20.1.1. Lecture - 21

Welcome to the 7th unit of the module we are discussing, that is on the control circuitry of the computer. So, this is 7th unit in which we are going to study about micro instructions and micro-program.

In this section, we begin our exploration of how computers manage control signals using microinstructions and microprograms. These elements play a crucial role in the operation of the control circuitry within a computer. Essentially, microinstructions provide the necessary directives for what the computer should do during each cycle of operation, akin to a script that the computer follows.

Consider microinstructions like the steps in a recipe. Just as a recipe details each step to create a dish, microinstructions guide the operations of the computer's components to achieve a specific task.

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. [...] In fact, for a given sequence of micro instructions and the control signals correspondingly we generate a finite state machine, and then we synthesis the circuit out of it.

This chunk contrasts two methods of generating control signals in computers: hardwired control and microprogrammed control. Hardwired control utilizes fixed circuitry, which is fast and efficient but inflexible; once designed, it cannot be modified. On the other hand, microprogrammed control offers flexibility by using a set of stored instructions (microinstructions) that can be altered, albeit at the cost of decreased speed.

Think of hardwired control like a traditional traffic light system where the signal lights are wired to change in a predetermined manner. In contrast, microprogrammed control is akin to a traffic system that can adapt its signal patterns based on live traffic data, allowing for dynamic responses.

So, basically a micro program consists of a sequence of instructions, and basically these instructions are nothing but which are the basically a micro-program corresponding to a basically a sequence of micro operations that is very well known, that a macro instruction has some micro instructions [...] you rather write into 1 word of the memory.

A microprogram is a sequence of micro instructions that together dictate a set of operations required to execute a macro instruction (a higher-level command). Each operation corresponds to control signals stored in memory, which are activated during processing to execute the desired task.

Imagine programming a robot to perform a dance. Each micro instruction would represent a specific movement (turning, jumping, clapping), and the collected sequence creates the whole dance routine. Instead of changing the wiring each time, you simply change the dance instructions in its program.

So, if we can put these control signals explicitly in some memory locations, then when you access that word of the memory such the corresponding control signals will become 1 and 0 as required.

This chunk explains how microprogrammed control units work. By storing control signals in memory, each signal can be accessed when required, allowing the computer logic to easily switch signals on or off (1s and 0s). This method facilitates a straightforward approach to generating control outputs.

Consider a light switch that can turn on different lights based on a control panel. Each button on the panel represents a specific memory location, and pressing a button sends a signal to turn a specific light on or off, similar to how control signals operate in a microprogram.

And therefore, you have to go in sequence that is one job of that logic, and then basically sequence I will tell you what is sequencing and control signal generation. It has 2 parts basically that is this control signals has to be generated in sequence.

The generation of control signals involves two main tasks: sequencing through the microinstructions and generating the corresponding control signals. Sequencing refers to the orderly execution of instructions stored in memory, while control signal generation is about firing those signals as needed during operation. This concept underscores the importance of the sequence of operations in computer processes.

Think of this as a conductor leading an orchestra. The conductor (microprogram counter) ensures that each musician (microinstruction) plays their part at the right moment, orchestrating a harmonious performance (the final output of the computer's operation).

But, whenever there is a jump instruction and you require also to check some flag conditions, which is simpler in finite state machine based approach. But here, we have to have separate arrangement.

Conditional branching in microprogrammed control poses extra challenges compared to hardwired systems. While hardwired systems easily transition between states based on fixed circuits, microprogrammed systems need to evaluate conditions and potentially redirect execution according to the results of those evaluations, which adds complexity.

Imagine navigating through a maze where you might encounter dead ends. If your path (microprogram) was predetermined (hardwired), you would move swiftly through the maze. However, if paths can change based on obstacles (conditions), you need to assess your surroundings continuously (conditional branching), complicating your route.

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