Arithmetic and Input · Lesson 6 of 30 · about 10 minutes
A Two-input Calculator
Build a reusable calculator instead of a fixed answer.
01 / Explain
Understand the idea
A useful program works for inputs it did not know when it was written. Two INPUT instructions consume the queue in order. Store their values in R1 and R2 so that both remain available.
Testing several cases checks behavior rather than one answer. Zero and negative values expose mistakes that a single positive example can hide. The challenge checker creates a fresh CPU for every case.
02 / Try
Watch it happen
Queue 3 and 4 before running. You should see their sum 7 and product 12.
INPUT
COPY R0 R1
INPUT
COPY R0 R2
LOAD R1
ADD R2
PRINT
LOAD R1
MUL R2
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read integers a and b. Print their sum and then their product. Use the supplied values rather than fixed answers.
The checker runs your current editor program in a fresh machine for each of 3 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
INPUT
COPY R0 R1
INPUT
COPY R0 R2
# Use R1 and R2 twice.
HALTNeed a hint?
Reload a before the second calculation.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
COPY R0 R1
INPUT
COPY R0 R2
LOAD R1
ADD R2
PRINT
LOAD R1
MUL R2
PRINT
HALTEmoji CPU lab
Emoji program
Type LOAD, ADD, or another opcode then Space to insert emoji. Ctrl/⌘ + Enter runs or pauses; Escape pauses; Ctrl/⌘ + ] indents. Tab moves focus. Labels use a colon. Jumps use zero-based instruction addresses.
Instruction map and breakpoints (0)
Breakpoints stop before an instruction. Run resumes past the stopped breakpoint once; Step executes it directly. Editing source clears old breakpoints and machine state.
CPU registers
- R0
- 0
- R1
- 0
- R2
- 0
- R3
- 0
- R4
- 0
- R5
- 0
- R6
- 0
- R7
- 0
Stacks and loop frames
SP = 255 − data depth − call depth. The stack is separate from memory.
Data stack (bottom → top)
Empty
Call return addresses (bottom → top)
Empty
Loop frames
Empty
Output and input
Run a PRINT instruction to see output.
Queued input: Empty
Memory · 256 bytes · 0 nonzero
Each cell shows address:value. R = read this step; W = written this step. Select a cell to inspect or initialize it before execution. Use arrow keys to move, Home/End for the row, and Ctrl/⌘ + Home/End for the whole memory.
Execution trace · 0 entries
Recent entries below. Inspect any zero-based index to see complete detached before/after state.
Check your challenge
You can run this check any time. Every case must pass to record completion.
Progress uses localStorage only. It stays on this browser and is never sent to a server.