DOS

DOS (Disk Operating System) is a program that serves to configure a computer to execute the command given as input.
Commonly used operating system is MS DOS, because in addition to its capacity to small also commands that are used relatively easier when compared with other systems.
Some functions of the operating system (DOS) is:
1.Organize or control the computer activity
2.Setting the Memory
3.Set the process input and output data
4.Menegement file
5.Management directory

A. File and Directory Processing


In the DOS operating there Command-command/perintah grouped into two groups:

Internal Command / Internal Commands
Is a command that no longer requires a special file, because all the internal instructions are accommodated in the file command.com

External Command / External Commands
The command has its own file and no longer be accommodated in the file command.com

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DMA (Direct Memory Access Controler)

DMA

As was explained earlier that the interruption mechanism is not efficient to do large data transfers. Data transfer is done per word. On the mechanism of interruption, for each data word is transferred, the processor will not wait for the data available on the device that sends data and the data was completed by a device that receives data. In such situations the processor will replace the current process execution (which conduct data transfer) with another process (context switch).

If the size of data transferred is large enough, the processor will repeatedly perform context switch, but will cause a context switch overhead. Hence the weakness mechanism to handle data transfer interruptions that are caused by a context switch.

To address these weaknesses, used a special control unit provided for direct data transfer between external devices and main memory without the intervention of the processor continuously. Special control unit is DMA.

Modern systems can reduce the burden on the CPU to perform I / O, namely USING DMA controllers. Thus the CPU can perform other tasks while operating the I / O performed. Each controller equipment can only have its own DMA hardware.

Another alternative is to have a DMA controller on the motherboard is set to transfer to a variety of equipment.
To start the data transfer is DMA, the driver equipment will write DMA command block of memory pointed to a data source, destination, and the number of bytes to be transferred. CPU then sends this command block address on the DMA controller. DMA controller will process this information to then operate the memory bus.

Transfer as much as 1 byte / word per unit time by the controller referred to as cycle stealing DMA controller uses the bus because it's CPU cycles. With a cycle stealing bus by the CPU usage will be delayed several times because the bus is used for the DMA.

Three steps in the transfer DMA:
1. processor set up DMA transfers by providing data from the device, the operation will be displayed, the memory address of the source and destination data, and the number of bytes transferred.
2. DMA controller to start the operation (setting up a bus, provide the address, write and read data) samapai entire block has been transferred.
3. DMA controller interrupts the processor, in which further action will be determined next.

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Interruptions

Interrupt occurs when an I / O device wants to tell the processor that he was ready to take orders, output is produced, or in case of error.

Handling Interruptions

There are several stages in the handling of interrupts:
1. Controller sends an interrupt signal via interrupt-request-line
2. The signal detected by the processor
3. The processor will first store the information about the state of his state(tentangproses information that is being done)
4. The processor identifies the cause of interruptions and access the interrupt vectortable to determine the interrupt handler
5. Transfer control to the interrupt handler
6. After the interruption successfully resolved, the processor will return to the state as before the interruption and continue the work that had been delayed.

Interrupt Request Line

On most CPUs, there are two interrupt request line. First, nonmaskable interruptions, interruptions are usually derived from the hardware and must be immediately implemented, such as errors in memory. Second, maskable interrupts, interrupt type can be serviced by the processor or could not be served. Even if served, should be seen as the state of the processor. There is the possibility of directly addressing the processor if the time, the processor preemptive, nonpreemptive if, then must wait for the process of being worked over.

If there is an interrupt signal on interrupt request line, how an interrupt handler
knowing the source of the interruption? Do you have to trace all sources of interrupts one by one? No need, because every interrupt handler has a memory address of each. This address is offset in a table called the interrupt vector.

Interrupt Vectors and Vector Chaining

If there is an interrupt signal on interrupt request line, how an interrupt handler
knowing the source of the interruption? Do you have to trace all sources of interrupts one by one? No need, because every interrupt handler has a memory address of each. This address is offset in a table called the interrupt vector.

In accordance with the times, computers have more devices (and more interrupt handlers) than elements in the vector address. This can be overcome by vector chaining technique. Each element in the interrupt vector pointing to the head of a list of interrupt handlers, so if there is an interrupt, the handler found on the designated list will be called one by one until obtained handler that can handle the interrupt in question.
high-priority interrupts to interrupt the execution of low priority interrupts. This system also
allows the I / O devices that require immediate care should take precedence over
Other devices have a lower priority. Setting priorities and the handling of devices based on priorities set by the processor and interrupt controller.

