20.3.6. Comparison with Hardwired Control

Basically, in the last unit we have seen that basically how to generate the control signals, if the hardware for this is exactly fabricated as a hard-coded non-modifiable circuit. Which you actually call as the hardwired control unit, that that is the sequence of micro instructions and the control signals to be generated corresponding to that can be generated using a hard-coded circuit, which is synthesized from its finite state machine control. So, that part we have seen that the circuit we generates out of this is a non-flexible circuit, but it is very fast, but it is non-flexible and it cannot be changed.

Hardwired control units are circuits designed to produce control signals using fixed wiring or logic gates, which make them very efficient and fast. The downside is that once they are constructed, they cannot be modified or reprogrammed. This means that for any change in functionality, a physical alteration of the circuit would be necessary. In essence, they can only execute the sequences they were designed for, which corresponds to a finite state machine model.

Imagine a traditional washing machine with mechanical buttons for specific cycles (wash, rinse, spin). Once built, if you wanted a new cycle to be added, you would have to replace the entire control system, similar to how a hardwired control unit requires physical changes if you want to change its operations.

In the next units basically, that is on micro-program control, that is these 2 units. So, basically, we are going to study how the same thing that is generation of the control signals can be done in a more flexible way and in terms of a program. So, it is not it’s very similar to what we understand by a normal computer program, which we have already looking throughout these lectures. But instead of the macro instructions we will be using micro instructions, and that control we will actually call micro-program based control.

Microprogrammed control units allow for the generation of control signals through a programmable memory, which means they can easily adapt to changes. Unlike hardwired units, microprogrammed control units use sequences of micro instructions that can be modified by altering the program stored in memory. Thus, changing control signals or functionalities can be done without physical modifications to hardware. This flexibility makes them suitable for various tasks and operations.

Think of this like software on your smartphone. You can download apps or update the operating system to add new features or fix bugs. In contrast to the washing machine example, your smartphone allows you to change how it operates without needing to replace any physical components.

If the control signals are generated from a dedicated circuit we call it as a hardwired control. And alternative approach is basically which we can generate such signals which are basically programmed into some kind of a memory. So, it is if it is a memory-based logic in which each of the memory cells or each of the memory word has the corresponding control signals to be generated, then that approach is actually called the micro-programmed control unit.

Control signals dictate how processors and computer systems operate at a low level. In hardwired systems, these signals are predetermined and essentially fixed in the hardware. In contrast, microprogrammed control units store the required control signals in memory cells. When needed, these signals can be retrieved and executed, allowing flexible programming and execution flow based on the current needs of the system rather than being hard-coded.

Consider a programmable thermostat. It can learn from user settings and environmental changes, adjusting the temperature based on a programmed schedule rather than just being stuck at a single preset temperature. This is akin to how microprogrammed units can adapt control signals based on programmed data.

So, this actually unit is focused on the generating of the control signals using a micro-programmed control which is more flexible, you can change it, but of course it will be slower than a hardwired circuit.

While microprogrammed control units provide greater flexibility, this adaptability often comes at the cost of speed. Hardwired controls are quick because they operate through fixed circuits without the need to fetch instructions from memory. Microprogrammed units must retrieve and interpret sequences from memory, which can introduce delays in execution, making them slower in certain applications. Understanding this trade-off is crucial when designing systems that require either speed or flexibility.

Think about driving a standard car vs. a hybrid vehicle. The standard car is straightforward and quick, just like a hardwired control unit; it has fewer components to slow it down. On the other hand, hybrids are designed for efficiency and flexibility in how you drive and use fuel, similar to microprogrammed controls, which can switch between modes but may lag behind in pure speed.

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