22.6.1. Horizontal Micro-program Technique

This is the control function field. So, this is very important; the control signals are like program counter in, program counter out, all these things will be there.

The control function field is essential in micro-programming. It manages signals that control operations like loading values into the program counter (PC) and handling program counter outputs. In micro-programmed control units, these signals direct how data is processed within the unit, ensuring that instructions are executed correctly.

Think of the control function as a traffic director at an intersection. It signals which direction cars should go, just as control signals dictate the flow of data in a micro-program. Without proper direction, traffic (or data) would be chaotic and could lead to crashes.

Branch address fields mean, it says that from this instruction if some conditions are true, then the next address may be say 101.

The branch address field indicates where to go next in the instruction sequence based on certain conditions. If the conditions are met (true), the micro-program counter (MPC) will take a predetermined jump to a specific address (e.g., 101) instead of incrementing sequentially. This mechanism allows for decision-making in program execution, akin to the 'if-then' logic in programming.

Imagine a choose-your-own-adventure book where each decision leads you to a different page. If you choose 'yes,' you turn to page 101; if 'no,' you turn to page 102. The branch address field functions similarly by determining which instruction to execute next based on preceding conditions.

Now, how can I tell that I have to go to this branch? This is very simple; 101 means you have to update the value of micro-program counter to 101.

In this chunk, it is emphasized that updating the micro-program counter is straightforward: when a branch decision is made, the MPC directly reflects this change by loading the next address (like 101) if the condition is satisfied. This updating mechanism is crucial for enabling the execution of different paths within a program, allowing for more dynamic processing.

Think of the MPC like an address book that you flip to find the next friend's house. If you decide to go to John’s instead of Mary’s, you simply mark the page (update the counter) with John's address. This keeps you on track as you navigate through your visits (or instructions).

There is a control to MPC. One is load and increment; that means if this line is 1... it is just going to make it +1.

This portion talks about the control mechanism that determines whether to load a specific value into the MPC or to increment it by one. If the control signal is active (set to 1), the MPC loads the new address; if inactive (0), it simply increments the current address. This binary choice is fundamental for executing conditional branches.

Imagine a set of train tracks; if the switch is turned one way (1), the train will head to a new station (load a value); if not (0), it continues on its current path, moving to the next station (increment by one). This control determines the path of program execution, similar to how train switches direct trains.

How to decide that this line will be 0 and 1 is decided by this condition select field.

The condition select field plays a critical role in deciding whether the program counter should load a new address or simply increment. This field utilizes a multiplexer, which channels the correct control signal based on the current conditions. Multiple conditions can lead to varied outcomes, reinforcing the need for specific logic to handle different scenarios.

Think of the condition select as a decision maker at a crossroad that evaluates the surrounding conditions (like traffic lights). The choice of direction (to proceed straight or turn) depends on what the lights signify; similarly, the condition select determines which control signal to engage.

Say for example, I have only 4 condition codes... a 4:1 multiplexer as simple as that.

Multiplexers streamline the decision-making process by allowing multiple inputs but producing a single output based on the condition select. In this case, if there are 4 conditions, a 4:1 multiplexer helps to select which condition's outcome dictates how the micro-program counter behaves, leading to either a jump or increment.

Imagine a remote control with four buttons, each for a different function. Pressing the active button (based on the situation) allows you to perform one action out of many options, much like how a multiplexer selects between several inputs for the MPC.

If I want to go for a jump unconditional... literally just jump to a specified address.

Unconditional jumps allow the program to jump to a specified address regardless of any conditions. This means that even without checking the state of other inputs, the processor can be instructed to execute a specific instruction immediately.

This is akin to a teacher directing a student to go directly to the library regardless of what's happening in class. The student doesn't check for conditions; they just follow the instruction given by the teacher, just like an unconditional jump in programming.

We have seen how this circuit basically implements the sequencing using a memory...

The structure of a micro-program control unit orchestrates program execution by leveraging control logic and memory sequencing. This flexibility allows the micro-program to manage complex instructions efficiently by allowing branching and direct execution choices based on specified conditions.

Think of the micro-program control unit as a conductor leading an orchestra. Each musician responds according to the signals from the conductor (the micro-program); together, they create a harmonious performance (the execution of program instructions). Without a conductor, coordination would be chaotic.

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