Hardwired control

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To execute instructions, a computer's processor must generate the control signals used to perform the processor's actions in the proper sequence. This sequence of actions can either be executed by another processor's software (for example in software emulation or simulation of a processor) or in hardware. Hardware methods fall into two categories: the processor's hardware signals are generated either by hardwired control, in which the instruction bits directly generate the signals, or by microprogrammed control in which a dedicated microcontroller executes a microprogram to generate the signals.

Before microprocessors, hardwired control usually was implemented using discrete components, flip-chips, or even rotating discs or drums. This can be generally done by two methods.

Method1: The classical method of sequential circuit design. It attempts to minimize the amount of hardwire, in particular, by using only log₂p flip flops to realize a p state circuit.

Method2: An approach that uses one flip flop per state and is known as one hot method. While expensive in terms of flip flops, this method simplifies controller unit design and debugging.

In practice, processor control units are often so complex that no one design method by itself can yield a satisfactory circuit at an acceptable cost. The most acceptable design may consist of several linked, but independently designed, sequential circuits.

Microprogramming made it possible to re-wire, as it were, a computer by simply downloading a new microprogram to it. This required dedicated hardware or an external processor. For example, some of DEC's PDP-10 processors used a PDP-11 as a front-end which uploaded a microprogram to the main processor at boot time.

Traditionally, a sewing machines' stitch patterns and a washing machine's wash programs were implemented as hardwired, usually mechanical, controls. In modern machines, these are instead implemented as software which controls a computer which controls the machine hardware. This makes it possible, for example, to download additional stitch patterns for a small fee or upgrade a machine without having to buy a complete new machine. It also opens up for intellectual property rights issues.