Teacher answer key

Meet the Machine

Gather four kinds of evidence: prediction of relevant state; behavior across varied cases including boundaries; explanation using trace/state evidence; and a reasoned correction of one mismatch. Mark each as emerging, with support or independent using paper or your school’s existing approved process. A challenge pass is machine-behavior evidence, not proof of authorship or complete mastery. Accept equivalent correct programs; the public key is a model, not the only possible answer.

Public teaching material. Solutions are examples; equivalent correct programs may also pass the actual checks.

1. Your First Emoji Program

Move a number into the CPU, print it, and stop.

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1. Predict and trace

Before running the Try program, predict its output and trace the first three executed instructions. Track the relevant registers, flags or memory as needed. Then step the program to compare.

📥 42
🖨️
⏹️

Queue input: None

Initialize memory: All bytes initially zero

Try prediction key

Output: 42

Instruction	PC before	PC after	R0 before	R0 after	Relevant state	Output
LOAD	0	1	0	42	`{"registers":[42,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None
PRINT	1	2	42	42	`{"registers":[42,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	42
HALT	2	2	42	42	`{"registers":[42,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	42

2. Build and check

Write a program that prints 42 exactly once and halts. Use LOAD, PRINT, and HALT.

Required instruction types: LOAD, PRINT, HALT

Example challenge solution

LOAD 42
PRINT
HALT

Actual checker fixtures

Case 1

Input: None
Initial memory: All bytes initially zero
Expected output: 42

3. Explain the machine

What is in R0 and output just before PRINT? What PC does HALT leave?

Reasoning and teaching note

R0=42; output empty before PRINT; output 42 after it; HALT at compiled index2.

2. Understanding Registers

Use the eight fast workspaces without losing a value.

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1. Predict and trace

Before running the Try program, predict its output and trace the first three executed instructions. Track the relevant registers, flags or memory as needed. Then step the program to compare.

LOAD 17 R1
LOAD 99
COPY R1 R2
LOAD R2
PRINT
HALT

Queue input: None

Initialize memory: All bytes initially zero

Try prediction key

Output: 17

Instruction	PC before	PC after	R0 before	R0 after	Relevant state	Output
LOAD	0	1	0	0	`{"registers":[0,17,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None
LOAD	1	2	0	99	`{"registers":[99,17,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None
COPY	2	3	99	99	`{"registers":[99,17,17,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None

2. Build and check

Keep 17 in R1 and copy it into R2. Print 17, then leave R0 at zero before halting. R1 and R2 must still hold 17.

Required instruction types: COPY, PRINT, HALT

Example challenge solution

LOAD 17 R1
COPY R1 R2
LOAD R2
PRINT
LOAD 0
HALT

Actual checker fixtures

Case 1

Input: None
Initial memory: All bytes initially zero
Expected output: 17
Expected final registers: R0: 0, R1: 17, R2: 17

3. Explain the machine

Why can R0 end at0 while R1 and R2 retain17?

Reasoning and teaching note

Registers are independent workspaces; COPY preserves source; clearing R0 does not clear R1/R2.

3. Following the Program Counter

Read a trace and separate source lines from instruction addresses.

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1. Predict and trace

Before running the Try program, predict its output and trace the first three executed instructions. Track the relevant registers, flags or memory as needed. Then step the program to compare.

# A comment is not instruction zero.
LOAD 7
NOP
PRINT

LOAD 9
PRINT
HALT

Queue input: None

Initialize memory: All bytes initially zero

Try prediction key

Output: 7, 9

Instruction	PC before	PC after	R0 before	R0 after	Relevant state	Output
LOAD	0	1	0	7	`{"registers":[7,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None
NOP	1	2	7	7	`{"registers":[7,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	None
PRINT	2	3	7	7	`{"registers":[7,0,0,0,0,0,0,0],"flags":{"zero":false,"negative":false,"overflow":false},"sp":255,"stack":[],"callStack":[],"loopStack":[],"memoryReads":[],"memoryWrites":[]}`	7

2. Build and check

Print 7 and then 9, in that order, exactly once each. Include a NOP between loading 7 and printing it.

Required instruction types: NOP, LOAD, PRINT, HALT

Example challenge solution

LOAD 7
NOP
PRINT
LOAD 9
PRINT
HALT

Actual checker fixtures

Case 1

Input: None
Initial memory: All bytes initially zero
Expected output: 7, 9

3. Explain the machine

Why do blank/comment source lines not change the output order?

Reasoning and teaching note

They create no compiled instructions; PC counts instruction addresses, output remains7,9.