Cause Interruptions

Interrupt mechanism is not only used to handle operations related to
device I / O. The operating system uses an interrupt mechanism for several reasons, among them:
1. exception handling
Exception is a condition where something happens, or the results obtained from an operation
considered certain special and should get more attention, for example, the division
by zero, accessing memory address that restricted or invalid, etc.
2. Set the virtual memory paging.
3. Handle software interrupt.
4. Handles the control flow kernel.
If the interruption occurs is a demand for large data transfers, then the use of
interruptions become inefficient, DMA is used to overcome them.

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Communication Devices With Processors

Hello guys, we've talked about the device and how they are connected to the computer, now focus on next is how the processor communicates with the device. There are two approaches:

1. Instruction Input / Output directly.
Each device is numbered I / O ports along the 8 / 16 bit unique. On the transfer of data between the device registers and CPU registers used I / O instructions specific. I / O instructions is different than just regular instruction memory because the address I / O ports do not use the same location as the memory address. Therefore, the two following instructions:-in R3, 0x200, 4, - and-mov R3, 0x200 --- has two different meanings. The first instruction is an instruction I / O specifically requesting the CPU to read the value of register number 4 of the device I / O at 0x200 port number and then put it on number 3 in the CPU registers. The second instruction is an instruction memory that just copying the contents of ordinary memory address 0x200 to the three registers in the CPU.

2. Memory mapped.
This approach uses the address mapping I / O to memory. Data registers and data buffers
mapped into the address space used CPU.
The advantages memory mapped are:

 1. Processors will have fewer number of instructions that the processor does not need
provide instruction input I / O specifically.

2. Access to memory is done by ordinary memory instruction, so the drivers for the equipment can be written in C / C + + (for normal memory instruction) rather than assembly language (for I / O instructions specifically).

3. The operating system can control access to device I / O, ie by not putting the device address space on the virtual address space the process.
However, there is a significant problem in the memory mapped approach to the problem of caching.
In caching, the process of storing the contents of the new memory location is referenced, so if any instructions that refer to the same address do not need to take to memory again, thus caching can improve system performance. There was a problem on a system that repeatedly read the status registers on the device to see if the device is readily accessible. The first reading, the contents of status registers will be stored in the cache.
However, the next iteration will take the value from the cache rather than the status registers on the equipment. As a result, will not be known when the device is ready for use because
values ​​continue to read the first value stored in the cache, instead of updating the status of the device itself. To overcome this problem, the caching process is still being done except for a memory location where the registers of I / O mapped.

Some systems use both these techniques. For example in the use of graphics controller. Graphics controller has a port address I / O (for the approach to I / O instructions directly), but he also has a memory-mapped region to accommodate the large screen display. The process of changing the look with a write to memory-mapped region and the controller will adjust the screen display based on information from the memory mapped. Much faster to write millions of Bytes into graphics memory instead of loading millions of instructions. However, his weakness, is when the pointer is pointing to the wrong memory area and writing it.

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Polls

When a processor wants to access the device, one approach is to let processor does all the work. The processor interacts with the controller through an elaborate protocol, but the basic handshaking is quite simple, namely:
1. CPU continuously reads the status bits until the bits that indicates the device is ready to receive commands the CPU.

2. CPU-write enable bit in a register command as the initial sign of the CPU gives a command and writes a byte in the data-out.

3. CPU activates the command-ready bit, meaning that the command is available to run the controller.

4. The controller saw the command ready bits are set so that bit of work on-set.

5. The controller reads the command registers and see the command write the data read-out and
ordered the I / O CPU to do what was ordered.

6. Controller erase command ready bits, bit and bit errors in the status of work.
Step 1 is called polling or busy waiting. Processor continuously reads the status bit, hoping a bit that indicates when the device is ready to receive his orders. Basically polling can said to be efficient when the performance of the device and its controller quickly. The weakness of this method is if the long waiting time, then it is better that the other processor tasks. As for knowing whether the device is ready to take orders again or not, is used
interruptions.

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Categories of I / O

At this present time, there are various kinds of I / O devices such as storage devices (disk, tape), transmission devices (network cards, modems), and devices with a user interface (screen, keyboard, mouse). In general, I / O devices can be divided into two categories namely:



1. Block device.
A device that stores information in a certain-sized blocks (typically 512 to 32768 bytes) and each block has a respective address. Each block on this device can be accessed and written independently. Examples of block device is a disk.

