Build Small Programs · Lesson 29 of 30 · about 18 minutes

A Stack-powered Buffer Utility

Combine input, loops, the stack, and indexed storage.

01 / Explain

Understand the idea

The stack naturally reverses a sequence. Read each byte and push it, then pop each byte and write it into a new buffer. Two loops separate the collection phase from the output phase.

The input begins with N, the number of following bytes. Store N in a register used by both loops. An empty buffer has zero iterations in both phases and should create no output or accidental memory writes.

02 / Try

Watch it happen

Expect 30, 20, 10 in output and memory cells 0–2.

LOAD 3 R3
LOAD 10
PUSH
LOAD 20
PUSH
LOAD 30
PUSH
LOAD 0 R1
LOOP R3
POP
STORE R1
PRINT
LOAD R1
ADD 1
COPY R0 R1
RETURN
HALT

Use Step to follow one instruction at a time. You can change the example and replay it.

03 / Challenge

Make it work

Read N (0–5), then exactly N byte values. Use the data stack to reverse them. Print the reversed sequence and store it beginning at memory address 0.

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
COPY R0 R3
LOOP R3
  # Read and push each byte.
RETURN
LOAD 0 R1
LOOP R3
  # Pop, store, print, and move destination pointer.
RETURN
HALT

Need a hint?

Preserve the byte in R0 until STORE and PRINT have both used it; only then increment the destination pointer.

Reveal a worked solution

Read the program, predict each instruction’s effect, then step through it in the lab.

INPUT
COPY R0 R3
LOOP R3
INPUT
PUSH
RETURN
LOAD 0 R1
LOOP R3
POP
STORE R1
PRINT
LOAD R1
ADD 1
COPY R0 R1
RETURN
HALT

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.

INPUT
COPY R0 R3
LOOP R3
  # Read and push each byte.
RETURN
LOAD 0 R1
LOOP R3
  # Pop, store, print, and move destination pointer.
RETURN
HALT

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.

Use Number

CPU registers

R0: 0
R1: 0
R2: 0
R3: 0
R4: 0
R5: 0
R6: 0
R7: 0

Program counter (PC): 0
Stack pointer (SP): 255
Flags: Zero: off · Negative: off · Overflow: off
Delay request: 0 ms

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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