22.6.2. Vertical Micro-program Techniques

We basically have a memory. So, we have actually 3 fields. So, this is the control function field. So, this is very very important the control signals are like program counter in, program counter out all these things will be there basically that is the main part of it. There are 2 other parts basically one is called the condition select and one in the branch address field.

This part introduces the concept of memory in micro-program controls, highlighting that it comprises three fields. The control function field is crucial as it manages signals like the program counter inputs and outputs. Additionally, there are two other fields: the condition select which indicates the conditions that need to be evaluated, and the branch address field which specifies the address to jump to conditionally based on prior evaluations.

Think of a memory as a library system. The control function field is like the librarian who manages what books are checked in or out (program counter signals), while the condition select field determines which books can be accessed based on specific criteria (like whether a book is available), and the branch address field points to where you need to go next to find a specific book.

Branch address fields means, it says that from this instruction if some conditions are true or something then the next address may be say 101. So, this is going to say that I am not going maybe at this present in the instruction called say 001. So, I will go to 101, if some condition is true.

The branch address field allows the micro-program to decide where to go next based on certain conditions. If these conditions are met, the program counter (PC) updates to the branch address, skipping the usual increment behavior. For example, if a condition points the PC to address 101 instead of incrementing from 001 to 002, it shows how programming logic can allow for non-linear flows in execution.

Imagine you're following a map to a destination. If you reach a checkpoint (condition) and there’s a detour (branch address), you might need to take that detour to go to a different location instead of just following the straight path.

This part of the control signals will directly go to the ALU the PC in and all these ports, but basically the branch address field is going to be basically one input to the MPC.

The control signals and the branch address field work together to direct operations in the Arithmetic Logic Unit (ALU) and to manage how the micro-program counter (MPC) processes inputs. When conditions signal that a branch should occur, the MPC will adopt the corresponding branch address, facilitating efficient execution of instructions as specified.

It’s akin to a traffic signal system where different signals dictate the flow of traffic (data) at intersections (conditional operations). If a signal allows right turns (a branch condition), then cars (data) can change directions (memory locations) accordingly.

So, here you see this is very interesting there is a control to MPC. So, what is the control to MPC one is load and increment bar; that means, if this line is 1. So, what is this going to do it is going to load the value which is available at this port so; that means, if this is 1 you are going to take the value of 101 in the micro-program control.

The control inputs to the micro-program counter (MPC) essentially dictate whether the MPC should load a specific address (like 101) or just increment the current address. If the control line activates (line is 1), the MPC is instructed to load the new address; otherwise (if the line is 0), it continues incrementing the address to the next in sequence. This logic allows for dynamic instruction execution flow.

Envision a remote-controlled car: if you press 'go' (control line is 1), it speeds to a designated location (loads a new address); if you press nothing (control line is 0), it just rolls forward slowly (increments the address).

How to decide that whether this line will be 0 and 1 then my job will be done.

The condition select field plays a critical role in determining whether the control to the MPC will load a new address or allow incrementing based on certain conditions (like flags or status). Implemented often using multiplexers, this mechanism helps optimize how decisions are made regarding control instructions during execution.

Consider a vending machine that requires you to select a drink (condition select field). If you choose soda (a specific condition), it will dispense soda, but if you choose water, it will simply remind you to add more money before fulfilling your request (incremental operation).

So, they actually say that say for example, I have only 4 condition codes. So, you have a 4:1 multiplexer as simple as that.

The text describes a multiplexer setup that simplifies control signal selection based on condition codes. In this scenario, four possible conditions can help determine the control line's state (1 or 0). Depending on the combination of input values, this mechanism allows the proper signals to be selected to either increment the address or load a specific instruction.

Imagine a light switch that can control four different light settings depending on which switch you flip. Each condition (input) corresponds to a unique light operation, allowing for versatile outcomes based on simple control inputs.

So, therefore, we have seen how this circuit basically implements the sequencing using a memory, because we have to remember that unlike if I said we do not have some flexibility like this, where some condition checks are here and you go from this state to this state based on some condition.

The text concludes with an explanation of vertical micro-programming techniques that focus on optimizing memory use by shortening micro instructions. This approach minimizes wasted memory by ensuring that only necessary bits are stored, leveraging the sparse nature of control requirements and leading to a more efficient system overall.

Consider packing a suitcase efficiently for a trip. Instead of throwing in random items, you prioritize and only take what you need, thus saving space (memory) and ensuring everything fits neatly without waste, aligning with the principles of vertical micro-programming.

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