Arithmetic and Input · Lesson 5 of 30 · about 9 minutes
Division and Remainders
Explain floor division and signed remainder, including negatives.
01 / Explain
Understand the idea
DIV rounds toward negative infinity: 18 divided by 4 gives 4, while −9 divided by 4 gives −3. MOD returns the signed remainder: 18 MOD 4 is 2 and −9 MOD 4 is −1. These operations follow the documented machine rules, which differ for negative values.
Neither DIV nor MOD accepts a zero divisor. Save the original input in another register when you need both results, because each operation replaces R0.
02 / Try
Watch it happen
Expect −3 then −1. Try replacing 4 with zero to inspect the diagnostic and its source line.
LOAD -9 R1
LOAD R1
DIV 4
PRINT
LOAD R1
MOD 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 an integer. Print its floor quotient by 4, then its signed remainder by 4. Keep the original long enough to calculate both.
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
# Print quotient, restore the input, then print remainder.
HALTNeed a hint?
LOAD R1 restores the saved input to R0.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
COPY R0 R1
DIV 4
PRINT
LOAD R1
MOD 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.
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