Operating Systems - Computer-System Structures (Night Class)

Compiled By Aurelie A. Peralta 

Computer-System Structures

We need to have a general knowledge of the structure of a computer system before we can explore the details of system operation.

The operating system must also ensure the correct operation of the computer system. To ensure that user programs will not interfere with the proper operation of the system, the hardware must provide appropriate mechanisms to ensure correct behavior.

Computer-System Operation

A modern, general-purpose computer system consists of a CPU and a number of device controllers that are connected through a common bus that provides access to shared memory. Each device controller is in charge of a specific type of device. The CPU and the device controllers can execute concurrently, competing for memory cycles. To ensure orderly access to the shared memory, a memory controller is provided whose function is to synchronize access to the memory.

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 is typically stored in read-only memory (ROM) such as firmware or EEPROM within the computer hardware. It initializes all aspects of the system, from the CPU registers to device controllers to memory contents. The bootstrap program must know how to load the operating system and to start executing the system. To accomplish this goal, the bootstrap program must locate and load into memory the operating-system kernel. The operating system then starts executing the first process, such as “init”, and 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 or a monitor call.

Modern operating systems are interrupt driven. A trap or an exception is a software-generated interrupt caused either by an error or by a specific request from a user program that an operating-system service be performed. An interrupt service routine is provided that is responsible for dealing with the interrupt. When a 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.

Interrupts are an important part of computer architecture. Each computer design has its own interrupt mechanisms, but several functions are common. An array of addresses or interrupt vector is a table of pointers to interrupt routines and is generally stored in low memory.

I/O Structure

A general-purpose computer system consists of a CPU 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, there may be more than one attached device.

SCSI – Small Computer-System Interface

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.

I/O Interrupts

To start an I/O operation, the CPU 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. Device controllers inform the CPU that a certain operation is finished by triggering an interrupt.

Synchronous I/O – waits for an operation to complete before returning the control to the user.

Asynchronous I/O – returns control to the user program without waiting for the I/O to complete. The main advantage of asynchronous I/O is increased system efficiency.

DMA Structure

Direct Memory Access (DMA) is used for high-speed I/O devices. After setting up buffers, pointers, and counters for the I/O device, the device controller transfers an entire block of data directly to or from its own buffer storage to memory, with no intervention by the CPU.

Storage Structure

Computer programs must be in main memory (also called random-access memory or RAM) to be executed. Main memory is the only large storage area (millions to billions of bytes) that the processor can access directly. It is implemented in a semiconductor technology called dynamic random-access memory (DRAM), which forms an array of memory words.

Ideally, we want the programs and data to reside in main memory permanently. This arrangement is not possible for the following two reasons:

1. Main memory is usually too small to store all needed programs and data permanently.
2. Main memory is a volatile storage device that loses its contents when power is turned off or otherwise lost.

Thus, most computer systems provide secondary storage as an extension of main memory. The main requirement for secondary storage is that is be able to hold large quantities of data permanently.

Secondary Storage

Magnetic Disks – provide the bulk of secondary storage for modern computer systems. The storage capacity of common disk drives is measured in gigabytes. Disk speed has two parts. The transfer rate is the rate at which data flow between the drive and the computer. The positioning time, sometimes called the random-access time, consists of the time to move the disk arm to the desired cylinder, called the seek time, and the time for the desired sector to rotate to the disk head, called the rotational latency.

A disk drive is attached to a computer by a set of wires called an I/O bus. Several kinds of buses are available, including enhanced integrated drive electronics (EIDE), advanced technology attachment (ATA), and SCSI buses. A disk controller is built into each disk drive.

Magnetic Tapes – was used as an early secondary-storage medium. Although it is relatively permanent and can hold large quantities of data, its access time is slow in comparison to that in main memory. Tapes are used mainly for backup, for storage of infrequently used information, and as a medium for transferring information from one system to another.

Storage Hierarchy

The wide variety of storage systems in a computer system 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.

Registers

Cache

Main Memory

Electronic disk

Magnetic disk

Optical disk

Magnetic tapes

Caching

Caching is an important principle of computer systems. Information is normally kept in some storage system. As it is used, it is copied into a faster storage system – the cache – on a temporary basis. Because caches have limited size, cache management is an important design problem.

