disappearance because it is too slow and not very practical, it dates from the whole beginning of the century.
This system, like the precedent, allows only one writing (there is no question of stopping the holes !).
1. 3. - MAGNETIC STORAGES
1. 3. 1. - MAGNETIC TAPES A NUMERICAL CODING
On these magnetic tapes, one writes the information in the form of 1 and of 0.
a) Magnetic tape
The magnetic tape consists of a plastic mylar support covered with chromium or iron oxide in fine agglomerated particles with a binder.
It can be conditioned in band, in bulk, or in cassettes.
b) Principle of the writing
On the tape, the magnetic field produced by the electromagnet in the air-gap “e” makes it possible to direct the oxide particles which preserve after the stop of the excitation, a remanent field (figure 3).
These remanent fields can contrary being transformed into pulses, it is what occurs to the reading.
There are various codes of writing. As on the punched tape, a code is written transversely on several longitudinal tracks.
The data are put in the form of blocks of 80 or 200 characters (figure 4).
In order to detect the reading and clerical errors, one uses a bit of parity (or of disparity) which gives the transverse parity, like two called additional characters LRCC (Length Redundancy Character Check), in French, control character of longitudinal parity and CRCC (Cyclic Redundancy Character Check), in French, control character of redundancy cyclic.
These the last two characters with the writing, are deduced from the data appearing in the record, by means of a logical operation ; by making with the reading the opposite operation, one can know if the record were indeed written correctly on the tape.
1. 3. 2. - MAGNETIC TAPES A CODING IN SOUND FREQUENCY (STANDARD KANSAS CITY)
This mode of writing is used mainly in the systems general public and all the cases in the systems of low capacities (computers or automatisms).
One uses a standard cassette (standard Philips) or a cartridge of the 3M type (figure 5) on which one converts bits 1 and 0 into sound frequencies (figure 6).
4 periods of 1 200 Hz account for the 0
8 periods of 2 400 Hz account for the 1.
This type of writing is thus analogical.
These memories are of series type ; the reading and the writing are possible there.
1. 3. 3. - MAGNETIC CYLINDERS (ANCESTORS OF THE MAGNETIC DISKS) (FIGURE 7)
They are abandoned today.
1. 3. 4. - MAGNETIC DISKS
This technique in full evolution tends to supplant all the other systems of memories to mechanical drive.
The magnetic medium is composed of a hard aluminum disk or a mylar floppy disc, one like the other covered with a magnetic oxide coating, itself covered with an anti-static and lubricating layer.
There are various types of disc :
a) Simple discs
Irremovable hard disks known as WINCHESTER.
Discs with fixed heads: they are fast but very expensive and today abandoned (figure 8).
Hard disks with mobile heads simple face (figure 9).
Floppy discs or diskettes known as “Floppy discs” with a diameter 8 inches, 5 inches 1/4, 3 inches 1/2 (figure 10).
b) The multiple discs or stackings of still called discs “say-packs” (figure 11).
One calls “say-pack” a whole of several hard disks connected by their center. As for the diskettes, there are several diameters of discs : 14 inches, 8 inches, 5 inches, 3 inches.
Foot-note : 1 inch (English measurement) = 2,54 cm.
1. 3. 5. - ORGANIZATION OF A MAGNETIC DISK (FIGURE 12)
The general organization is common to all the discs.
A disc includes/understands concentric tracks divided into sectors as represented figure 12. On each segment thus determined, one can write a certain number of bytes (in general 128 to 256).
Each segment is characterized by an address including/understanding the number of the track and that of the sector like in the case of discs doubles face or of multiple discs, the number of the face used.
To go to read on the disc, it will be thus necessary to move the read head to position it on the selected sector and the track. It will be said that the system is addressed.
The displacement of the read head is ordered by means of a suitable mechanism. You defer on figures 9 and 10.
The writing itself is carried out according to the same general principle as that used for the writing on tape, but this time, information being written only on only one track, the bits are written in series the ones behind the others. There is also a control character similar to the CRCC.
Figure 13 makes it possible to compare the characteristics of various types of mass memories.
| Support | Standard capacity (bytes) | Average access times in seconds |
| Punch card (80 columns with 600 charts / mn) | 80 | 150 |
| Punched tape | 120 K | 500 |
| Magnetic card | 5 000 | 10 |
| Magnetic cassette | 300 K | 10 to 100 |
| Minicassette | 64 K | 20 to 150 |
| Diskette (1 face, simple density) | 300 K | 0,45 |
| Minidiskette | 100 K | 0,8 |
| Magnetic cartridge | 2 M | 20 to 60 |
| Minis cartouche | 270 K | 15 |
| Mini hard disk 8 inches | 5 M | 0,1 |
| Hard disk 14 inches | < 5 M | 0,08 |
1. 4. - OPTICAL DISCS
These systems are used for the memorizing of the sounds, they tend to replace the old discs with analogical, said recording microgrooves 33 or 45 turns / minute which function thanks to the piezoelectric effect.
The aural signal to record is sampled then quantified and recorded in numerical form. The 1 and the 0 thus obtained are materialized by the absence or the presence of holes of 0,5 µm of width and 0,1 µm of depth engraved on a nickelled synthetic matter disc. The bits are written in series according to a track in spiral as in a traditional microgroove, as represented figure 14. This writing is carried out by dieing as for the long-playing records.
The reading is carried out thanks to the beam of coherent light emitted by a laser like illustrating figure 15. This one comes to cross the transparent protective coating covering the disc and to be reflected on the metal film.
An electric eye analyzes the considered beam and can thus detect the presence or the absence of holes on the portion of explored track.
The signal highlighted by the photoelectric sensor appears in the form of 1 and of 0.
These memories with reading authorized only, are destined for other developments such as the memorizing of images (Philips Videodisk) or to even memorize any type of information.
These memories are currently with access series.
|
Click
here for the following lesson or in the synopsis envisaged to this end. |
High
of page |
 Preceding
page |
Following
page |