2. Character device.
A device that sends and receives a row of characters regardless of the block structure. Examples of character devices are printers, network interfaces and devices rather than disk. However, this division is not entirely true because there are devices that do not meet both these criteria the clock. Clock is a device that does not have the address block, do not send and receive interrupt the sequence of characters but only within a certain timeframe.

Component I / O
Unit I / O consists of two components, namely:
1. Mechanical Components. Mechanical components are I / O devices such as the mouse itself,
monitors, etc..
2. Electronic Components. Electronic component is also called the controller device.
The device is not connected directly with the processor, the controller was associated with the processor.

How To Connect To The Computer

I / O devices communicate with the computer system through signals transmitted through wires or air (wireless). I / O devices connected to the engine through a point called a port. Port I / O consists of four registers, namely:
1. Data-in register. This register will read the CPU to get input.
2. Data-out registers. CPU write a bit here as output data.
3. Status. CPU will read this register to know the status of the device. Device status
could indicate whether the available inputs in the data-in registers, the device completed the
duties properly or an error in the device.
4. Control. These registers are written by the CPU to initiate a command or to change the mode
device. One example is the replacement of the device mode bits contained in control registers
in serial port that serves to select the transfer rate supported by the serial port
them. The bus is a collection of wires and protocols that define a set of messages that can be sent over the wire. Some devices can be connected to the same bus. When the device A to device B is connected, the device B is connected to the device C and so on until the last device connected to the computer, the circuit device is called a daisy chain. Daisy chain also serves as a bus.

The structure of the bus on the PC

PCI (Peripheral Component Interconnect) bus is a high-speed bus that connects the processor to the memory subsystem, high-speed devices and the Expansion bus associated with a slower device such as keyboard, serial ports and port paralell. SCSI or Small Computer System Interface (read: skazi) is the bus that connects multiple disks to the SCSI controller. With SCSI, you can get up to seven devices connected to the computer but it will slow down your computer at start-up.

We all are familiar with storage devices like floppy drives, hard drives and CDROM drives. Typically these devices are connected to the computer via the IDE (Integrated Drive Electronics). The interface brings together the controller to the drive, so with simpler instructions and routes that are closer between the drive and controller, making it faster and easier to use. Buses, ports and devices can be operated by a controller which is a collection of electronic devices. Serial-port controller is one of the controller device that is simple because only a chip that controls the signal from the cable in the port. On the other hand, there is also a complex device controller SCSI controller that is so complex as to be implemented specifically as a separate circuit board inside the computer. This is called a host adapter. SCSI controller contains a processor, microcode, and some of its own memory.

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I / O System

Basically, the computer's main task is the processing and I / O. In fact, most of the time used to process I / O while the processing is only incidental. So, in the context of I / O, the role of the operating system is to regulate and control the device I / O and I / O.  I / O devices vary widely. Therefore, how to control these devices received a major in computer organization. Imagine, I / O devices are very many in number and each device has a function and its own pace, of course, require different methods.
Therefore, the known classification of the device I / O to block devices and character devices, although there are devices that do not belong to any of these two categories. The device is connected to a computer through a port, set by the device controller and communicates with the processor and other devices through the bus.

The device communicates with the processor via two approaches and instruction memory mapped I / O directly. When a processor wants to access a device, he will continue to check the device to determine its status, if allowed to be accessed. How this is done repeatedly called to the polls.

Meanwhile, when a device wants to tell the processor when readily accessible, the device uses an interrupt. Both methods has advantages and disadvantages of each. The existence of Direct Memory Access (DMA) can reduce the CPU load due to the transfer of data between devices and memory without going through the CPU. Differences details for each tool will be encapsulated in a kernel module called a device driver. As for knowing when and how long a process to use clock and timer.

Categories of  I / O

At this present time, there are various kinds of  I / O devices such as storage devices (disk, tape), transmission devices (network cards, modems), and devices with a user interface (screen, keyboard, mouse). In general, I / O devices can be divided into two categories namely:



1. Block device.
A device that stores information in a certain-sized blocks (typically 512 to 32768 bytes) and each block has a respective address. Each block on this device can be accessed and written independently. Examples of block device is a disk.

