ECE 353 Fall 2012
Lab C
Pipeline Simulator
October 18, 2012
ECE 353
Aims of Lab C
 Reinforce your understanding of pipelining
 Provide additional experience in C programming
• Managing queues
 Introduce you to time-driven simulation
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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 ``quasiinstructions”
• 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
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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
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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
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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)
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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
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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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
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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.
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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)
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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
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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
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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
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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
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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
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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
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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
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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.
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Tips from TA
Doug Frazer
ECE 353
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
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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.
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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
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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., enum OPCODE 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
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