Decisions and Flags · Lesson 10 of 30 · about 10 minutes

Conditional Logic: Making Decisions

Compare values, then branch on a stored flag.

01 / Explain

Understand the idea

CMP value compares R0 with a value by setting flags from R0 − value. It leaves R0 unchanged. Equal values set the zero flag. JUMP_IF_ZERO tests that stored flag, which can come from CMP even when R0 itself is not zero.

A label names an instruction address. JUMP same follows that address without counting comments or source lines. Be careful: LOAD, COPY, and arithmetic update flags, so place the conditional jump immediately after the comparison it should use.

02 / Try

Watch it happen

R0 is seven at CMP, yet the zero flag is true. The program prints 1.

LOAD 7
CMP 7
JUMP_IF_ZERO same
LOAD 0
JUMP show
same: LOAD 1
show: PRINT
HALT

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. Print 1 when they are equal, otherwise print 0. Handle both equal and unequal inputs.

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
INPUT
CMP R1
# Add two labeled paths.
HALT

Need a hint?

The unequal path needs a JUMP past the equal path.

Reveal a worked solution

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

INPUT
COPY R0 R1
INPUT
CMP R1
JUMP_IF_ZERO same
LOAD 0
JUMP show
same: LOAD 1
show: PRINT
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 R1
INPUT
CMP R1
# Add two labeled paths.
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.

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