2. Character device.
A device that sends and receives a row of characters regardless of the block structure. Examples of character devices are printers, network interfaces and devices rather than disk. However, this division is not entirely true because there are devices that do not meet both these criteria the clock. Clock is a device that does not have the address block, do not send and receive interrupt the sequence of characters but only within a certain timeframe.

Component I / O
Unit I / O consists of two components, namely:
1. Mechanical Components. Mechanical components are I / O devices such as the mouse itself,
monitors, etc..
2. Electronic Components. Electronic component is also called the controller device.
The device is not connected directly with the processor, the controller was associated with the processor.

Note : I  = input, O = output, /  = or.

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Virtual Memory

Over the years, the implementation of memory management in essence is to put all the parts of the process to be run into the memory before the process can begin execution. Thus all parts of the process must have its own allocation in physical memory. In reality, not all parts of the program will be processed, for example:

• There are statements or options that will be executed only if certain conditions are met 
• There are functions that are rarely used 
• Allocating more memory than is actually needed. 

In a large-capacity memory, these things would not be a problem. But in memory with a very limited capacity, this will decrease the utility of space optimization of physical memory (main memory). Any programs that run must be in memory. Memory is a place main storage (primary storage) are temporary (volatile). Memory size Restricting the airflow can cause the problem of how to put the program size larger than the size of physical memory (main memory) and application of multiprogramming problem that requires a bigger place in memory. Virtual memory is a technique that separates between logical memory and physical memory.

Logical Memory is a collection of the entire page of a program. Without virtual memory, logical memory will immediately be brought into physical memory (main memory). This is where virtual memory is done by placing the separation of logical memory to secondary storage (secondary disk) and only bring the required page into main memory (physical memory).

This technique puts the whole program on a secondary disk and bring the necessary pages into physical memory so that main memory will only store the most frequently used addresses the process and others will be stored in secondary disk and can be taken as needed. So if the processes that are running require instruction or data contained on a particular page then the page will be sought in the main memory. If the desired page does not exist it will be searched to the secondary disk.

Picture 1. Virtual Memory 

In the picture above indicated a virtual memory space is divided into equal parts and identified with the number of virtual pages. Physical memory is divided into page frames of the same size and are identified by number page frames. Frame  storing data from the page. Or virtual memory to map virtual pages to number number page frames. Mapping  lead to a virtual page can have only one physical address location. In a paging system, if a space is needed for the process and the pages in question are not being used, then the pages of the process would have paged out (saved to disk) or swap out, the memory will be empty for the current page to another. Pages that moved from disk into memory called paged in when required (returned to memory) or swap-in. When an item can have paging, then the item is included in the items that occupy a virtual space, which is accessed with virtual addresses and the room is allocated for mapping information. The operating system allocates the address of the item only when items are experiencing paging in. Gains derived from the storage program is only partially on physical memory it is:

• Decreasing the M / K is needed (traffic M / K is low)
• Space to be more liberal because of reduced physical memory that is used
• Increased response due to reduced loads I / O and memory
• Increased number of users that can be served. Memory space that is still widely available allows the computer to receive more requests from users.

Virtual memory techniques will facilitate the work of a programmer when large data and the program is beyond the capacity of main memory. A multiprogramming can implements virtual memory techniques so that multiprogramming system becomes more efficient. For example: 10 programs with a size of 2 MB can be running in memory capacity of 4 MB. Each program is allocated 256 Kbyte and section - part of the process (swap in) enter into physical memory as needed and will be out (swap out) when not needed. Principle of virtual memory is that "The maximum speed ekseskusi process in virtual memory can be the same, but will never exceed the speed of execution the same process in a system that does not use virtual memory".
Virtual memory can be implemented in two ways:

1. Demand Paging is by applying the concept of pages in the process
2. Demand segmentation, more complex segments of varying sizes are applied.

If you want to know about  Demand Paging and Demand segmentation. pleas read my other article.

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Memory mapping

Operating system and various processes the user is located in the main memory. Therefore, we must keep the process between the two do not mix by allocating a number of the memory for operating system and user processes. This memory is usually divided into 2 parts. One for the operating system, and one for the user process. Mapping memory (memory mapping) require a relocation register.

As already described in the previous article, the relocation register is added to the base register in each address of the user process when sent to memory. In this memory mapping there is a limit registers consisting of the logical address value range. With the limit registers and relocation registers, each logical address must be smaller than the limit register. Process mapping carried out by the MMU (Memory Management Unit) by adding up the value of relocation to a logical address register. Which has been mapped address is then sent to memory. In the picture below we can see that a process that has a base and limit registers 30 004 12 090 registers will be mapped to physical memory with the first according to the base address registers (30 004) and ends at address (30004 + 12090 = 42094).

