--- title: "A tiny processor" subtitle: "From gates to a processor · A machine that remembers · Lesson 4" --- # A tiny processor **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Your games, apps and websites all end up as simple instructions like ADD and JUMP. What does a processor actually do with them? :::teacher-note **In one sentence:** Students step a tiny processor through fetch, decode and execute one clock tick at a time, change its program and data, follow a loop, and write a program of their own (AC9TDI8K04 node 3, extended; links to AC9TDI8K01). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 8: load 5, add 3, output the accumulator, stop. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: one instruction, three ticks :::keep-together ### LOAD 6, tick by tick 1. $\text{fetch: IR} = \text{cell } 0 = 22,\ PC = 1$ — LOAD 6 is stored as 1 × 16 + 6 = 22. The PC moves on, ready for next time. 2. $\text{decode: } 22 = 1 \times 16 + 6$ — Operation 1 is LOAD; the address is 6. 3. $\text{execute: ACC} = \text{cell } 6 = 5$ — Copy the number in cell 6 into the accumulator. 4. $\text{next: fetch cell } 1$ — The PC says where to go next: ADD 7. Now count the ticks for the whole program. **Each instruction takes three clock ticks. How many ticks does the whole program (LOAD, ADD, OUT, HALT) take?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 12 4 instructions × 3 ticks = 12. ::: :::keep-together ### 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. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 3 Line 3: instructions are numbers, kept in the same memory as the data. If the PC ever reached cell 6, the processor would run 5 as an instruction. ::: :::keep-together ### Clock speed **A processor runs at 3 GHz. About how many clock ticks is that each second?** ☐ 3 billion ☐ 3 ☐ 3 thousand ☐ 3 million ::: :::teacher-note **Answer:** 3 billion 3 GHz is 3 billion ticks a second, so this 12-tick program would take 4 billionths of a second. ::: :::typical ## Vary: change the program **Change the data.** **Change an instruction.** **Store the answer.** **A loop.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## The cycle :::keep-together ### 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 ::: :::teacher-note **Answer:** 5 Fetch copies the instruction and adds 1 to the PC. ::: :::keep-together ### Loops **Which instruction lets a program repeat steps?** ☐ HALT ☐ OUT ☐ ADD ☐ JUMP: it changes the PC to an earlier cell ::: :::teacher-note **Answer:** JUMP: it changes the PC to an earlier cell A jump back makes a loop; a JZ (jump if zero) can leave it. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** A row of full adders, made of gates Eight full adders side by side, as in Adding with gates. ::: ## 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? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Name the three steps a processor repeats, and say what the program counter does.** {{lines-3}} ::: :::teacher-note **Answer:** Fetch, decode, execute. The program counter holds the cell of the next instruction and goes up by one after each fetch (or jumps). :::