Operating System : Organization

COMPUTER SYSTEM ORGANIZATION

Computer system organization has computer-system operations, Storage structure, I/O structure 

1. Computer-System Operations: 

A modern general-purpose computer system consists of one or more CPUs and a number of device controllers connected through a common bus that provides access to shared memory. Each device controller is in charge of a specific type of device (for example, disk drives, audio devices, or video displays. 

For a computer to start running—for instance, when it is powered up or rebooted—it needs to have an initial program to run. This initial program, or bootstrap program, tends to be simple. Typically, it is stored within the computer hardware in read-only memory (ROM) or electrically erasable programmable read-only memory (EEPROM), known by the general term firmware. It initializes all aspects of the system, from CPU registers to device controllers to memory contents. 

The bootstrap program must know how to load the operating system and how to start executing that system. To accomplish this goal, the bootstrap program must locate the operating-system kernel and load it into memory. Once the kernel is loaded and executing, it can start providing services to the system and its users. Some services are provided outside of the kernel, by system programs that are loaded into memory at boot time to become system processes, or system daemons that run the entire time the kernel is running. Once this phase is complete, the system is fully booted, and the system waits for some event to occur.

The occurrence of an event is usually signaled by an interrupt from either the hardware or the software. Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus. Software may trigger an interrupt by executing a special operation called a system call.

When the CPU is interrupted, it stops what it is doing and immediately transfers execution to a fixed location. The fixed location usually contains the starting address where the service routine for the interrupt is located. The interrupt service routine executes; on completion, the CPU resumes the interrupted computation.

2. Storage Structure:             

Memory is form of array of bytes. The CPU can load instructions only from memory, so any programs to run must be stored there. General-purpose computers run most of their programs from rewritable memory, called main memory (also called random-access memory, or RAM). Main memory commonly is implemented in a semiconductor technology called dynamic random-access memory (DRAM). Computers use other forms of memory is read-only memory, ROM) and electrically erasable programmable read-only memory, EEPROM).

All forms of memory provide an array of bytes. Each byte has its own address. Interaction is achieved through a sequence of load or store instructions to specific memory addresses. The load instruction moves a byte or word from main memory to an internal register within the CPU, where as the store instruction moves   the content of a register to main memory. Aside from explicit loads and stores, the CPU automatically loads instructions from main memory for execution.

The wide variety of storage systems can be organized in a hierarchy according to speed and cost. The higher levels are expensive, but they are fast. As we move down the hierarchy, the cost per bit generally decreases, whereas the access time generally increases.

The most computer systems provide secondary storage as an extension of main memory. The main requirement for secondary storage is that it be able to hold large quantities of data permanently. The most common secondary-storage device is a magnetic disk, which provides storage for both programs and data.

In storage device hierarchy, there is increase in space but decrease in transfer speed and as we go up the hierarchy, increase in transfer speed but decrease in memory.

·        Registers: These are the smaller storage devices and they store data in bits. It means 0’s and 1’s, so they can be accessed quickly.

·        Cache: The storage size of the cache is bigger than the register but it is slow.

·        Main Memory: example- RAM, it has limited size but it is fast in speed.

·        Registers, cache and main memory belongs to volatile memory means their contents could be lost when power is removed.

·        Electronic disk, magnetic disk, optical disk, magnetic tapes are the secondary storage devices.

·        Electronic disk, magnetic disk, optical disk, magnetic tapes are the non-volatile memory which does not loses the content even when the power is removed.

3. I/O Structure:

Storage is only one of many types of I/ O devices within a computer. A large portion of operating system code is dedicated to managing I/ O, both because of its importance to the reliability and performance of a system and because of the varying nature of the devices.

A general-purpose computer system consists of CPUs and multiple device controllers that are connected through a common bus. Each device controller is in charge of a specific type of device. Depending on the controller, more than one device may be attached. For instance, seven or more devices can be attached to the small computer-systems interface (SCSI) controller. A device controller maintains some local buffer storage and a set of special-purpose registers. The device controller is responsible for moving the data between the peripheral devices that it controls and its local buffer storage. Typically, operating systems have a device driver for each device controller. This device driver understands the device controller and provides the rest of the operating system with a uniform interface to the device.

      To start an I/ O operation, the device driver loads the appropriate registers within the device controller. The device controller, in turn, examines the contents of these registers to determine what action to take (such as “read a character from the keyboard”). The controller starts the transfer of data from the device to its local buffer. Once the transfer of data is complete, the device controller informs the device driver via an interrupt that it has finished its operation. The device driver then returns control to the operating system, possibly returning the data or a pointer to the data if the operation was a read. For other operations, the device driver returns status in formation. This form of interrupt-driven I/ O is fine for moving small amounts of data but can produce high overhead when used for bulked at a movement such as disk I/ O. To solve this problem, direct memory access (DMA) is used.