ECE 353 Fall 2011 Lab C Pipeline Simulator

1. ECE 353 Fall 2011Lab CPipeline Simulator October 20, 2011

2. Aims of Lab C • Reinforce your understanding of pipelining • Provide additional experience in C programming • Managing queues • Introduce you to time-driven simulation

3. Outline of Lab • Write a simulator for the MIPS five-stage pipeline (covered in ECE 232) that does the following: • Implements a subset of the instruction set (LW, SW, BEQ, ADD, SUB, MUL, ADDI) as well as ``quasi-instructions” • Reads from a file an assembly language program • Simulates, cycle by cycle, the activity in all registers associated with that program • Displays the values of all the integer registers (except for register 0) and the PC • Gives the user the option of stepping through the simulation, cycle by cycle, or just checking the registers at the end of the program and the utilization of each stage

4. Types of Simulator • Event-Driven Simulator: Identifies events of interest in the system being simulated, orders them in time, and moves down the ordered list of events, simulating the machine as it goes. • Example: Queuing network simulator for a computer network. • Time-Driven Simulator: Has a central clock and moves from one clock cycle to the next, simulating all the activity of interest in that clock cycle. • Example: Architecture simulator

5. Five-Stage Pipeline • IF Stage: Fetches instructions • If a conditional branch is encountered, the fetch unit stops fetching until that branch is resolved. • The IF stage places fetched instructions in an instruction queue, to be consumed by the ID stage. • The instruction queue can hold up to q instructions, where q is an input to the simulator. • Assume the PC has its own dedicated adder to increment the byte address by 4 (or word address by 1). • ID Stage: Decodes the instructions in the instruction queue, one by one. • Immediate operands are sign-extended to 32 bits • Makes operands available to the EX stage • Generates control signals

6. Five-Stage Pipeline (contd.) • EX Stage: • Executes arithmetic/logical instructions • BEQ instructions are resolved in this stage • MEM Stage: • Carries out access to the data memory • WB Stage: • Writes back into the register file (if necessary)

7. Simplifying Assumptions • Separate instruction and data memories; all memory accesses are hits. No need to model cache misses. • There is just one ALU and it is NOT pipelined. It takes • m cycles for MUL • n cycles for all other arithmetic and logic operations • m and n are input parameters to the simulator • Integer operations only: FP is not implemented • No forwarding is available in this pipeline • Ignore all interrupts • Register writes are done in the first half of a clock cycle; register reads in the second

8. Interaction between IF and ID stages • IF must not overrun the ID unit: if the instruction queue fills up, IF must stop fetching until there is at least one empty slot in the queue • ID must not overrun the IF unit

9. Parsing Instructions • The assembly language program must be read by the simulator • You can use a case statement associated with each possible instruction, which tells the simulator what to do with that instruction. • When an instruction reaches the ID stage, controls governing the rest of its activity will be generated.

10. A Straightforward Approach • Mimic within the simulator what happens in the MIPS pipeline • Generate the control signals associated with each instruction in the ID stage • Pass these signals along, stage by stage, along with the instruction. • Don’t forget to check for data hazards • Use a branch_pending signal to guide the IF unit. • Every clock tick, mimic what is supposed to happen in each of the five stages and collect statistics on which stages are doing anything useful

11. Maintain Data Memory State • Keep track of the contents of the data memory • Each data memory access takes c cycles • Some data (as specified by the lab handout) will already be in the data memory when the program starts • Make any reasonable assumption about the addresses of your data • No virtual memory: assume all addresses are physical • Data and instruction address spaces are separate

12. Instruction Memory • The program starts at location 0 of the instruction memory • The PC will point to this location at the beginning of the simulation • All addresses are physical: no virtual addresses (no need for TLBs or page tables) • Each memory access takes c cycles

13. Test Program 1 • Multiplication of two 10X10 matrices: C=A X B. • Starting address of matrices A, B, and C are already in registers $1, $2, and $3 when execution begins (since you don’t have a virtual memory management system, don’t worry about how these addresses were loaded into these registers): pick any appropriate starting data addresses • The values of A and B for the test program are: A[i][j] = B[i][j] = i*2+j. Initialize your simulator by loading these values into the data memory: should be a separate function called d_initialize.

14. Simplifications • To make it easier for you to read in the assembly language program: • The assembly language is redefined to do away with commas; spaces will do instead. • No register names will be used: register numbers only. • Example: add 3 2 1 means add register 1 contents to those of register 2 and put the result in register 3. • The SW and LW instructions are redefined to remove their offset field • Example: lw 5 3 means to load into register 5 from the memory at the location pointed to by register 3. • For 10% extra credit, remove all these limitations and enable the simulator to accept the standard MIPS assembly language format.

15. Simplifications (contd.) • Don’t use labels in your program: instead the beq instruction will identify its target by an offset. • Remember from ECE 232 that the offset is with respect to the instruction following beq and is in units of words, not bytes • Example: beq 4 5 -2 means that if the contents of registers 4 and 5 are identical, we branch to the instruction immediately preceding beq. • For 5% extra credit, endow the simulator with the ability to handle instruction labels.

16. Reading the Assembly Language Program • C provides a number of ways in which to read input. For example, fscanf(fp, “%s %d %d %d”, opcode, &field2, &field3, &field4); where fp is a file pointer, opcode is a character array, and field2, field3, and field4 are integers

17. Parsing the Assembly Language Program • Some useful library functions (remember to #include <string.h>): • *strcpy: Copies one string into another • strcmp: Compares two strings. Note that this will give you an output of 0 if the two strings match.