Coherency and Consistency

In a multiprocessor environment various CPUs can all execute concurrently, we must make sure that an update in one cache is immediately reflected in all other caches where a certain value also resides. This situation is called cache coherency, and is usually a hardware problem.

Hardware Protection

Early operating systems were called resident monitors, and starting with the resident monitor, the operating system began to perform many of the functions, especially I/O, for which the programmer had previously been responsible.

In addition, to improve system utilization, the operating system began to share system resources among several programs simultaneously. With spooling, one program might have been executing while I/O occurred for other processes; the disk simultaneously held data for many processes. Multiprogramming put several programs in memory at the same time.

This sharing both improved utilization and increased problems. When the system was run without sharing, an error in a program could cause problems for only the one program that was running. With sharing, many processes could be adversely affected by a bug in one program.

Without protection against these sorts of errors, either the computer must execute only one process at a time, or all output must be suspect. A properly designed operating system must ensure that an incorrect program cannot cause other programs to execute incorrectly.

Many programming errors are detected by the hardware. These errors are normally handled by the operating system. If a user program fails in some way then hardware will trap to the operating system. The trap transfers control through the interrupt vector to the operating system, just like an interrupt. Whenever a program error occurs, the operating system must abnormally terminate the program. This situation is handled by the same code as is a user-requested abnormal termination. An appropriate error message is given, and the memory of the program may be dumped. The memory dump is usually written to a file so that the user or programmer can examine it, and perhaps can correct and restart the program.

Dual-Mode Operation

At the very least, we need two separate modes of operation: user mode and monitor mode (also called supervisor mode, system mode, or privileged mode). A bit, called the mode bit, is added to the hardware of the computer to indicate the current mode: monitor (0) and user (1). The dual mode of operation provides us with the means for protecting the operating system from errant users, and errant users from one another. We accomplish this protection by designating some of the machine instructions that may cause harm as privileged instructions. The hardware allows privileged instructions to be executed only in monitor mode. If an attempt is made to execute a privileged instruction in user mode, the hardware does not execute the instruction, but rather treats the instruction as illegal and traps it to the operating system.

I/O Protection

A user program may disrupt the normal operation of the system by issuing illegal I/O instructions, by accessing memory locations within the operating system itself, or by refusing to relinquish the CPU. We can use various mechanisms to ensure that such disruptions cannot take place in the system.

To prevent users from performing illegal I/O, we define all I/O instructions to be privileged instructions. Thus, users cannot issue I/O instructions directly; they must do it through the operating system. For I/O protection to be complete, we must be sure that a user program can never gain control of the computer in monitor mode. If it could, I/O protection could be compromised.

Memory Protection

To ensure correct operation, we must protect the interrupt vector from modification by a user program. In addition, we must also protect the interrupt-service routines in the operating system from modification.

We see then that we must provide memory protection at least for the interrupt vector and the interrupt-service routines of the operating system. To separate each program’s memory space, we need the ability to determine the range of legal addresses that the program may access, and to protect the memory outside that space. We can provide this protection by using two registers, usually a base and a limit.

This protection is accomplished by the CPU hardware comparing every address generated in user mode with the registers. The base and limit registers can be loaded by only the operating system, which uses a special privileged instruction.

CPU Protection

We must prevent a user program from getting stuck in an infinite loop or not calling system services, and never returning control to the operating system. To accomplish this goal, we can use a timer. A timer can be set to interrupt the computer after a specified period. The period may be fixed or variable. A variable timer is generally implemented by a fixed-rate clock and a counter. The operating system sets the counter. Before turning over control to the user, the operating system ensures that the timer is set to interrupt. If the timer interrupts, control transfers automatically to the operating system, which may treat the interrupt as a fatal error or may give the program more time. Clearly, instructions that modify the operation of the timer are privileged.

Thus, we can use the timer to prevent a user program from running too long. A simple technique is to initialize a counter with the amount of time that a program is allowed to run. A more common use of a timer is to implement time sharing. Another use of the timer is to compute the current time.

Network Structure

There are basically two types of networks: local-area networks (LAN) and wide-area networks (WAN). The main difference between the two is the way in which they are geographically distributed. The differences of these two type of networks imply major variations in the speed and reliability of the communications network, and they are reflected in the distributed operating-system design.

Reference: Operating System Concepts by Silberschatz, Galvin, and Gagne, 2003