static   or   dynamic. MOS   means   metal   oxide semiconductor, and LSI means large scale integration. Thin Film Thin   film   memory   consists   of   Permalloy,   a ferromagnetic  material,  deposited  on  a  supporting material (substrate) of thin glass. This is done under controlled  conditions  in  a  vacuum  chamber.  When  all air  has  been  removed  from  the  chamber,  a  shutter arrangement  is  opened,  and  vapors  from  molten Permalloy pass through a mask and are deposited on the supporting material. The pattern thus formed is determined by the shape of the mask. The thickness of  each  spot  (magnetized  area)  is  controlled  by  the amount of time the shutter is open. A  magnetic  field  is  applied  parallel  to  the  surface of  the  substrate  during  deposition.  The  film  spots become easier to magnetize in the direction parallel to that in which the magnetic field was applied during the  deposition  process.  This  direction  is  known  as  the preferred  direction;  likewise,  the  axis  of  this magnetism is called the preferred axis. Magnetic  Drums The magnetic drum storage device is a cylinder that  rotates  at  a  constant  velocity.  Information  is written on or read from the drum when its magnetic surface  passes  under  magnetic  heads,  which  are similar to the magnetic heads found on commercial tape  recorders. Magnetic drums provide a relatively inexpensive method of storing large amounts of data. A magnetic drum (fig. 8-3) is made from either a hollow cylinder (thus the name drum) or a solid cylinder. The cylinder may  consist  entirely  of  a  magnetic  alloy,  or  it  may have  such  an  alloy  plated  upon  its  surface.  Many drums are made by spraying on magnetite, an iron oxide. The surface is then coated with a thin coat of lacquer,  and  buffed. Representative  drums  have  diameters  ranging from 12.7 to 50.8 centimeters (about 5 to 20 inches respectively). The surface of the drum is divided into tracks or channels that encircle the drum. A number of READ and WRITE heads are used for recording and reading. There is at least one head per track. The drum is rotated so that the heads are near, but not touching, the drum surface at all times. As the drum rotates, the tracks are continuously passing  under  their  respective  head.  Each  track  is subdivided  into  cells,  each  of  which  can  store  one binary bit. All the cells that are positioned under the heads of a multitrack drum at the same time are called a “slot.” With some drums, each head reads or writes one bit of a word. Thus, when a word is written into or read from a slot, each track contains one bit of that word. The number of heads used depends on the size of the word that the computer is designed to handle. One of the tracks provides timing signals for the drum  rotation. The  timing  track  determines  the location of each set of storage cells around the drum. Each  timing  signal  denotes  a  unit  of  time  of  drum rotation. For example, if the timing track is 80 inches long and timing signals are recorded at 120 pulses per inch, there are 9,600 locations for bit storage on the track. If the drum has 32 tracks in addition to the timing track, the drum has the capacity to store a total of 307,200 bits. Some  drums  use  two  or  even  three  timing  tracks. The   timing   tracks   are   used   for   synchronization purposes  and  are  sometimes  called  “control”  or “clock” tracks. The timing pulses establish the time scale to which all circuits through the computer are synchronized. The retrieval of data from a rotating drum can be a  rather  involved  process,  as  can  be  realized  by drawing   a   comparison   to   the   core   memory   of   a computer. When core memory is used, all the data is stored in the cores in a static condition. The data can be located at a given place at any instant, and can be easily read from that location in serial or parallel form Figure 8-3.-Magnetic drum. 8-7