Machine Instructions

Which of the following instructions correctly tests if the value stored in M[100] is zero and jumps to label END if it is?

• @100

  D=M

  D;JEQ

  @END

  0;JMP

• @100

  D=M

  @END

  D;JEQ

• @END

  @100

  D=M

  D;JEQ

• @100

  D=M

  @END

  M;JEQ

Answer explanation

@100 loads address 100 into A.
D=M loads value at RAM[100] into D.
@END loads END's address into A.

D;JEQ jumps to END if ALU output (here D) == 0.

Which of the following correctly describes the Hack registers?

• • A: Arithmetic register only

  • D: Data register only

  • M: Memory location at RAM[0]

• • A: Address/Data register

  • D: Data register

  • M: Memory value at address stored in A

• • A: Program counter register

  • D: Destination register

  • M: Memory value at D

• • A: Address register only
    D: Data register only

  M: Memory value at address stored in D

Answer explanation

• A register: Can hold either a value or an address.

• D register: General-purpose data register.

• M: Not a physical register; it represents RAM[A]

@10

D=A

What will be stored in D?

• 10

• Value at RAM[10]

• Address of instruction

• Contents of D unchanged

Answer explanation

@10 loads 10 into A.
D=A copies the value of A (which is 10) into D.

If the code is:

@7

D=M

and RAM[7] = 15, what will D contain?

• 7

• 15

• Address of next instruction

• 0

Answer explanation

• @7 sets A = 7.

• D=M copies RAM[A] (i.e., RAM[7]) → D.

Which statement is FALSE about A, D, and M registers?

• D cannot be used to address memory directly.

• M is not a physical register.

• A can hold either an address or a constant.

• M can directly store an address to another memory location.

Answer explanation

• M refers only to RAM[A]. It does not store an address; it stores a value.