## Department of Electrical and Computer Engineering

### The University of Texas at Austin

EE 360N, Fall 2005
Problem Set 4
Due: 9 November 2005, before class
Yale N. Patt, Instructor
Aater Suleman, Linda Bigelow, Jose Joao, Veynu Narasiman, TAs

Instructions:
You are encouraged to work on the problem set in groups and turn in one problem set for the entire group. Remember to put all your names on the solution sheet. Also remember to put the name of the TA in whose discussion section you would like the problem set returned to you.

1. In class, we discussed two types of busses: "pending bus" and "split transaction bus". What is the advantage of a split-transaction bus over a pending bus?

2. In class, we discussed the asynchronous finite state machine for the device controller of an input-output device within the context of a priority arbitration system. Draw the state diagram for this device controller (as drawn in lecture), identify the input and output signals, and briefly explain the function of each input and output signal.

As mentioned in class, the finite state machine has some race conditions. Identify the race conditions and show what simple modifications can be made to eliminate them.

3. In class we discussed asynchronous buses with central arbitration. Our job in this problem is to design the state machine for a synchronous bus using distributed arbitration. Recall that with distributed arbitration, each device receives the Bus Request signals from all other devices, and determines whether or not it is the next Bus Master. Assume all bus transactions take exactly one cycle, and that no device may be the Bus Master for two consecutive cycles.

Assume four devices, having priorities 1, 2, 3, and 4 respectively. Their respective controllers request the bus via asserting BR1, BR2, BR3, and BR4 respectively. Priority 4 is the highest priority.

1. Show the interconnections required for distributed arbitration for the four devices and their controllers connected to the bus. Be sure to label each signal line and designate by arrows whether the signals are input or output with respect to the device.
2. Is it possible for starvation to occur in this configuration? Describe the situation where this can occur.
3. Assume each I/O Controller is implemented using a clocked finite state machine. Draw a Moore model state machine for the controller operating at priority level 2. Label each state clearly. Label all necesary inputs and outputs. You do not need to show the clock signal on the state machine diagram. State transitions are synchronized to the clock.
4. Given the following code:
```MUL R3, R1, R2
MUL R7, R8, R9
MUL R10, R5, R6
```
Note: Each instruction is specified with the destination register first.

Calculate the number of cycles it takes to execute the given code on the following models:

1. A non-pipelined machine.
2. A pipelined machine with scoreboarding and five adders and five multipliers.
3. A pipelined machine with scoreboarding and one adder and one multiplier.
Note: For all machine models, use the basic instruction cycle as follows:
Fetch (one clock cycle)
Decode (one clock cycle)
Execute (MUL takes 6, ADD takes 4 clock cycles)
Write-back (one clock cycle)

Do not forget to list any assumptions you make about the pipeline structure (e.g., data forwarding between pipeline stages).

5. Suppose we have the following loop executing on a pipelined LC-3b machine.
```
DOIT     STW   R1, R6, #0
AND   R3, R1, R2
BRz   EVEN
BRp   DOIT
BRp   DOIT
```

Assume that before the loop starts, the registers have the following decimal values stored in them:
```       R0:       0
R1:       0
R2:       1
R3:       0
R4:       0
R5:       5
R6:    4000
R7:       5
```

Fetch-stage takes 1 cycle, Decode-stage takes 1 cycle, Execute-stage takes variable number of cycles depending on the type of instruction (see below), and Store-stage takes 1 cycle.

All execution units (including the load/store unit) are fully pipelined and the following instructions that use these units take the indicated number of cycles:

```       STW:       3
AND:       2
BR :       1
```
• Data forwarding is used wherever possible. Instructions that are dependent on the previous instructions can make use of the results produced right after the previous instruction finishes the Execute-stage.
• The target instruction after a branch can be fetched when the BR instruction is in ST stage. For example, the execution of an ADD instruction followed by a BR would look like:
```ADD       F | D | E1 | E2 | E3 | ST
BR            F | D  | -  | -  | E1  | ST
TARGET                                 F  | D
```
• The pipeline implements "in-order execution". A scoreboarding scheme is used as discussed in class.

1. How many cycles does the above loop take to execute if no branch prediction is used?
2. How many cycles does the above loop take to execute if all branches are predicted with 100% accuracy.
3. How many cycles does the above loop take to execute if a static BTFN (backward taken-forward not taken) branch prediction scheme is used to predict branch directions? What is the overall branch prediction accuracy? What is the prediction accuracy for each branch?

6. A five instruction sequence executes according to Tomasulo's algorithm. Each instruction is of the form ADD DR,SR1,SR2 or MUL DR,SR1,SR2. ADDs are pipelined and take 9 cycles (F-D-E1-E2-E3-E4-E5-E6-WB). MULs are also pipelined and take 11 cycles (two extra execute stages). The microengine must wait until a result is in a register before it sources it (reads it as a source operand).

The register file before and after the sequence are shown below (tags for ``After'' are ignored). 1. Complete the five instruction sequence in program order in the space below. Note that we have helped you by giving you the opcode and two source operand addresses for instruction 4. (The program sequence is unique.) 2. In cycle 1 instruction 1 is fetched. In cycle 2, instruction 1 is decoded and instruction 2 is fetched. In cycle 3, instruction 1 starts execution, instruction 2 is decoded, and instruction 3 is fetched.
Assume the reservation stations are all initially empty. Put each instruction into the next available reservation station. For example, the first ADD goes into ``a''. The first MUL goes into ``x''. Instructions remain in the reservation stations until they are completed. Show the state of the reservation stations at the end of cycle 8.

Note: To make it easier for the grader, when allocating source registers to reservation stations, please always have the higher numbered register be assigned to SR2. 3. Show the state of the Register Alias Table (V, tag, Value) at the end of cycle 8. 