Diode matrix
Encyclopedia
A diode matrix is a two-dimensional grid of wires, where each "intersection" where one row crosses over another either has a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

 connecting them, or the wires are isolated from each other.
It is one of the most popular techniques for implementing a read-only memory.

A diode matrix is used as the control store
Control store
A control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. Early control stores were implemented as a diode-array accessed via address decoders, a form of read-only memory. This tradition dates back to the program timing...

 or microprogram in many early computers.
A logically equivalent transistor matrix is still used as the control store or microprogram or decode ROM in many modern microprocessors.

At any one instant, a single row of the diode matrix (or transistor matrix) is activated.
Charge flows through each diode connected to that row.
That activates the column corresponding to each row.
The only activated control signals during that instant were those whose corresponding column wire was connected with a diode to that row.

History

Diode matrix control circuits were invented by Bashir Rameyev
Bashir Rameyev
Bashir Iskandarovich Rameyev was a Soviet inventor and scientist, one of the founders of Soviet computing, author of 23 patents, including the first officially registered in the USSR patent in the field of electronic computers—a patent for the Automatic Electronic Digital Machine...

 in (year).

A diode matrix ROM was used in many computers in the 1960s as well as electronic desk calculator
Calculator
An electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...

s and keyboard encoders for terminal
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...

s.

The microsequencer
Microsequencer
In computer architecture and engineering, a sequencer or microsequencer is a part of the control unit of a CPU. It generates the addresses used to step through the microprogram of a control store....

 of many early computers, perhaps starting with the Whirlwind (computer)
Whirlwind (computer)
The Whirlwind computer was developed at the Massachusetts Institute of Technology. It is the first computer that operated in real time, used video displays for output, and the first that was not simply an electronic replacement of older mechanical systems...

,
simply activated each row of the diode matrix in sequence,
and after the last row was activated, started over again with the first row.

The technique of microprogramming was first described by Maurice Wilkes in terms of a second diode matrix added to a diode matrix control store.

Later computers used a variety of alternative implementations of the control store, but eventually returned to a diode matrix or transistor matrix.

A person would microprogram the control store on such early computers by manually attaching diodes to selected intersections of the word lines and bit lines.
(In schematic diagrams, the word lines are usually horizontal and the bit lines are usually vertical).

The control store on some minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...

s was one or more programmable logic array
Programmable logic array
A programmable logic array is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output...

 chips.
The "blank" PLA from the chip manufacturer came with a diode matrix or transistor matrix with a diode (or transistor) at every intersection.
A person would microprogram the control store on these computers by destroying the unwanted connections at selected intersections.

Some modern microprocessors and ASICs use a diode matrix or transistor matrix control store.
Typically a blank grid is designed with a diode (or transistor) at every intersection,
and then a mask is prepared that leaves out the unwanted connections at selected intersections.
When reverse engineering integrated circuits that include such a mask-programmed decode ROM,
one of the key steps is to take photographs of that ROM
with enough resolution to separate each intersection site
and enough color depth to distinguish between the "connected" and "not connected" intersections.

Since the control store is in the critical path of computer execution, a fast control store is an important part of a fast computer.
For a while the control store was many times faster than program memory, allowing a long, complicated sequence of steps through the control store per instruction fetch, leading to what is now called complex instruction set computing.
Later techniques for fast instruction cache sped that cache up to the point that the control store was only a few times faster than instruction cache, leading to fewer and eventually only one step through the control store per instruction fetch in reduced instruction set computing.

A keyboard matrix (music)
Keyboard matrix (music)
Most electronic keyboards used in synthesizers, electronic organs, and digital pianos use a keyboard matrix circuit to connect the switches for each key. In this matrix circuit, the rows and columns are made up of wiring...

has a very similar grid of diodes, but is used differently.
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