Types of Computers Week 8 Micros to Mainframes

Computer Architecture

Typical Computer Architecture

The basic elements in typical computers are

1. 
   shared memory
   
2. 
   CPU
   
3. 
   IO devices
   

• 
  Central Processing Unit (CPU)
  
  • 
    Arithmetic and Logic Unit (ALU) for arithmetic & logic operations
    
  • 
    Registers = very fast pieces of memory used to store the data the ALU is using right now
    
    • 
      Program Counter (PC)  address in memory of current instruction
      
    • 
      Memory Address Register (MAR)  address in memory to read from or write to
      
    • 
      Memory Buffer Register (MBR)  data read from or written to memory
      
    • 
      Instruction Register (IR)  the instruction currently being executed
      
    • 
      general purpose registers (R0, R1...)  used to temporary store data
      
  • 
    Micro-memory  (sometimes called firmware) contains microcode for running CPU
    
  • 
    CPU is usually contained entirely on one chip
    

• 
  Storage
  
  • 
    RAM  (read & write, volatile) - stores programs & variables
    
  • 
    ROM  (read, non-volatile) - holds start-up instructions
    
  • 
    Cache Memory
    
    • 
      Stores frequently & recently used data that is too large to leave in registers
      
    • 
      Faster than regular memory but slower than registers
      
  • 
    Secondary Storage  (non-volatile)
    
    • 
      For storing large quantities of data
      
    • 
      eg: Disk drives (both hard drives & floppies), CDROM, Tape drives
      
    • 
      Slower but cheaper than RAM
      
  • 
    The data speed/size/cost pyramid:
    

• 
  I/O Devices
  
  • 
    Hardware for interfacing between the computer and the "real world"
    
    • 
      eg: disk, keyboard, screen, mouse, printer, speakers, sensors of various types
      
  • 
    I/O devices are very slow compared to the CPU
    
    • 
      Control chip interfaces with the CPU
      
    • 
      CPU sends requests to the control chip
      
    • 
      Control interrupts CPU when task is completed
      

• 
  Communications Bus
  
  • 
    Communication link between different components of the computer
    
    • 
      Get an instruction from the keyboard and send it to the CPU
      
    • 
      Transfer data from disk to memory
      
    • 
      Transfer data from memory to the CPU
      
    • 
      Transfer some data from memory to the monitor
      
  • 
    A central bus connects the CPU to all other components
    
  • 
    Other buses connect specific pairs of components together, e.g., a bus may connect main memory & disk drives so files can be quickly transferred
    
  • 
    Usually the bus is duplex but atomic
    

• 
  Transferring data between memory & the CPU
  
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    Reading data from memory
    
    • 
      The memory address is placed in the MAR
      
    • 
      A read signal is sent to memory
      
    • 
      The data is transferred from memory to the MBR
      
    • 
      The data is retrieved from the MBR
      
  • 
    Writing data to memory
    
    • 
      The memory address is placed in the MAR
      
    • 
      The data is placed in the MBR
      
    • 
      A write signal is sent to memory
      
    • 
      The data is transferred from the MBR to memory