Algorithms with State · Lesson 18 of 30 · about 15 minutes

Reverse a Memory Buffer

Copy a buffer backward using source and destination pointers.

01 / Explain

Understand the idea

A pointer is a value used as an address. To reverse four bytes without destroying the input, read source addresses 3, 2, 1, 0 and write destination addresses 16, 17, 18, 19. The two pointers move in opposite directions.

This is a reverse copy, not an in-place reverse. Keeping source and destination regions separate avoids overwriting data you have not read yet. The checker seeds different buffers and inspects both the reversed result and the unchanged source.

02 / Try

Watch it happen

The source remains 1, 2, 3, 4 while cells 16–19 become 4, 3, 2, 1.

STORE 1 0
STORE 2 1
STORE 3 2
STORE 4 3
LOAD 3 R1
LOAD 16 R2
LOOP 4
LOAD [R1]
STORE R2
LOAD R1
SUB 1
COPY R0 R1
LOAD R2
ADD 1
COPY R0 R2
RETURN
HALT

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

03 / Challenge

Make it work

Four bytes are seeded at addresses 0–3. Copy them in reverse order into addresses 16–19, leaving the originals unchanged. Use a loop and register addresses; no output is required.

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.

LOAD 3 R1
LOAD 16 R2
LOOP 4
  # Read through R1, write through R2, move both pointers.
RETURN
HALT

Need a hint?

STORE R2 uses the value in R2 as the address.

Reveal a worked solution

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

LOAD 3 R1
LOAD 16 R2
LOOP 4
LOAD [R1]
STORE R2
LOAD R1
SUB 1
COPY R0 R1
LOAD R2
ADD 1
COPY R0 R2
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.

LOAD 3 R1
LOAD 16 R2
LOOP 4
  # Read through R1, write through R2, move both pointers.
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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