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ARITHMETIC-LOGIC UNIT
Magnetic Drums

Aviation Electronics Technician 1 (Organizational)
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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







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