Build Small Programs · Lesson 28 of 30 · about 18 minutes
A Tiny Target Game
Build a three-round game with stored state and scoring.
01 / Explain
Understand the idea
A game is a program with rules, input, state, and feedback. Memory 200 holds the target number. In each of three rounds, the player submits a guess and earns one point for equality. The score changes only on a hit.
Separate the game rules from the current target and guesses. Tests change both. This tiny game prints a numeric final score; no randomness, network, account, or secret student data is involved. A game can be interesting without pretending the VM prints text or graphics.
02 / Try
Watch it happen
Queue guesses 5, 2, 5. The final score is 2 in both output and memory 201.
STORE 5 200
LOAD [200] R1
LOAD 0 R2
LOOP 3
INPUT
CMP R1
JUMP_IF_ZERO hit
JUMP next
hit: LOAD R2
ADD 1
COPY R0 R2
next: NOP
RETURN
LOAD R2
STORE 201
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Memory 200 contains a target byte. Read exactly three guesses. Count how many equal the target, store the score at memory 201, and print that score once.
The checker runs your current editor program in a fresh machine for each of 4 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
LOAD [200] R1
LOAD 0 R2
LOOP 3
INPUT
CMP R1
# Add one point only on a hit.
RETURN
LOAD R2
STORE 201
PRINT
HALTNeed a hint?
The miss branch must skip the score increment but still reach the loop’s RETURN.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
LOAD [200] R1
LOAD 0 R2
LOOP 3
INPUT
CMP R1
JUMP_IF_ZERO hit
JUMP next
hit: LOAD R2
ADD 1
COPY R0 R2
next: NOP
RETURN
LOAD R2
STORE 201
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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