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# A tiny processor
From gates to a processor · A machine that remembers
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## Today
**We are learning to** a processor repeats one cycle: fetch the instruction the program counter points at, decode it, execute it, one step per clock tick. The program is numbers in the same memory as the data. Step a tiny processor, change its program, and write your own.
- I can describe the fetch, decode and execute cycle.
- I can say what the program counter and accumulator hold.
- I can explain that instructions are numbers stored in memory.
- I can write a short program for a tiny processor.
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## Why it matters
> Your games, apps and websites all end up as simple instructions like ADD and JUMP. What does a processor actually do with them?
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💬 Discussion
## Predict
**Memory holds: cell 0 LOAD 6, cell 1 ADD 7, cell 2 OUT, cell 3 HALT, cell 6 = 5, cell 7 = 3. What does the program output?**
- 8
- 53
- 5, then 3
- Nothing: HALT comes first
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fsystems%2Flogic%2Fprocessor%2Fstep-2&code=widget%3A+cpu%0Aprogram%3A+LOAD+6%3B+ADD+7%3B+OUT%3B+HALT%0Adata%3A+6%3D5%2C+7%3D3%0Aexpect%3A+8%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/systems/logic/processor/?full=step-2#step-2)
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✅ Check
## What happened?
* 8: load 5, add 3, output the accumulator, stop.
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💬 Discussion
## LOAD 6, tick by tick
Now count the ticks for the whole program.
$$\text{fetch: IR} = \text{cell } 0 = 22,\ PC = 1$$
1) $\text{decode: } 22 = 1 \times 16 + 6$ — Operation 1 is LOAD; the address is 6.
2) $\text{execute: ACC} = \text{cell } 6 = 5$ — Copy the number in cell 6 into the accumulator.
3) $\text{next: fetch cell } 1$ — The PC says where to go next: ADD 7.
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✅ Check
## LOAD 6, tick by tick
**Each instruction takes three clock ticks. How many ticks does the whole program (LOAD, ADD, OUT, HALT) take?**
* Answer: 12
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✅ Check
## Find the mistake
**Sam explains the memory. Click the line where it first goes wrong.**
```
1 Cell 0 holds LOAD 6, which is stored as the number 22.
2 Cell 6 holds the number 5.
3 Instructions and numbers are different kinds of thing in memory.
4 So the processor could never run cell 6 by mistake.
```
* Answer: line 3
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✅ Check
## Clock speed
**A processor runs at 3 GHz. About how many clock ticks is that each second?**
- 3 billion
- 3
- 3 thousand
- 3 million
* Answer: 3 billion
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✍️ Activity
## Vary: change the program
- **Change the data:**
- **Change an instruction:**
- **Store the answer:**
- **A loop:**
[Open this step](https://studio.learningwaypoints.com/tutorials/systems/logic/processor/?full=step-4#step-4)
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✅ Check
## The PC
**The PC is 4. The processor fetches. What is the PC afterwards (if the instruction isn't a jump)?**
- 4
- 0
- 5
- It depends on the accumulator
* Answer: 5
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✅ Check
## Loops
**Which instruction lets a program repeat steps?**
- HALT
- OUT
- ADD
- JUMP: it changes the PC to an earlier cell
* Answer: JUMP: it changes the PC to an earlier cell
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✅ Check
## Review: adding
**Review: ADD 7 adds two 8-bit numbers. What inside the processor does the adding?**
- The memory
- The clock
- The program counter
- A row of full adders, made of gates
* Answer: A row of full adders, made of gates
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✍️ Activity
## Make: write a program
Then: can you do it with a loop instead, so it would work for 2, 4, 6, 8, 10 without more cells?
[Open this step](https://studio.learningwaypoints.com/tutorials/systems/logic/processor/?full=step-6#step-6)
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✅ Check
## Exit ticket
**Name the three steps a processor repeats, and say what the program counter does.**
* Fetch, decode, execute. The program counter holds the cell of the next instruction and goes up by one after each fetch (or jumps).