Meet the Machine · Lesson 3 of 30 · about 7 minutes
Following the Program Counter
Read a trace and separate source lines from instruction addresses.
01 / Explain
Understand the idea
The CPU follows program order, not the position of a value on the screen. A comment explains your intent to a person; it creates no instruction. NOP advances the PC while preserving registers, flags, and output.
A trace records each instruction’s before and after state. Step Back restores that earlier state, so you can inspect a mistake and replay it. A breakpoint pauses before its instruction; Step still executes that instruction explicitly.
02 / Try
Watch it happen
Set a breakpoint on the second LOAD. Run, step back once, and compare the trace with the source line highlight.
# A comment is not instruction zero.
LOAD 7
NOP
PRINT
LOAD 9
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Print 7 and then 9, in that order, exactly once each. Include a NOP between loading 7 and printing it.
The checker runs your current editor program in a fresh machine for each of 1 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
LOAD 7
NOP
# Finish the two outputs.
HALTNeed a hint?
PRINT does not change R0; loading the next value does.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
LOAD 7
NOP
PRINT
LOAD 9
PRINT
HALTEmoji CPU lab
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.
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.
CPU registers
- R0
- 0
- R1
- 0
- R2
- 0
- R3
- 0
- R4
- 0
- R5
- 0
- R6
- 0
- R7
- 0
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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