18. Test Program 2 • Use quasi-instructions: these don’t do anything other than take up time in the pipeline stages. • A quasi-instruction: • Consumes one cycle to decode and takes one cycle to pass through each of the MEM and WB stages. • Takes a random amount of time in the EX stage: picks any of the integers in {a, a+1, …, b} with equal probability and uses that as its execution time. • Has no impact on the register contents. • Does not suffer from data hazards (since it neither uses nor produces register contents)

19. Generating Random Numbers • Computers are deterministic, so how can we use them to generate random numbers? • Approach 1: Provide random input. • Example 1: Use time interval between keystrokes. • Example 2: Have the user move the mouse randomly in a given area and extract numbers from that. • Example 3: Use a device (e.g., vacuum tube) whose noise characteristic is known and measure the noise to generate the random number • Approach 2: Generate pseudo-random numbers. • This is the technique we will use

20. Pseudo-Random Numbers • These are generated iteratively, from a given seed. • Example: X_n+1 = (aX_n + b) mod M • X_1, X_2,… are the outputs of the generator • X_0 is the seed, which is given to the generator • a, b and M are integer constants: these determine how good the generator is. • The above stream of numbers is: • Clearly NOT random: given X_i, you can predict the rest of the stream • The stream repeats itself: the cycle cannot be more than M long

21. Pseudo-Random Numbers (contd.) • Given that pseudo-random numbers are not random, how can we use them as if they were random? • We check the statistical properties of these numbers. Some of these are: • Probability distribution function. • Measures of correlation between random numbers • We can use pseudo-random numbers in simulations if their statistical properties are sufficiently close to a truly random stream

22. Pseudo-Random Numbers (contd.) • Random Number Generators (RNGs) are an active area of research: there are many available. • Using a good RNG is often vital to the correctness of the simulation • The C standard library (accessed by including stdlib.h in the header) has a function, rand() • rand() is an integer function, generating values from 0 to RAND_MAX, where RAND_MAX is a constant defined within the library • It is not the best RNG around, but it is available and easy to use and good enough for our purposes

23. Using rand() to pick exec time at random • Quasi-instructions take a random amount of time in the set {a, a+1, …, b} in the EX stage. • rand() generates random integers uniformly distributed over the set {0, 1, …., RAND_MAX}. • Use rand() to generate the execution time of each of the quasi-instructions

24. Intermediate Checkpoints Show a TA the following items: • Code for reading the assembly program (by November 2 – carries deadline-sensitive 5% credit): • Write assembly code to multiply two 10X10 matrices • Demonstrate that your code for reading the assembly code from a file works correctly • Code for Instruction Queue Management: (by November 8 – carries deadline-sensitive 5% credit): • Interaction between IF and ID units • Handling unresolved branches • Demonstrate that your code works correctly

25. Simulator Output • The simulator can be set to operate in either of two modes: single-step or batch • Output consists of • Contents of each of the 31 registers $1 to $31 and that of the PC (in decimal) • The number of clock cycles that have elapsed • Utilization of each stage IF, ID, EX, MEM, WB. • In single-step mode, this will be the utilization so far; in batch mode, this will be the utilization for the entire program • The utilization of a stage is the number of cycles during which that stage did anything useful divided by the total number of cycles

26. Lab Report • Description of the structure of your simulator: Include • How the IF stage manages branches and how it knows that branches have been resolved • How hazards are handled • How instructions are prevented from progressing if there’s a data or structural hazard • How you implement single-step mode • Source code for your simulator • Fully document your code • Keep the code as well-structured as possible • Matrix multiplication program: For the parameter values specified in the lab document, plot the following: • Assembly language code • Total execution time • Fraction of fetch buffer utilized • Utilization of each pipeline stage • Your strategy for testing the simulator for correctness.

27. Tips from TA Doug Frazer

28. Tips from TA (Doug Frazer) • Start with the queue: • Write enqueue and dequeue functions • Test them thoroughly against a controlled set of instructions • Be sure to test things such as: • Calling dequeue on an empty queue • Handling NULL next and prev pointers on dequeue and enqueue • Handle dependencies on queue • Handle different types of MIPS commands • Test with MIPS

29. Tips from TA • The assembly and C are relatively independent • One person can work on each • If you need to test the C and do not have working assembly, try running it against MIPS from your hardware book.

30. Intro to Structures and enums • Generic data structure: struct • Define necessary information to be grouped together • Precompiler ‘labels’: enum • Makes the code easier to read and debug by using BEQ instead of an integer to represent BEQ • Gets compiled to an integer, first element is 0 and it increments by 1 unless you specify a different value • Useful for this lab: • Define a struct for each instruction • Define an enum for registers • Define an enum for instruction type

31. Enumerated Data Types • enum OPCODE {add, addi, sub, mul, lw, sw, beq} • Defines OPCODE as a data type, taking values just from the above set • With this definition, you can declare variables to be of type OPCODE, e.g., enumOPCODE op1, op2, op3; • Internally represents them as integers, starting from 0 • Can use equality checks or case statements • if (op1 == add) • case op2: • You can define enum types for the opcode and the registers • You’ll still need to read the instruction opcode from the file as a character string