The Stack and Functions · Lesson 20 of 30 · about 12 minutes

Calling a Subroutine

Reuse code and return to the instruction after CALL.

01 / Explain

Understand the idea

CALL label saves the next instruction address in a separate call frame, then jumps to the subroutine. A RETURN outside a LOOP uses that call frame to resume the caller. Main code needs HALT before the function definitions so it does not fall through into them.

A function can use R0 for an input and a result. This is a convention agreed by the caller and function, not a new VM instruction. A function with no active caller cannot RETURN successfully.

02 / Try

Watch it happen

Watch the return addresses in call frames. The outputs are 9 and 16.

LOAD 3
CALL square
PRINT
LOAD 4
CALL square
PRINT
HALT
square: MUL R0
RETURN

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

03 / Challenge

Make it work

Read two integers. Call the same square subroutine for each and print the two squares in input order.

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
CALL square
PRINT
INPUT
CALL square
PRINT
HALT
square:
  # R0 in, R0 squared out.
RETURN

Need a hint?

MUL R0 multiplies the accumulator by its current value.

Reveal a worked solution

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

INPUT
CALL square
PRINT
INPUT
CALL square
PRINT
HALT
square: MUL R0
RETURN

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
CALL square
PRINT
INPUT
CALL square
PRINT
HALT
square:
  # R0 in, R0 squared out.
RETURN

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.

Progress uses localStorage only. It stays on this browser and is never sent to a server.