In traditional operating systems, each process has an address space and a single thread of control. In fact, that is almost the definition of a process. Nevertheless, there are frequently situations in which it is desirable to have multiple threads of control in the same address space running in quasi-parallel, as though they were separate processes (except for the shared address space). In the following sections we will discuss these situations and their implications.
EEPROM which stands for Electrically Erasable Programmable Read-Only Memory is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed. When larger amounts of static data are to be stored a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices.
An EPROM, or ErasableProgrammableRead-Only Memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light
A programmable read-only memory is a form of digital memory where the setting of each bit is locked by a fuse or anti fuse. Such PROMs are used to store programs permanently.
Read-only memory is a class of storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified, it is mainly used to distribute firmware.
Every computer has some main memory that it uses to hold executing programs. In a very simple operating system, only one program at a time is in memory. To run a second program, the first one has to be removed and the second one placed in memory.
More sophisticated operating systems allow multiple programs to be in memory at the same time. To keep them from interfering with one another (and with the operating system), some kind of protection mechanism is needed. While this mechanism has to be in the hardware, it is controlled by the operating system.
The above viewpoint is concerned with managing and protecting the computer’s main memory. A different, but equally important memory-related issue, is managing the address space of the processes. Normally, each process has some set of addresses it can use, typically running from 0 up to some maximum. In the simplest case, the maximum amount of address space a process has is less than the main memory. In this way, a process can fill up its address space and there will be enough room in main memory to hold it all.
However, on many computers addresses are 32 or 64 bits, giving an address space of 232 or 264 bytes, respectively. What happens if a process has more address space than the computer has main memory and the process wants to use it all? In the first computers, such a process was just out of luck. Nowadays, a technique called virtual memory exists, in which the operating system keeps part of the address space in main memory and part on disk and shuttles pieces back and forth between them as needed.
Java Creation and Developement Java was conceived by James Gosling, Patrick Naughton, Chris Worth, Ed Frank and Mike Sheridan at Sun Microsystems, in 1991. It took 18 months to develop the first working version. This language was originally called “Oak” but it was renamed as Java in 1995. Between the initial implementation of Oak in the fall of 1992 and the public announcement in the spring of 1995, many more people contributed to the design and evolution of the language. It is a strongly typed language. First, every variable has a type, every expression has a type, and every type is strictly defined. Second, all assignments, whether explicit or via parameter in method calls, are checked for type compatibility.
The second major component in any computer is the memory. Ideally, a memory should be extremely fast (faster than executing an instruction so the CPU is not held up by the memory), abundantly large, and dirt cheap. No current technology satisfies all of these goals, so a different approach is taken. The memory system is constructed as a hierarchy of layers.
The top layer consists of the registers internal to the CPU. They are made of the same material as the CPU and are thus just as fast as the CPU. Consequently, there is no delay in accessing them. The storage capacity available in them is typically 32 x 32-bits on a 32-bit CPU and 64 x 64-bits on a 64-bit CPU. Less than 1 KB in both cases. Programs must manage the registers themselves, in software.
Next comes the cache memory, which is mostly controlled by the hardware. Main memory is divided up into cache lines, typically 64 bytes, with addresses 0 to 63 in cache fine 0, addresses 64 to 127 in cache line 1, and so on. The most heavily used cache lines are kept in a high-speed cache located inside or very close to the CPU. When the program needs to read a memory word, the cache hardware checks to see if the line needed is in the cache. If it is, called a cache hit, the request is satisfied from the cache and no memory request is sent over the bus to the main memory. Cache hits normally take about two clock cycles. Cache misses have to go to memory, with a substantial time penalty. Cache memory is limited in size due to its high cost. Some machines have two or even three levels of cache, each one slower and bigger than the one before it.
Main memory comes next. This is the workhorse of the memory system. Main memory is often called RAM (Random Access Memory). Old timers sometimes call it core memory, because computers in the 1950s and 1960s used tiny magnetizable ferrite cores for main memory. Currently, memories are tens to hundreds of megabytes and growing rapidly. All CPU requests that cannot be satisfied out of the cache go to main memory.
Next in the hierarchy is magnetic disk (hard disk). Disk storage is two orders of magnitude cheaper than RAM per bit and often two orders of magnitude larger as well. The only problem is that the time to randomly access data on it is close to three orders of magnitude slower. This low speed is due to the fact that a disk is a mechanical device, as shown in Fig. 1-8.
A disk consists of one or more metal platters that rotate at 5400, 7200, or 10,800 rpm. A mechanical arm pivots over the platters from the corner, similar to the pickup arm on an old 33 rpm phonograph for playing vinyl records. Information is written onto the disk in a series of concentric circles. At any given arm position, each of the heads can read an annular region called a track. Together, all the tracks for a given arm position form a cylinder.
Each track is divided into some number of sectors, typically 512 bytes per sector. On modern disks, the outer cylinders contain more sectors than the inner ones. Moving the arm from one cylinder to the next one takes about 1 msec. Moving it to a random cylinder typically takes 5 msec to 10 msec, depending on the drive. Once the arm is on the correct track, the drive must wait for the needed sector to rotate under the head, an additional delay of 5 msec to 10 msec, depending on the drive’s rpm. Once the sector is under the head, reading or writing occurs at a rate of 5 MB/sec on low-end disks to 160 MB/sec on faster ones.
A modern computer system consists of one or more processors, some main memory, disks, printers, a keyboard, a display, network interfaces, and other input/output devices. All in all, a complex system. Writing programs that keep track of all these components and use them correctly, let alone optimally, is an extremely difficult job. For this reason, computers are prepared with a layer of software called the operating system, whose job is to manage all these devices and provide user programs with a simpler interface to the hardware. We can tell it in simple word, the operating system controls the entire hardware and the application software. It provides the software platforms.
