Processors

#Computer-Science#CS250#Hardware

Main Computer Architectures §

There are two main computer architecture types

Harvard Architecture §

• Dedicated instruction memory
• Dedicated data memory
• Processor knows bit strings of instructions from data

Von Neumann Architecture §

• One memory for both data and program
• Can only access memory one item at a time, can’t get both data and instructions at same time

What is a processor? §

• General purpose processor
• Building hardware is expensive, so we like to have a generic processor that can be used for a lot of things
• Doesn’t make sense to build one processor for each problem, instead we make one general one and make a LOT of them
• Early electrical computers were just wall mounted circuits with wires connecting things.
• Design of a general purpose processor
  • Using collections of bit strings machine instructions to represent programs
  • Store machine instructions in memory
  • Then fetch a given machine instruction when needed for the next program step

How fast do things run on a processor? §

Clock rate, instruction rate, and time §

• Clock rate is upper bound by the worst case circuit propagation delay
  • Registers and mux/demux delays
  • ALU propagation delay
    • Can vary significantly among function circuits
    • Often complex functions have many circuit designs with very different costs
  • We don’t want to clock faster than the worst case. That way all instructions are guaranteed to finish before the next instruction
• Time to execute a benchmark program
  • $CPUTime=\frac{instructions}{program}\times \frac{clock\_cycles}{instruction}\times \frac{seconds}{clock\_cycle}$
  • Clock cycles per instruction depends on compiler and circuit design (both software and hardware)
  • Seconds per clock cycle is set by the worst case propagation delay (hardware only)

Starting/Stopping a Processor §

• Hardware isn’t designed to stop
• Software must be designed to always provide a machine instruction
• In a dedicated system, applications usually run endlessly (think like your microwave, it always is doing something inside)
• For a general purpose computer, the operating system contains an idle loop to run when no other process is ready to run
• OS always has an infinite loop to keep the hardware running

Starting a processor §

• Method is hardware dependent
  • Typically begins with a power or reset of latches to force them to a known state
  • How software starts depends on the memory
• Process is called bootstrapping
• Result is that dedicated application or OS running
• BIOS - basic input output system
• When hardware receives power it resets and runs the thing at memory address 0, which is the BIOS which then tells the computer what it should actually do
• Every time the computer starts it has a mini panic attack before the circuit tells it to check the BIOS instead of just doing stuff and that gives it proper instructions to do things