School of Physics Lecture Demonstration Manual

Eh-9 Core memory

Published: Thursday 07 June 2007 - Updated: Tuesday 29 March 2011


To demonstrate a piece of ferrite core memory from an old PDP11 computer.



Click on pictures to enlarge.

Core Memory Pic 1

Core Memory Pic 2

Core Memory Close Up Pic


Module 1 consists of 4 arrays of 32X64 ferrite beads. Module 2 consists of 36 arrays of 32X64 ferrite beads. In Module 1, each bead has three wires that run through it, vertically, horizontally and the sense wire giving 8192 bits of data. The word length used was 32 bits.

To write to a memory location a minimum current must flow through the ferrite core. Current in one direction will result in magnetic flux in a clockwise direction and opposite current will result in the second state or anticlockwise flux. The sense wire reads the state of the flux.

When in use, to alter the state of the magnetic flux the column and row wires each carry half of the current required to initiate the flux change. Core memory is non volatile i.e. it does not require refreshing power to hold a magnetic state.

Safety notes

Electrical Safety 


top of page