# Fetch-Decode-Execute Cycle ## What is the Fetch-Decode-Execute Cycle?

The FDE cycle is the continuous process the CPU follows to retrieve, interpret, and carry out instructions

- The CPU repeatedly executes this cycle to process instructions - There are **3 main stages**: - **Fetching** an instruction from memory - supplying the address and receiving the instruction from memory - **Decoding** the instruction - interpreting the instruction and then reading and retrieving the required data from their addresses - **Executing** the instruction - the CPU carries out the required action ## Which Registers Are Used in the FDE Cycle?

In the following sections, registers and CPU components appear in **bold** and assembly language is in *italics*

### Fetch Stage 1. The **PC** is loaded with `0` 2. The value from the **PC** (`0`) is copied to the **MAR** 3. The data from the **MAR** (`0`) is sent across the address bus with the instruction to read the data sent across the control bus 4. The data from that location in memory (`0`) is sent down the data bus to the **MDR** 5. The **PC** is incremented by `1` ### Decode Stage 1. The data is sent from the **MDR** to the **CIR** where it is split into the opcode and operand 2. This is sent to the **CU** to be decoded ### Execute Stage - Which registers are used here will depend on the instruction being executed #### Different Instruction Types | Instruction Type | Registers Used | Details | |-----------------|----------------|---------| | **INP** (Input) | **ACC** | The **ACC** will store the value | | **OUT** (Output) | **ACC** | This will be the value currently in the **ACC** | | **LDA** (Load) | **MAR**, **MDR** | Value sent across the data bus from RAM (in the address location in the **MAR**) to the **MDR** | | **STA** (Store) | **ACC**, **MDR**, **MAR** | Value from the **ACC** → **MDR** → data bus → RAM (to address location in the **MAR**) | | **ADD/SUB** (Arithmetic) | **ALU**, **ACC** | Values passed to the **ALU**, operation carried out, result stored in the **ACC** | | **BRA/BRZ/BRP** (Branch) | **ALU** | Comparison takes place in the **ALU** | ### Detailed Execution Examples - **If a value is being inputted** (*INP*) the **ACC** will store the value - **If a value is being outputted** (*OUT*) this will be the value currently in the **ACC** - **If a value is loaded from RAM** (*LDA*) this will be sent across the data bus from RAM (in the address location in the **MAR**) to the **MDR** - **If a value is to be stored** (*STA*) it will take the value from the **ACC**, send it to the **MDR** and then send it across the data bus to RAM (to the address location in the **MAR**) - **If a value is being added to or subtracted from another value** (*ADD/SUB*) the values will be passed to the **ALU**, the operation will be carried out, and the result will be stored in the **ACC** - **If the LMC code is to branch** (*BRA/BRZ/BRP*) the comparison will take place in the **ALU** ## Worked Example Consider a program written in the Little Man Computer instruction set: ```assembly INP STA num loop LDA total ADD num STA total LDA count ADD one STA count SUB num BRZ end BRA loop end LDA total OUT HLT one DAT 1 num DAT 0 count DAT 0 total DAT 0 ``` **Question:** Explain which registers are used and their values when the line `STA count` is executed and the accumulator is holding the value `9`. The label `count` refers to memory location `16`. ### How to Answer This Question The instruction being executed is `STA count`, so the registers used will be: - **ACC** - the accumulator is holding the value `9` - **MDR** - the value `9` from the **ACC** will be copied to the **MDR** - **MAR** - the value `16` will be stored here so the data is sent to memory location `16` #### Working Through the Problem - The value in the **accumulator (ACC)** is going to be stored in memory (RAM) - To work out where in memory it will be stored we need to know what `count` represents - In this question, we've been told it's `16` - "the label count refers to memory location 16" - You're not always told this in your exam, so you should also be able to count the lines of code - It's best to write the line numbers on the code in the question, to see which line `count` is on - Don't forget that the first line is memory location `0`! - `count DAT 0` is on line `16`, so the value `16` is what goes to the **MAR** register - It is always the value from the **accumulator (ACC)** that is stored, so the value `9` must go to the **MDR** as data can only be sent to memory from the **MDR** - Then the value is sent to the memory location, in this case `16` ### Example Answer **Full marks answer:** The contents of the accumulator (`9` in this case) will be copied to the **MDR** and then `9` is copied to location `16`. **Acceptable alternative:** The value `16` is copied to the **MAR**.

When answering questions about registers, be specific about the contents based on the question to ensure full marks

## Key Points to Remember

If your answer doesn't include specific values from the question (e.g., `16` and `9`), you won't get marks despite knowing which registers are used

- The question asks for which registers are used **and their values** when a specific line is executed - Always state: - **Which register** is being used - **What value** it contains at that moment - **Where that value** is going next (if applicable)