20.5. Micro-program Concept

In this unit, we focus on generating control signals using a micro-programmed approach, which is flexible compared to hardwired control units that generate signals from a dedicated circuit.

The micro-program control is a system that generates control signals required for the operation of the computer. Unlike hardwired control units, which are fixed and fast but inflexible, micro-programmed control allows for the control signals to be defined in a program-like manner. This flexibility comes at the cost of slower operation since it relies on accessing memory to fetch the required signals.

Think of hardwired control as a traditional watch with mechanical gears. It works very fast, but you cannot easily change its functionality. Meanwhile, micro-programmed control is like a smartwatch that runs applications. You can add new apps and features, but it might take longer to process given the reliance on software.

Micro-program memory is allocated separately for micro programming. Control signals are stored in specific memory locations, allowing them to be easily accessed and modified.

Micro-program memory serves as a dedicated storage space for control signals, similar to how a normal computer memory stores data and programs. Each word in this memory corresponds to certain control signals; for instance, bits are set to '1' or '0' to determine the action to be taken. When a micro instruction is executed, the relevant memory location is accessed, and the control signals are activated to carry out the instruction.

Imagine a library filled with books (micro-program memory) where each book contains detailed instructions (control signals) for various tasks. When you need to complete a task, you simply go to the specific book, read the instructions, and follow them. This system allows different tasks to be executed flexibly, much like accessing different books for various subjects.

Generating control signals in a micro-programmed control unit is straightforward. Memory values are fetched, and the corresponding signals are output without the need for complex circuitry.

In a micro-programmed control unit, generating control signals is relatively simple. The required values are read from memory, and these values dictate what control signals are activated. This is in contrast to hardwired systems, where signals depend on the intricate design of circuits and finite state machines.

Consider a vending machine where pressing a button (fetching memory values) results in a specific product being dispensed (generating control signals). The machine operates based on a straightforward mechanism – you select an item, and the corresponding mechanism is triggered without any complicated internal workings.

Sequencing in micro-programmed controls is a step-by-step process, but it can become complicated when jump instructions are needed based on specific conditions.

The control signals in a micro-program are usually executed in a sequential manner. For example, you might go from step 1 to step 2 and then to step 3. However, if a jump instruction is encountered, it necessitates checking certain conditions, which can complicate the sequencing process. This is where a micro-program counter, which functions similarly to a regular program counter, comes into play, managing which step to follow next.

Imagine following a recipe (micro-program) where typically you'd follow one step after another. But if you find that one ingredient is missing, the recipe instructs you to jump to a different part that may require alternative ingredients or steps. This additional decision-making complicates your cooking process, much like the sequencing in a micro-programmed control unit during a jump instruction.

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