Base and limit register

When the CPU scheduler selects a process to be executed, he will enter a relocation register and also limit its registers. Register relocation allows the operating system to resize partitions in memory dynamically. For example, the code and buffers are allocated to drivers equipment in the operating system can be removed from memory if the equipment is rarely used. Such code is called a transient operating system code, therefore this code can "come and away "from memory depending on when he is needed. So the use of this code can be transient change the size of the operating system during program execution takes place.

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Swap

A process that as already explained above, must be in memory before being executed. The process of swapping is:    swap a process out of memory for a while into a temporary storage with the other processes that are in need of a number of allocations memory for execution. A temporary storage area is usually in the form of a fast disk with a capacity that can accommodate all of the copies of all memory images and provides direct access to the picture. If the execution process issued earlier will be continued after a moment, then he will be brought back to the memory of the place temporary storage before. How does the system know which processes are to be executed? This can be done with a ready queue. Ready queue contains all the processes that lie well in temporary storage or memory ready for execution. When the CPU scheduler will executing a process, If the process is not in memory then the process of swapping will be performed as described above.

Picture. Swapping Process

An example to illustrate this swapping technique is as follows: Algorithm Round- Robin used in multiprogramming environments using a time quantum (unit CPU time) in the execution of its processes. When the time quantum expires, memory manager will issue a (swap out) finished the process that has undergone at the time kuantumnya a time and insert (swap in) into the memory of other processes that have been free to them. on the same time, the CPU scheduler will allocate time to other processes in memory. This concern is, the time quantum should be long enough so that the time of use CPU can be optimized when compared with the exchange that occurs between memory and disk. Swapping technique roll out, roll in which the priority-based algorithm when the process with higher priority arrives then the memory manager will issue a priority process Lower and load the process with higher priority is. When the process with higher priority is executed then the process has been completed which has a lower priority can be put back into memory and re-executed. Most of the time swapping is the time of transfer. As an example we see the following illustration: a user process has a size of 5 MB, while the temporary storage area a harddisk(hard drive) has a data transfer speed of 20 MB per second. Then the time required to transfer the process for 5 MB of or into the memory is equal 5000 KB / 20 000 KBps = 250 ms The above calculation does not include latency time, so if we assume the latency time for 2 ms then the swap is equal to 252 ms.


Therefore there are two instances where one is the process of spending a process and another is the process of entry into the memory process, then the total time of the swap to be 252 + 252 = 504 ms. In order for swapping techniques can be more efficient, better processes in the swap is just the process that is really needed so as to reduce the time the swap. Therefore, system must always be aware of any changes that occur in meeting the needs of memory. This is where a process requires a system call function, namely to tell the system operation when he was asked when freeing memory and the memory space. If we want to do the swap, there are some things to watch. We must avoid exchange process with M / K is delayed (the operating assumption M / K is also being lined up in the queue for equipment M / Knya is busy). Examples such as this, if the process P1dikeluarkan from memory and we want to enter a process P2, then the operation of M / K which is also in the queue will take the freed memory space quota is P1. This problem can be overcome if we do not swap with the operation of M / K is postponed. In addition, the execution of the operation M / K should be performed on the operating system buffer.

 Each operating system has a version of each on swapping technique it uses. As example on UNIX, basically swapping is not enabled, but will start if many processes which requires a lot of memory allocation. Swapping will be disabled again if the number process that included reduced. In Microsoft Windows 3.1 operating system, if a process newly inserted and turned out there was not enough space in memory to contain it, a process that first is in memory will be transferred to disk. The operating system is basically not apply swapping technique in full, this is due to the user larger role in determining the process which will be exchanged instead of the CPU scheduler. With provisions like these processes has been issued will not return again to the memory until the user selects a process to executed.

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Memory allocation

Memory is one of the resources that are important in executing a process. Memory consists of word or byte array, each of which has an address. A process can executed if he has been in memory before the CPU fetches the instructions on the address designated by the program counter.

Part of the system in charge of organizing the memory is called memory manager. memory manager to set which parts of memory should be used and which are not in a time, but it also allocates memory manager for memory processes who need it and download dealokasikannya back when these processes are no longer need it.

