21.2.1. Micro-Program Organization

So, basically the micro-programmed control unit ensures that the signal will be generated in correct sequence. Sequencing here is a bit tricky, because generating control signals is directly they are encoded, directly they are actually put in the memory location.

Micro-programmed control units generate control signals in a specific order to ensure that the operations of a computer are performed correctly. These control signals are pre-encoded and stored in memory. When an instruction is executed, the control unit accesses the corresponding signals from memory, ensuring that the correct sequence of operations is followed.

Think of a recipe in a cookbook where each step must be followed in order. If the recipe says to mix ingredients before baking, performing those steps in a different order could lead to a failed dish. In the same way, a micro-programmed control unit must generate control signals in the correct sequence to execute instructions properly.

Sequencing is actually very important here because, many times we will depend on the condition codes and status flags.

Sequencing becomes critical when the execution of instructions depends on various conditions, identified by status flags like zero flag or carry flag. These flags inform the control unit about certain states within the system. Based on these conditions, the control unit may need to jump to a different instruction rather than just proceeding to the next one in line.

Imagine you're driving and you come across a traffic light. If the light is green, you go straight, but if it's red, you must stop. Similarly, the control unit checks the status flags (like traffic lights) to determine whether to proceed with the next instruction or take a different path.

So, you have to have also arrangement for something called a branch decision. In right normal programs we have micro instructions which actually which is conditional, and also, we have something which is unconditional.

In micro-programming, some instructions (called micro instructions) direct the control unit to check conditions before proceeding (conditional) while others proceed without checking (unconditional). When a conditional micro instruction is executed, software must specify which micro-program memory address to jump to based on the results of previous conditions.

Consider choosing a snack: if you’re not allergic to nuts, you reach for a nut bar (conditional). Otherwise, you grab an apple (unconditional, because you always eat it if the nut bar isn’t an option). The choice is based on your situation, similar to how the micro-program decides its next action based on flags.

So, we will take some examples then it will be more clear. For the time being let’s take the fact that instruction fetch, decode and execute.

Every instruction in a micro-program goes through three phases: fetching the instruction, decoding it, and executing it. The fetch phase retrieves the micro instructions related to the instruction. The decode phase interprets which operation is to be performed. Finally, the execution phase performs the required action based on the decoded instruction.

Think of learning a new dance move: first, you watch a video to learn (fetch), then you understand the steps (decode), and finally practice it to perform the dance (execute). This three-step process ensures you master the move effectively, just like the control unit processes instructions.

So, micro-program memory architecture and normal memory architecture there is not much difference, they are almost the same thing.

Micro-program memory is structured similarly to regular memory but is specifically allocated for storing micro instructions that correspond to macro instructions. Each instruction has a specific section in this memory, allowing for organized access and execution of the appropriate micro instructions as needed.

Imagine a library where books are sorted by genre. Each genre (like fiction, non-fiction) has its own section—this organization helps you quickly find the book you want. Similarly, in micro-program memory, the instructions are sorted to facilitate fast access during execution.

So, now, if you look at this slide which actually tells in more formal manner which I was saying that during the execution of a micro-programmed control, the MPC is always incremented.

During the execution of micro-programs, the micro-program counter (MPC) typically increments to point to the next instruction. However, this incrementing is conditional and can be interrupted by branch instructions or when reaching the end of a micro-program.

Think of navigating through a playlist on your phone. You normally progress to the next song automatically (increment), but if you choose a specific song from a list (branch), you jump to that song instead of continuing to the next in line. This mechanism ensures the sequence of actions aligns with your choices.

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