Computer Systems Exam 1

What does PC mean and what does it do?

• Program Counter

  • Stores the address of next instruction

What are condition codes? 

• They store status information about most recent arithmetic or logical operation 

  • isnt used in MIPS but used in other ISA’s 

Register File?

• Used in program data

Memory?

•  A byte addressable array 

• holds code and user data 

• stack to support procedures 

What is the Assembly/machine code view of CPU and Memory? Draw it !!!

What are computers made of?

• They are made of circuits

  • Use tow symbols to represent information

    • Low voltage (off, or zero)

    • High voltage (on, or one)

  • They are also base 2 (binary)

What is a not gate and its truth table? 

What is an AND gate and its truth table?

What is does an OR gate look like and whats its truth table?

What does an XOR gate look like and whats its truth table?

What are the components of the system? 

• Processing

  • Where computation happens

• Memory

  • where data is stored

• Peripherals

  • communication beyond the computer

What are the components of circuits and what do they do?

• Power

  • may act as a 1 and or high voltage (on)

• Ground

  • acts like a 0 and or low voltage (off)

• Resistors

  • allow a change of voltage

• Transistors

  • Are the building blocks of computers

  • allow all circuits to compute values dynamically

  • The gate controls the connection between source and drain

    • sending voltage to the gate connects source and drain

    • removing voltage to the gate disconnects source and drain

What is a multiplexer?

• A hardware unit that acts as selectors

What does a multiplexer do?

• Takes multiples inputs 

  • 1 selection input

  • multiple possible choices

• 1 output

  • value is equal to the choice input chosen by selection input

• They make it possinle for several input signals to share one device or resource

What is a clock signal?

• A signal that oscillates between high and low state at a constant frequency (think metronome)

  •  it is used to synchronize actions by using latches

What does a latch (AKA registers) do?

• They manage the data transfer between components on rising edges 

  • They store a bit of data temporarily until its updated by new input signals

What are peripherials and name a few examples

• They are how computers communicate with the outside world

  • are used by memory-mapping

• Examples:

  • monitor

  • mouse

  • keyboard

  • internet

  • printer

  • speaker

  • webcam

What is the hardware in a processor? 

• storage 

  • Instruction memory

    • fetch 32-bit instructions

  • Data memory

    • Load/store 64-bit data

  • Register Array

    • storage for 32 integer registers

    • has 2 read ports/ can read 2 registers at once

    • 1 write port

• Functional Units

  • +4

    • PC incrementer

  • Xtnd

    • sign extender

  • ALU

    • Arithmetic and logical instructions

  • Zero test

    • detector whether operand == 0

What happens when executing an instruction? (Ex)

The ALU performs the specified operation from the instruction opcode bits on its input values. 

What happens when fetching an instruction? (IF)

The required data is fetched from main memory with an increment of +4 

What happens when decoding an instruction?(ID)

Read source registers and generate control signals 

what happens when using memory with an instruction?(MEM)

Read or write the data memory

What happnes in write back with an instruction?(WB)

Store a result in the desired register

What is latency?

• The time between the beginning and completion of an instruction

• = number of stages * maximum stage time (ns)

  • remember when trying to get the maximum stage time include the latch time with that

What is throughput (AKA bandwidth) 

• The rate of instruction completion 

• = 1 / maximum stage time (GHz)

What is Amdahl’s Law?

S_overall = T/ T’ = 1/ ((1-F_enhanced) + (F_enhanced/ S_enhanced))

What are the 5 stages of pipelining

• Fetch (IF)

• Decode (ID)

• Execute (EX)

• Memory (MEM)

• Write back (WB)

What is a data hazard?

Events thats occur when the result of a previous instruction and that result of instruction has not yet been computed.

Pairs of instructions that would change output if reversed

What are the types of data hazards?

• RAW (read after write)

• WAW (write after write)

• WAR (Write after read)

what is a control hazard?

When a choice on what instruction to fetch hasnt been made in time for the next instruction

What is stalling?

The idea behind stalling is to delay the dependent instruction until data is ready.

what are pipeline stage transitions?

• links between pipeline stages that can be disabled or enabled

  • bubble

  • transfer

  • stall

  • latches

What is branch prediction?

technique used in hardware to guess the outcome of conditional branch instructions

How does a static prediction work?

• Makes a prediction independent of code execution

How does a static not-taken prediction work?

Assumes that a branch will never be taken 

How does branch prediction improve performance?

The more successful predictions you have the faster your system can run (usually only good for low-latency systems).

How is correctness maintained in branch prediction?

• Store computation without committing it

  • if wrong, rollback and pay a penalty

  • otherwise commit it

Why are taken branches harder to predict well then non-taken branches?