26.5.2 - Condition Select and Branch Address
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Introduction to Micro Instruction Execution
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Today, we are diving into the execution of macro instructions through micro instructions. Can anyone tell me what micro instructions are?
Aren't micro instructions the specific commands that control the operation of the ALU and other parts of the CPU?
Exactly! Micro instructions are the low-level commands executed by the control unit to carry out higher-level operations defined by macro instructions. Let's consider an example of addition and subtraction.
So, they are similar except for maybe one bit in the control signal, right?
Correct! This similarity allows us to potentially optimize and write a single micro program for both operations. This leads to more efficient processing.
Understanding Jumps in Micro Instructions
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Now, let's discuss jumps. In micro routines, we can have explicit jumps, which are directly linked to the macro instruction. Can anyone provide an example?
Jump on zero would be an example, right? It tells the CPU to jump to a specific address if a condition is met.
Exactly! Implicit jumps, on the other hand, occur when we are optimizing for multiple similar macro instructions within a common micro program. Why do you think we would need to implement implicit jumps?
It allows us to save space and makes the execution faster by avoiding repetition of similar micro instructions!
Great insight! By sharing routines, we minimize the need for separate handling for every single macro instruction.
Control Signals and Branching Conditions
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Next, let’s talk about control signals, particularly condition select. How do you think control signals influence the branching address in micro routines?
They tell the CPU which condition should trigger an instruction to jump or just move to the next instruction.
Right! For instance, selecting the output from the condition register determines whether we should jump or increment the program counter. If we set a flag, how does that influence our controls?
If the flag is set, it will lead to loading a new branch address, but if not, it will just increment the PC!
Exactly! Understanding these relationships is vital for programming micro instructions effectively.
Practical Application of Jumps
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Let's discuss a practical scenario of a jump instruction like 'jump on zero.' What happens if the zero flag is set during execution?
The program counter would get updated to the target location, continuing the program from there!
That's correct! And if the zero flag is not set, what would happen?
Then the program would skip the jump and simply move to the next instruction in the sequence.
Great job! This emphasizes the importance of conditions in dictating the flow of execution.
Review and Summary
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To wrap up, can someone summarize what we've learned about implicit vs explicit jumps?
We learned that explicit jumps are directly tied to macro instructions while implicit jumps optimize our micro routines by allowing shared processing.
Excellent! And what role do control signals play in this?
Control signals determine which actions to take based on conditions, like whether to jump or increment the program counter.
Exactly! Knowledge of these aspects is crucial for microprogramming effectively.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
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In this section, we explore the execution of macro instructions through micro instructions, focusing on the concepts of condition selection and branching. We discuss implicit and explicit jumps in micro routines and how similar macro instructions can share a common micro program to optimize execution.
Detailed
Detailed Summary
This section focuses on the execution of macro instructions using micro instructions, particularly through condition selection and branching techniques. It begins with the concept that similar macro instructions (like addition and subtraction) prompt similar micro instruction patterns, allowing for the consolidation of routines.
A critical part of execution involves understanding both explicit and implicit jumps. Explicit jumps are those dictated by the macro instruction itself (such as a jump on zero instruction), while implicit jumps occur when micro programs share common routines and require internal branching based on control signals.
The distinction between condition selection for branches is emphasized through examples, notably the jump on zero (JMPZ) versus jump on carry instructions, which share fundamental processes but require different handling for their specific conditions. The integration of control signals into these decisions ensures that micro instructions execute correctly, reflecting the macro instruction's intent. By consolidating these instructions into common micro programs, efficiency can be greatly improved, allowing for more streamlined processing in computing architectures.
The section closes by recapping how returning to a common routine after executing a branch instruction is implemented, ensuring clarity of execution paths and the roles of various flags like the zero and carry flags in executing jumps.
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Overview of Micro and Macro Instructions
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Chapter Content
Now, let us again now actual I will tell you two things, two things are very important over here. So, as I told you in the last class that in this unit we will also see how a complete macro instruction is executed in terms of micro instructions. Because in the last unit we just saw that how to do a fetch here we will see how a total instruction is executed. Secondly, as I told you we also give some idea that how micro instructions can be macro instruct micro instructions corresponding to different type of similar macro instructions can be clubbed.
Detailed Explanation
This chunk introduces the core concepts of micro and macro instructions. Macro instructions are high-level commands that perform specific operations, while micro instructions are the lower-level instructions that the CPU executes to perform these commands. By clubbing similar types of macro instructions, like addition and subtraction, we can optimize the execution process. This section aims to clarify that while executing a macro instruction, we can view the sequence of micro instructions that it comprises, thus providing a more detailed understanding of how CPUs operate at a low level.
Examples & Analogies
Imagine a recipe (macro instruction) that contains multiple steps (micro instructions). Just as certain steps can be combined or executed in a sequence to streamline cooking, similar computer instructions can be grouped to improve performance in executing programs.
