and other data that are to be used in the calculations. The  process  by  which  these  instructions  and  data  are read into the computer is called  loading. The first step in loading instructions and data into a computer is to manually place enough instructions into memory by using the console or keyboard. These instructions   are   then   used   to   bring   in   more instructions  as  desired.  In  this  manner,  a  few instructions are used to “bootstrap” more instructions. Some  computers  use  an  auxiliary  (wired)  memory, which  permanently  stores  the  bootstrap  program, thereby  making  the  manual  loading  unnecessary. These  instructions  may  be  stored  in  chips.  These chips  are  referred  to  as  “read  only  memories”  or ROMs. The memory section of a computer is essentially an  electronically  operated  file  cabinet.  It  is  actually  a large  number  of  storage  locations.  There  are generally between 1 and 40,000 locations. Each one is referred to as a storage address or register. Every computer word that is read into the computer during the  loading  process  is  stored  or  filed  in  a  specific storage address and is almost instantly accessible. The types of memory storage devices used most frequently  in  present-day  computer  technology  are magnetic  cores,  semiconductor,  thin  film,  magnetic drum, magnetic tape, and magnetic disks. Magnetic Cores One of the methods for storing internal data in a computer is realized by using magnetic cores. Cores are generally constructed by two methods. The first type of core, called a tape wound core, is fabricated by  wrapping  a  tape  of  magnetic  material  around  a nonmagnetic  toroidal  form.  A  toroid  is a term used to describe a doughnut-shaped solid object. The second type of core is called a ferrite core, and it is made by molding  finely  ground  ferrite  into  a  toroidal  form. The ferrite used in this application is a ceramic iron oxide  possessing  magnetic  properties.  The  ferrite particles  are  then  heat-fused  or  “sintered”  by  the application of heat and pressure. In magnetic core memories, each data bit is stored in the magnetic field of a small, ring-shaped magnetic core  (fig.  8-2).  Magnetic  cores  generally  have  four wires running through them. Two wires are used for READ selection. These same two wires are used for WRITE  by  reversing  the  direction  of  current  flow.  An inhibit  wire  prevents  writing  a  1  when  a  0  is  to written. The sense wire picks up the signal voltage generated  by  the  shifting  of  core  from  1  to  0  in  a READ  cycle. Since a single core stores only one bit of a word, a large number of cores are required to handle all the bits  in  every  word  to  be  stored.  These  cores  are arranged in arrays to assign memory address locations and quickly write data and locate data for read-out purposes.  The  technique  used  most  frequently  for writing and reading data in magnetic core arrays is known  as  the  coincident-current  technique. In  computer  memory  applications,  the  ferrite  core is magnetized by a flux field produced when a current flows in a wire (drive line) that is threaded through the core. The core retains a large amount of this flux when  the  current  is  removed.  Flux  lines  can  be established  clockwise  or  counterclockwise  around  the core, depending on the direction of the magnetizing current.  A  current  in  one  direction  establishes  a magnetization  in  a  given  direction.  Reversing  the direction of the current flow reverses the direction of the flux field and the core magnetization. These two unique states represents 0 and 1, respectively. Semiconductor Memories Semiconductor  memories  are  used  in  many modern    computers. Most  of  the  semiconductor memories  are  of  the  MOS  LSI  type,  which  may  be Figure 8-2.-Magnetic core showing X, Y, inhibit, and sense lines. 8-6