20.1.2. Microinstructions and Microprograms

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.

This section introduces the study of microinstructions and microprogramming, which are essential concepts in understanding the control circuitry of a computer. In this unit, the focus is on how control signals within a computer can be generated and managed, specifically through the use of microinstructions and a microprogram.

Think of a microprogram as a recipe used in cooking. Just as a recipe outlines the ingredients and steps to prepare a dish, a microprogram outlines the control signals and steps needed to execute a specific operation within a computer.

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...

In previous lessons, we learned about hardwired control units, where control signals are generated through non-changeable circuits based on finite state machines. Though this method is fast, it lacks flexibility, meaning any changes to control signals require significant hardware alterations.

Imagine a vending machine programmed to release only certain snacks. If you want to add a new snack option, you'd need to physically rewire the machine, which is cumbersome. This is similar to a hardwired control unit that lacks flexibility.

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...

Microprogrammed control units offer a more flexible approach, allowing control signals to be generated through programming rather than fixed hardware circuits. Much like coding in a programming language, programmers can write instructions (microinstructions) that dictate the required control signals, leading to a more adaptable computing architecture.

Think of microprogramming like writing a custom script in a software application. Instead of modifying a complex machine structure, you adjust the script as needed to perform different tasks, highlighting the ease of changes in microprogrammed systems.

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...

A microprogram consists of a sequence of microinstructions that correspond to specific microoperations required to execute a larger instruction. Each microinstruction is stored in a dedicated memory (micro-program memory), where specific control signals are organized logically to facilitate execution. This design allows for easy access and modification of control signals as they are stored in memory like any conventional data.

Consider a playlist of songs saved on a music app. Each song represents a microinstruction, and the playlist is the micro-program memory. You can play, skip, or reorder songs (microinstructions) freely, illustrating the flexibility of microprogrammed systems compared to fixed hardware setups.

So, control signal generation, generation of the control signal is very simple, that is just this values has to be dumped from the memory and it has to go to the corresponding locations...

The process of generating control signals in a microprogrammed control unit is straightforward. The control signals are fetched directly from the micro-program memory, meaning that the system can quickly output the necessary signals without the complexity of a hardwired approach. This results in a simpler, more efficient signal generation process.

Imagine a light switch system in a house. If you have a remote that can turn on different lights in different rooms without needing to connect each light with wires, it's like how microprogrammed systems can activate control signals quickly just by retrieving them from memory.

Sequencing actually slightly tricky, which is somewhat very easy in a finite state machine approach because there you have the flow of states which can take care very easily...

Sequencing in microprogrammed control units is more nuanced compared to hardwired systems. When executing control signals, the system generally moves from one microinstruction to the next in a sequential manner. However, conditional jumps or branching within the control signal generation require that the system evaluates flags and conditions to determine the next microinstruction, adding complexity.

Think of reading a book. If you come across a chapter where you are told to jump to a conclusion based on certain decisions in earlier chapters, you have to remember where to go based on previous content. This jump requirement in microprogramming alters the normal flow, similar to how microinstruction sequencing demands condition checking before progressing.

So, basically micro-program based control is very simple, whatever signals you require you directly put them in the memory location...

While microprogrammed control units offer significant flexibility by allowing changes to the control signals through programming, they also introduce some complexities in sequencing and branching. The micro-programmed architecture enables modifications without significant hardware changes, making it easier to adapt to new requirements compared to hardwired systems.

If you were to change a game’s rules, it’s much simpler to adjust the game manual (microprogram) than to redesign the game pieces (hardwired). Hence, microprogramming gives users the ability to enhance or adapt systems more easily while requiring careful management of the execution flow.

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