Thirty small steps into a computer
Understand an idea, try it in a visible CPU, and solve a challenge checked by the machine. Start with three instructions and finish with your own tiny programs.
No account or installation. A lesson is completed only when your program passes every challenge case. Progress and badges stay in this browser.
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Complete a unit to earn its badge. Badges record checked challenges, not visits.
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Unit 1 · about 20 minutes
Explain fetch/execute, registers, PC, output and stopping.
0 of 3 checked challenges complete
Move a number into the CPU, print it, and stop.
Use the eight fast workspaces without losing a value.
Read a trace and separate source lines from instruction addresses.
Unit 2 · about 28 minutes
Build integer calculations with saved inputs and test cases.
0 of 3 checked challenges complete
Combine addition, subtraction, and multiplication as ordered steps.
Explain floor division and signed remainder, including negatives.
Build a reusable calculator instead of a fixed answer.
Unit 3 · about 28 minutes
Distinguish values from addresses and read/write byte storage.
0 of 3 checked challenges complete
Store and actually retrieve a byte after overwriting R0.
Use an address as data and read a lookup table.
See why memory wraps while registers do not.
Unit 4 · about 29 minutes
Use comparison, remainder and labeled paths to express decisions.
0 of 3 checked challenges complete
Compare values, then branch on a stored flag.
Use remainder and a branch to classify input.
Make a decision from stored data and record the result.
Unit 5 · about 34 minutes
Use bounded loops, zero cases, counters and accumulators.
0 of 3 checked challenges complete
Pair LOOP with RETURN and treat zero repeats correctly.
Print 1 to 10 and generalize the count.
Keep an accumulator separate from the loop counter.
Unit 6 · about 42 minutes
Describe and inspect factorial, Fibonacci and reverse-copy invariants.
0 of 3 checked challenges complete
Use a multiplicative accumulator and explain why 0! is 1.
Keep two previous values and update them in a safe order.
Copy a buffer backward using source and destination pointers.
Unit 7 · about 39 minutes
Use LIFO data, call frames and save/restore conventions.
0 of 3 checked challenges complete
Save values on the data stack and restore them in reverse order.
Reuse code and return to the instruction after CALL.
Use a save/restore convention when a function needs scratch space.
Unit 8 · about 39 minutes
Select and toggle bits and explain signed 32-bit behavior.
0 of 3 checked challenges complete
Select individual bits with AND and set them with OR.
Use XOR’s reversible change to update a saved setting.
Understand NOT and test a signed difference with a sign mask.
Unit 9 · about 45 minutes
Distinguish scheduling policy, work partition and handler state restoration.
0 of 3 checked challenges complete
Take turns between tasks and distinguish policy from CPU instructions.
Divide independent jobs and combine their results.
Handle an event and restore the interrupted foreground value.
Unit 10 · about 56 minutes
Combine the instruction set into a game, buffer utility and job dispatcher.
0 of 3 checked challenges complete
Build a three-round game with stored state and scoring.
Combine input, loops, the stack, and indexed storage.
Dispatch queued jobs to reusable services and record their results.
First-time programmers welcome
Follow a short recipe and predict its output. Open that exact program in the lab, then take its lesson challenge to check your own solution.
Build a print-42 recipe, then follow its trace.
📥 42
🖨️
⏹️Expected output: 42
Calculate 10 + 5 and watch the accumulator change.
📥 10
➕ 5
🖨️
⏹️Expected output: 15
Count from five to one with a labeled exit.
📥 5
again: 🖨️
➖ 1
❓ done
⏭️ again
done: ⏹️Expected output: 5, 4, 3, 2, 1