Repetition and Counters · Lesson 14 of 30 · about 12 minutes
Building a Counter
Print 1 to 10 and generalize the count.
01 / Explain
Understand the idea
A counter is state that changes once per iteration. Print the current value, add one, and save the new value for the next iteration. The starting value and repeat count determine the whole sequence.
A countdown can use a backward jump and exit when SUB makes the zero flag true. Labels name the exit and repeat destinations without counting source lines. A bounded LOOP is useful when the number of repetitions is known.
02 / Try
Watch it happen
Expect 5, 4, 3, 2, 1. Step Back exposes each decrement.
LOAD 5
again: PRINT
SUB 1
JUMP_IF_ZERO done
JUMP again
done: HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read N from 0 through 10 and print every integer from 1 through N in order. For N = 0, print nothing.
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 R2
LOAD 1 R1
LOOP R2
# Print and advance R1.
RETURN
HALTNeed a hint?
Keep the count in R2 and the changing counter in R1.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
COPY R0 R2
LOAD 1 R1
LOOP R2
LOAD R1
PRINT
ADD 1
COPY R0 R1
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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