Computer Organization: Hardware

                       _______________
                       |  _________  |
                       |  |  CPU  |  |
                       |  ----|----  |
                       |      |      |           
                       |      |b     |          
                       |      |u     |         
                       |      |s     |           
                       |  ____|____  |        
Secondary Storage  ----|--|   MM  |--|---- Output Devices
Communications     ----|--|rom____|  |
                       |------|------|      
                              |
                              |
                        Input Devices

• Main Memory (MM in the diagram).
  Also known as Primary Storage and RAM. This is where active data and programs live. Main memory is divided into bytes, each one with a unique address. On runner, addresses are represented by 32-bit integers, so there are 2 to the 32nd power addresses, enough for about four billion bytes. Only some of the addresses are used, about 512 million (2 to the 29th).
  
  Main memory is volatile, i.e., it loses its data as soon as you turn off the power.
  
  Access time to main memory is on the order of nanoseconds, i.e., billionths of a second. A typical DRAM (dynamic random access memory) chip will have an access time of about 10 to 100ns to retrieve memory from one address.
  
  ROM (read-only memory) is a special part of main memory that doesn't lose its data when the computer is shut off. It contains instructions and data used when the computer is first turned on. The instructions basically tell the computer how to load the operating system from the hard disk or network.
• Secondary Storage
  These are things like
  • fixed disk (hard disk)
  • removeable media (floppy disk, large removeable hard-disk things, writeable CD-ROMs, "flopticals")
  • tape (used mainly for backup)
  Data are arranged in blocks of many bytes, usually 512 or 1024. A hard disk is a random access device. Several platters with rotate under several read/write heads that can move to different parts of the media. You specify the particular block you want, the head seeks over there and gets it.
  
  A tape is a sequential access device; to get to block 100 you have to "fast forward" past blocks 1 through 99. So it is slower than hard disk, but tapes are cheaper.
  
  The access time for hard disks is on the order of milliseconds, or thousandths of a second. This is literally a million times slower than main memory!
  
  Secondary storage is non-volatile. This means that the data is still there when you turn off the power. Your files, computer programs, and other data are stored on secondary storage.
• Input devices
  These encode human actions or other real world phenomenae, such as
  • keystrokes on a keyboard,
  • movement and clicks on a mouse,
  • paper going through a scanner,
  • voice or music over a microphone
  into bits that go over the wires connecting them to the CPU/MM via the computer's bus. Think of some examples of input devices.
• Output devices
  These take electrical signals fron the computer and translate them into real world actions, such as
  • printing somthing on a page,
  • drawing a picture on the screen,
  • making sound come out of a speaker.
• Communications (not present on all computers)
  This allows the computer to communicate with other computers. For example:
  • network interface card + media,
  • modem + phone line,
  • digital phone line + adapter.
• The CPU
  This is the part that executes programs. Programs are the instructions that tell the computer what to do.
  
  The CPU interprets the instructions, in a simple binary language called machine language. The CPU starts executing instructions at a specified address on startup, then does the instructions in sequence until it finds an instruction that tells it to execute code at a different address. It might loop, doing the same thing over and over many times.
  
  The CPU may do numerical or other types of computations, using its registers and MM as temporary storage for the results.
  
  The CPU may instruct the other devices in the computer to do something, such as get information from the disk or see if something is being typed at the keyboard.
  
  The ROM (read-only memory) has the initial program the computer needs to start up. The ROM usually tells the CPU to immediately load the operating system from the secondary storage.
  
  A program lives on the disk until the operating system (perhaps at the request of the user) loads a program into main memory and begins executing it. When a program begins running, it is known as a process.

Software

Computer Languages

	program	number
		integer	i

		do 10 i=1,10
			print*, i
 10 		continue
		stop
	end