Decisions and Flags · Lesson 11 of 30 · about 9 minutes
Even or Odd
Use remainder and a branch to classify input.
01 / Explain
Understand the idea
Every even integer has remainder zero when divided by two. For negative odd integers the remainder is −1, which is still nonzero. Testing for zero works for both signs.
A branch turns one program into two possible paths. The lab trace shows only the path taken for the current input. Test inputs from each path, including zero.
02 / Try
Watch it happen
The example prints 0 because −3 is odd. Change it to −4 or 0 to take the other path.
LOAD -3
MOD 2
JUMP_IF_ZERO even
LOAD 0
JUMP show
even: LOAD 1
show: PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read an integer. Print 1 for an even integer and 0 for an odd integer.
The checker runs your current editor program in a fresh machine for each of 4 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
INPUT
MOD 2
# Branch using the zero flag.
HALTNeed a hint?
MOD already updates the zero flag; CMP is optional here.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
MOD 2
JUMP_IF_ZERO even
LOAD 0
JUMP show
even: LOAD 1
show: 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.
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