CS/ECE 252 Introduction to Computer Engineering

Let's assume that PC = 0x0101, R1 =0x250F, R2=0xCAFE, R3 = 0xBEEF and any other accessed memory contains 0x0000
For the following LC-3 instruction

0001001010000001

1. What type of instruction is it?
   ADD instruction
2. Which registers are used?
   R1 and R2
3. What is the value in R1, R2 and R3 after the execution of this instruction?
   R1 = 0xF00D
   R2 = 0xCAFE
   R3 = 0xBEEF
   
4. How are the n, p or z flags affected by the execution of this instruction?
   n = 1
   p = 0
   z = 0
   

Problem 2 (6 points)

The following table shows a part of the LC-3's memory:

Address	Content
x3100	1001 001 001 111111
x3101	0101 010 010 000001
x3102	1001 010 010 111111
x3103	0000 010 111 111100

If the conditional branch redirects control to location x3100, state what is known about R1 and R2 before the execution of the program.
Branch is taken means that the result of the 3rd instruction; NOT(R2)=0, was zero. So, R2 was 0xFFFF at the end of the 2nd instruction. R1 AND R2 =0xFFFF implies that both R1 and R2 were 0xFFFF after the instruction 1. So at the start R1 was 0x0000 and R2 was 0xFFFF

Problem 3 (4 points)

1. LD
   2 ( one to fetch instruction + one to fetch data)
2. LDR
   2 ( one to fetch instruction + one to fetch data)
3. LDI
   3 ( one to fetch instruction + one to fetch address + one to fetch data)
4. LEA
   1 ( only to fetch instruction )

Problem 4 (6 points)

Assume we have a 256 bytes memory with 128 different addresses and 2 bytes are stored at each address

1. How many bits are required for the memory addressing?
   log₂(128) = 7bits
2. Assume we want to address 20 locations in both directions, how many bits are required for an instruction to specify the offset in Base+Offset addressing mode?
   In 2's complement we can represent -2^(n-1) to 2^(n-1) -1
   20 = 2^(n-1) -1
   n = 6 bits
3. Assume we want to address 20 locations in both directions, how many bits are required for an instruction to specify the offset in PC-relative offset addressing mode?
   20 = 2^(n-1) - 1
   n = 6 bits

Problem 5 (6 points)

Write out LC-3 instructions (in hex) for the implementing following tasks

1. Subtract the value of R4 from R1 and store the result in R1
   R4 = NOT (R4)
   R4 = R4 + 1
   R1 = R1 + R4
   
2. Take the OR of R3 and R4 and store the result in R3
   R3 = NOT(R3)
   R4 = NOT(R4)
   R3 = R3 AND R4
   R3 = NOT(R3)
   

Problem 6 (3 points)

We would like to have an instruction that does nothing. Many ISAs actually have an opcode devoted to doing nothing. It is usually called NOP, for NO OPERATION. The instruction is fetched, decoded and executed. The execution phase is to do nothing! Which of the following three instructions could be used for NOP and have the program still work correctly?

1. 0001 001 001 1 00000
   ADD R1, R1, #0 differs from a NOP because it sets modifies the flags
2. 0000 111 000 0 00001
   BRnzp #1, Unconditionally branch to one after the next address in the PC. Therefore this instruction is not the same as NOP
3. 0000 000 000 0 00000
   Same as NOP

Problem 7 (3 points)