Arithmetic and Input · Lesson 4 of 30 · about 9 minutes
Emoji Math
Combine addition, subtraction, and multiplication as ordered steps.
01 / Explain
Understand the idea
ADD, SUB, MUL, and DIV change R0. Each instruction sees the previous instruction’s result, so instruction order expresses parentheses: LOAD 10, ADD 5, MUL 2 calculates (10 + 5) × 2.
INPUT reads a supplied decimal integer into R0. If the input queue is empty, the CPU waits at INPUT without consuming a cycle. Arithmetic keeps whole safe integers instead of silently wrapping into a byte.
02 / Try
Watch it happen
The first output is 15. The second is 26.
LOAD 10
ADD 5
PRINT
MUL 2
SUB 4
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read one integer x and print (x + 5) × 2 − 4. Your program must work for every supplied input, including negative numbers.
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
# Calculate the expression in R0.
PRINT
HALTNeed a hint?
Write one operation per instruction, in the same order as the parentheses.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
ADD 5
MUL 2
SUB 4
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.