Repetition and Counters · Lesson 13 of 30 · about 10 minutes
Loops: Repeating Things
Pair LOOP with RETURN and treat zero repeats correctly.
01 / Explain
Understand the idea
LOOP count starts a block that ends at its matching RETURN. A count of four executes that body four times. A count of zero skips the body entirely; negative counts produce a clear error.
This RETURN closes the loop block. A RETURN outside a loop has a separate role for subroutines, which we will learn later. Indenting the body makes the block easier to read even though whitespace does not change its behavior.
02 / Try
Watch it happen
Expect four eights. Change 4 to 0 and check that there is no output.
LOAD 8
LOOP 4
PRINT
RETURN
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read a repeat count from 0 through 5. Print 8 exactly that many times, then halt.
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
LOAD 8
LOOP R1
# Repeated instruction
RETURN
HALTNeed a hint?
Use the input count as the LOOP operand.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
COPY R0 R1
LOAD 8
LOOP R1
PRINT
RETURN
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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