The problem is what if the memory is no longer enough to accommodate all the processes to be executed? The solution to this problem is to transfer techniques process from memory to the disk and move it back to memory at the time of going executed more or better known as swapping.

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Dynamic linking

Reader can be static, known as the archive which consists of a collection of routines that duplicated to a program by the compiler, linker, or binder, resulting in a application executable (can be stand alone or walk alone). compiler provide standard libraries, such as C standard library, but the programmer can also make libraries to be used alone or disseminated. Static library is causing the memory to be weight. Therefore, in line with technological developments, there is a library that is dynamic. The mechanism is called dynamic linking, while libraries are called dynamically linked libraries.
Dynamic linking means that the data (code) in the libraries are not duplicated into the program at compile time, but living in separate files on disk linker just takes a little work at compile time. function linker is to record what is needed by the library for the execution and index names or numbers. Heavy work of linking to be completed at load time or during run time. Code liaison needed is a loader. At the appropriate time, the loader find the relevant literature on the disk and add data from the library into existing processes in the memory space. The advantage of linking is a dynamic program memory does not become unwieldy.

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Dynamic Loading

The size of physical memory is limited. In order for the memory utility goes well, then we using dynamic loading. In this way, routine-routine will only be called if required.

Following illustration, all routines are stored on disk in a format that can be reallocated (relocatable load format). The main program is placed in memory and executed. When a routine routine calls another, the first thing done is to check whether there are other routines already loaded. If not, relocatable linking loader is called to put the routines that and memory needed to renew the program address table. Then, control is given to the routine new call.

The advantage of dynamic loading is not used routine never gets called. This method is useful in code that amounted to much, when it emerged the wrong case as routine. Although a large code size, the portion that is used can be much smaller. The operating system does not create a dynamic loading mechanism, but only provide routine routine to implement this mechanism. Users who design its own program in order program uses dynamic loading system.

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Logic and Physical Address Space

Addresses generated by the CPU in the form of logical address, whereas the entry into the memory is the physical address. At compile time and load time, the physical and logical addresses are identical. Conversely, physical addresses and logical differences occur at execution time.

Collection of all logical addresses generated by the program is the logical address space / space virtual address. Collection of all the physical addresses corresponding to logical address is called physical address space.

At the time the program was in the CPU, the program has a logical address, then by the MMU mapped into physical addresses that are stored in memory. Following illustration, the value in this register will be added to every address made by the user process which is then sent to memory. For example in the relocation register 14000, logical address in the 346, then directly mapped into physical addresses in 14 346.

Using Dynamic image Relocation Relocation Register(register relocation).

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Address Binding

Definition of address bindings is a procedure to determine the physical address that will used by the programs contained in main memory. Address binding is done of a program can be done in three distinct phases, namely:


• Compilation time.
At this stage of a program initially will produce a form of address symbols, then these symbols will be directly converted into an absolute address or physical address that is static. When a shift occurs when the address of the program then to return to the address that should be done recompilation. example: file type. com which is the result of compiling the program


• Load time.
At this stage the program was originally produced in the form of address symbols random in nature (relative address), then a recount will be conducted in order to program is placed at addresses that can be reallocated (relocateble address). In short bindings occur at the time the program has finished being loaded. Example: File type. Exe. • Execution time.
Address is relative, the bindings will be done at run time. At run time it takes the help of the hardware MMU (Memory Management Unit).

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Operating system

Computer operating systems are computer software that is tasked to perform control and management of hardware and basic system operations, including run software applications such as data processing programs that can be used to facilitate human activity. Operating System or Usually called with OS.  

Operating system software on your computer is a first layer that is placed on computer memory, (computer memory in this case there are hard drive, not the ram memory) when the computer starts up. While other software is run after the computer operating system running, and the Operating System will perform the core public services for that software. Common core services such as access to the disk, memory management, task scheduling, and user interface. So that each software no longer need to perform the core duties of the public, because it can be served and performed by the Operating System. Section of code that perform the core duties and the general is called with an Operating System kernel.

Operating System serves as a liaison between the hardware layer and software layer. in addition, the computer Operating System is also doing all the important commands in the computer, and ensure that applications can run smoothly different functions simultaneously without a hitch. Computer Operating System guarantees other software applications can use the memory, perform input and output to other equipment, and have access to the file system. If several applications running concurrently, then the Computer Operating System would set a proper schedule, so that as much as possible all the processes running on computers that have sufficient time to use the CPU and not mutually interfere with other devices.

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