Decisions and Flags · Lesson 12 of 30 · about 10 minutes

A Memory-backed Decision

Make a decision from stored data and record the result.

01 / Explain

Understand the idea

Programs often combine input with saved state. Here memory address 40 contains a ticket code, and address 41 will hold the decision. A code is a learning-game value, not a password or real authentication mechanism.

Separate the rule from its current data. The same program should work when the stored ticket code changes. Save a 1 for a match and a 0 for a mismatch, then display that same decision.

02 / Try

Watch it happen

Watch cell 41 record 1. Change the guess to 22 and replay the other branch.

STORE 23 40
LOAD [40] R1
LOAD 23
CMP R1
JUMP_IF_ZERO match
LOAD 0
JUMP record
match: LOAD 1
record: STORE 41
PRINT
HALT

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

03 / Challenge

Make it work

Memory 40 holds a supplied ticket code. Read one guess, then print and store at address 41 a 1 for a match or a 0 otherwise.

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 [40] R1
INPUT
CMP R1
# Decide and record the result.
HALT

Need a hint?

Both paths can share the STORE and PRINT instructions.

Reveal a worked solution

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

LOAD [40] R1
INPUT
CMP R1
JUMP_IF_ZERO match
LOAD 0
JUMP record
match: LOAD 1
record: STORE 41
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.

LOAD [40] R1
INPUT
CMP R1
# Decide and record the result.
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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