Branching and Implicit Jumps
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So, this type of branching will be actually called the implicit jumps we have to do, because you are writing a common micro program for different macro programs. So, this is not a requirement of the macro program. So, for example, if I have add 𝑅1 and 𝑅2. So, if this instruction is there you do not require to explicitly write jump instructions in the micro routine because this is this is not a jump instruction.
Detailed Explanation
In this section, implicit jumps are introduced as a part of optimizing micro instruction routines. Implicit jumps allow programs to move from one instruction to another without explicitly stating a jump command. For instance, when executing a simple addition, the micro program does not need to specify jumps because the next instruction is understood based on the flow of the program, continuing the execution as intended.
Examples & Analogies
Think of a flow of activities in a day, like going from breakfast to work. You don’t need to announce that you are 'jumping' to work after breakfast; it's understood as the next step, just like in programming where the next instruction follows naturally.
Explicit Jumps in Micro Instructions
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So, there are basically 2 parts. One is called the explicit that was because of the instruction type if it is add, sub this will not be there, and there will be some implicit type which will be coming in to the micro routines because you are doing an optimization.
Detailed Explanation
This chunk explains that explicit jumps are deliberate commands in the micro instruction set that manage program flow based on specific conditions, such as checking flags. For example, the instruction "jump if zero" (e.g., JMPZ) requires an explicit micro instruction to check the zero flag and decide whether to jump to a specific memory address. This is different from implicit jumps, which happen automatically based on program flow.
Examples & Analogies
Consider a traffic signal at an intersection. When the light turns red, drivers must stop (explicit instruction), as opposed to simply driving straight through without needing to think about each stop.
Control Signals and Multiplexers
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So, this is actually the implicit jump routine, which will be there in the micro program, and then another type of jumps which will be there which are actually explicitly mention in the macro; macro instruction corresponding to that right for example, 𝐽𝑀𝑃𝑍. So, it says that if the 0 flag is set you jump to memory location 𝑀, that is an explicit jump instruction which has to be in built in the micro routine.
Detailed Explanation
In this section, control signals are discussed in the context of a multiplexer (MUX) configuration that directs the flow of execution. Different control signals will determine if the program should jump to a new address or continue executing. When a jump instruction like JMPZ is executed, the zero flag is checked, and if it indicates a condition to branch, the program counter is updated to point to the new address. The multiplexers play a critical role in selecting the correct address based on the control signals generated in the CPU.
Examples & Analogies
Think of a decision-making flowchart where at each step (a control signal) you choose a path (branching). The MUX acts like a traffic guide, directing which way to go based on current conditions and rules, ensuring you take the right path based on the traffic light status.
Examples of Jump Instructions
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So, assume that we have we are going to write a common routine corresponding to jump on 𝑍 to 𝑀 and jump on carry to 𝑀. So, this figure if you look at. So, this is already we have discussing in the last unit just a refresh.
Detailed Explanation
This chunk concludes the discussion with specific examples of commonly used jump instructions, focusing on jumps conditioned on specific flags (like jump on zero or carry). The goal is to demonstrate how these different instructions, though they may share similar structure in terms of micro programs, lead to distinctly different execution paths depending on the state of the system flags.
Examples & Analogies
Imagine a fork in a road: one path leads to a destination that's only accessible when the traffic light is green (jump on zero), while the other path is taken when a certain condition about traffic is met (jump on carry). Just like the conditional paths, programmed instructions decide the course of action based on specific conditions.
Key Concepts
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Execution of Macro Instructions: The process through which high-level instructions are translated into lower-level micro instructions.
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Branching Logic: The use of condition flags to determine the flow of execution, either through jumps or increments.
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Efficiency of Micro Programs: By sharing common routines for similar macro instructions, we can increase efficiency and reduce complexity.
Examples & Applications
Using a jump on zero instruction, if the zero flag is set, the program counter will update to the target instruction address rather than incrementing.
An implicit jump might occur when similar instructions are bundled; for example, jump on zero and jump on carry could share a micro routine.
Memory Aids
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Rhymes
In micro and macro, flags hold sway, jumping to instructions, guiding the way.
Stories
Once, a CPU faced a decision maze, it had to jump or just advance. With flags as guides, it chose the right path, optimizing its dance.
Memory Tools
Remember: JUMPS = Jumps Using Macro Processing Signals.
Acronyms
CUES for Control Under Execution Signals.
Flash Cards
Glossary
- Micro Instruction
A low-level instruction executed by the control unit to carry out operations defined by macro instructions.
- Macro Instruction
Higher-level assembly or machine code commands that consist of multiple micro instructions.
- Implicit Jump
A jump in micro routines that occurs based on conditions in shared routines without direct instruction from the macro program.
- Explicit Jump
A specifically defined jump instruction in a macro program that directs execution to a new address.
- Control Signals
Signals used by the CPU to determine which operations to execute based on current conditions.
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