21.1.2. Micro-Program Memory Architecture

So, basically the micro-programmed control unit ensures that the signal will be generated in correct sequence. So, sequencing here is a bit tricky, because generating control signals is directly they are encoded, directly they are actually put in the memory location. So, if you generate if you just take cell 1 memory row 1 memory row 2, memory 3 automatically control signals are generated as default because, they are already stored in the memory.

The micro-programmed control unit is responsible for generating control signals in a specific order. This sequencing can be complex because the control signals are predetermined and stored in designated memory locations. When an action needs to be performed, such as invoking a control signal from cell 1 or memory row 2, the micro-programmed control unit retrieves these signals from memory, where they are stored ahead of time.

Imagine a pre-recorded playlist of songs on a music player. Each time you play a song, the player retrieves it in the order it was recorded. Similarly, the micro-programmed control unit retrieves the control signals from memory in a predetermined sequence, ensuring that instructions are executed correctly.

Sequencing is actually very important here because, many times we will depend on the condition codes and status flags. Based on some storage that is some signal from the memory WMFC based on some case like some interrupt etcetera. Because of some case and also some status flag like zero flag, carry flag so all these things we must take into picture, and then you have to decide whether it will be the next phase, or it will go to some other instruction which is not consecutive.

Sequencing in the micro-programmed control unit is critical because the execution of instructions may depend on various flags and condition codes. For instance, the zero flag or carry flag can influence the flow of execution, determining whether the next instruction should be the next consecutive one or if it should jump to a different instruction altogether, often due to interrupts or specific conditions.

Think of a traffic light system where the light changes based on external conditions. If a pedestrian presses a button (a condition), the light must change to accommodate them. Similarly, in the micro-programmed control unit, the next instruction can change based on condition codes, much like how traffic lights react to pedestrians.

So, you have to have also arrangement for something called a branch decision. So, in right normal programs we have micro instructions where actually which is conditional, and also, we have something which is unconditional. In conditional it is required to specify the address of the micro-program memory where the control must direct.

Branching allows the micro-program to make decisions based on conditions during execution. There are two types of micro instructions: conditional and unconditional. Conditional instructions require specifying the address in the micro-program memory where control should be directed if certain conditions are met, while unconditional instructions execute without condition checking.

Consider a choose-your-own-adventure book where you must make choices at the end of each page. Depending on your choice (condition), you will turn to a specific page (address) to continue the story. Similarly, conditional micro instructions determine where to direct the next steps in processing based on defined conditions.

So, we will take with example then it will be easy, like for example if you have some instructions called ADD R1 R2. So, first is first phase is called the fetch an instruction. So, we have discussed many many times that for most of the instruction fetch phase is similar.

In micro-programmed control architecture, when an instruction like ADD R1 R2 is executed, the first phase is fetching the instruction. This phase is consistent across most instructions, meaning that the same sequence of micro instructions is followed for the fetching process, allowing for a standardized approach to instruction execution.

Think about a chef (the control unit) getting ingredients (instructions) from the pantry (memory). Each time a recipe calls for ingredients, the chef follows a familiar routine to fetch those ingredients. Similarly, the fetch phase standardizes how instructions are retrieved, making it efficient and predictable.

So, whenever instruction has to be fetched you can directly invoke that part of the memory, micro-program memory which has the micro instructions corresponding to fetch. After that the add will be decoded, and then different types of activities has to take place.

Once the instruction is fetched, it is then decoded to understand what operation is to be performed. For an operation like ADD R1 R2, the micro-programmed control unit will direct the micro-program counter to the specific micro instructions that correspond to the addition operation, enabling the execution of required steps for that operation.

Imagine a teacher (the control unit) who first reads a student’s name (fetch) and then checks their score (decoding) during a grading session. Once the teacher knows the score, appropriate actions follow, such as calculating grades (execution). This mirrors how micro instructions are fetched, decoded, and executed in a micro-program.

So, that part has to be a part of the sequencing circuit logic for micro-program control. Whenever there is a condition which depends on the input, depends on the flags, depends on some I/O input.

The micro-program counter plays a critical role in executing instructions by pointing to the current micro instruction to be executed. When branching conditions arise, the counter may either increment to the next instruction or jump to a different micro-program based on the evaluated conditions and flags.

Consider a GPS system that recalibrates its route based on real-time traffic (conditions). If the traffic is clear, it continues on the current route (increment). However, if heavy traffic is detected, it may direct you to a new path (branch). Similarly, the micro-program counter adjusts its path based on current program conditions.

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