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The LC-3 – Chapter 5

The LC-3 – Chapter 5

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The LC-3 – Chapter 5

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  1. The LC-3 – Chapter 5 COMP 2620 Dr. James Money COMP 2620

  2. Operate Instructions • Recall that there are three types of instructions: • Operate instructions • Data movement instructions • Control instructions • We first consider the operate instructions today

  3. Operate Instructions • Remember these types of instructions process data • It includes the arithmetic operations such as ADD, SUB, MUL, DIV • In addition, it includes the logical instructions such as AND, OR, NOT, XOR

  4. Operate Instructions • The LC-3 has three operate instructions • ADD • AND • NOT • We will have to essentially emulate the remaining operations that we need

  5. Operation: NOT • The NOT instruction is the only operate instruction that is unary - it requires only one source operand • The opcode is 1001 • The output of the operation is the 16-bit logical NOT of the source operand • NOT uses register addressing mode

  6. Note: The source and destination operands can be the same

  7. Operation: NOT • Consider the instruction: 1001 011 101 111111 • SRC = 101 = R5 • DST = 011 = R3 • Assume R5=1010 1111 0000 1111Then, after the instruction R3=0101 0000 1111 0000

  8. Operation: NOT

  9. Operations: ADD and AND • These are a binary operations – they require two inputs • The opcodes are • ADD – 1001 • AND - 0101 • The source operands are 16 bits • The first operand is a register

  10. Operations: ADD and AND • ADD performs a two’s complement addition of the two source operands • AND performs a bitswise AND operation on the two source operands • The second is either a register or immediate value • Bit[5] = 0 implies register • Bit[5] = 1 implies immediate mode

  11. Register Mode this zero means “register mode”

  12. Immediate Mode this one means “immediate mode” Note: Immediate field issign-extended.

  13. Operations: ADD and AND • Assume R4=6, R5=-18 • Consider the instruction 0001 001 100 000 101 • Then, R1=-12 • That is R1= R4+R5

  14. Operations: ADD and AND

  15. Operations: ADD and AND • What does the following instruction do? 0101 010 010 100 000 Register 2 is cleared • What does the following instruction do? 0001 110 110 100 001 R6=R6+1

  16. Operations: ADD and AND • Recall that the 2’s complement of a number for this example • Assuming R0 and R1 have values A and B, respectively, what sequence of instructions perform R2=A-B 1001 001 001 111 111 R1=NOT(R1) 0001 010 001 100 001 R2=R1+1 0001 010 000 000 010 R2=R0+R2