--- title: "Remembering a bit" subtitle: "From gates to a processor · A machine that remembers · Lesson 3" --- # Remembering a bit **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A gate's output changes the moment its inputs change. So how does a computer remember anything? :::teacher-note **In one sentence:** Students set and reset a latch made of two NOR gates, see it keep its bit with both switches off, and store a number in a 4-bit register of latches (AC9TDI8K04 node 3, extended; the kit names adder circuits as going further). **Students should be able to say:** I can explain how a latch remembers a bit. I can say what Set and Reset do, and what happens with both off. I can store a number in a register and explain why it stays. **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 **A latch has a Set switch and a Reset switch. Turn Set on: Q becomes 1. Now turn Set off again. What is Q?** ☐ It flickers between 0 and 1 ☐ 0: nothing is switched on ☐ Still 1: it remembers ☐ It depends on the time My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Still 1. Each gate's output keeps the other gate in the state that holds Q at 1, until Reset changes it. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: the latch's rules :::keep-together ### Four settings 1. $S = 1, R = 0 \Rightarrow Q = 1$ — Set: the bottom NOR's output goes to 0, which lets the top one make Q = 1. 2. $S = 0, R = 0 \Rightarrow Q \text{ stays}$ — Both off: each gate's output holds the other where it is. This is the memory. 3. $S = 0, R = 1 \Rightarrow Q = 0$ — Reset: the top NOR's output goes to 0. 4. $S = 1, R = 1$ — Not allowed: Q and not Q both become 0, which makes no sense for one bit. Now try Reset. **Q is 1. Reset goes on, then off again. What is Q now?** ☐ 0 ☐ 1 ::: :::teacher-note **Answer:** 0 Reset sets Q to 0; letting go keeps it at 0. ::: :::keep-together ### Find the mistake **Sam explains the latch. Click the line where it first goes wrong.** ``` 1 Set goes on, so Q becomes 1. 2 Set goes off again, so Q goes back to 0, like a lamp. 3 So the latch only shows 1 while Set is held on. 4 That means gates can't remember anything. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Line 2: a latch is not a lamp. With Set and Reset both off, Q keeps the 1: that is memory. ::: :::keep-together ### Why RAM forgets **Why does RAM lose everything when the power goes off?** ☐ Its bits are held by circuits like latches, which need power to keep feeding each other ☐ Power cuts damage it ☐ The processor deletes it ☐ It copies everything to storage first ::: :::teacher-note **Answer:** Its bits are held by circuits like latches, which need power to keep feeding each other No power, no loop, no bit. Storage keeps data a different way (magnetism or trapped charge). ::: :::typical ## Vary: latches and a register **Reset.** **Both on.** **Store a number.** **Keep it.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Memory :::keep-together ### Register size **A register is 8 latches side by side. What is the biggest number it can hold?** ☐ 8 ☐ 128 ☐ 255 ☐ 256 ::: :::teacher-note **Answer:** 255 8 bits: 0 to 255. ::: :::keep-together ### RAM in bits **A computer has 8 GB of RAM. About how many bits is that?** ☐ About 64 billion ☐ About 8 million ☐ About 8 billion ☐ About 8 ::: :::teacher-note **Answer:** About 64 billion 8 billion bytes × 8 bits = about 64 billion bits, each held by a tiny circuit. ::: :::keep-together ### Review: RAM and storage **Review: which part keeps your files when the computer is switched off?** ☐ Storage ☐ RAM ☐ The processor's registers ☐ The clock ::: :::teacher-note **Answer:** Storage Storage (SSD or hard disk) keeps data without power. ::: ## Make: hold a number Then: what would you need to store 16 bits? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Set goes on then off, then Reset goes on then off. What is Q after each step, and why does the latch count as memory?** {{lines-3}} ::: :::teacher-note **Answer:** 1, 1, 0, 0. With both switches off it keeps its last value, so its output depends on what happened before. :::