STORAGE STRUCTURE

Main memory

The Main Memory of the computer is also known as RAM, standing for Random Access Memory. It is constructed from integrated circuits and needs to have electrical power in order to maintain its information. When power is lost, the information is lost too! It can be directly accessed by the CPU. The access time to read or write any particular byte are independent of whereabouts in the memory that byte is, and currently is approximately 50 nanoseconds (a thousand millionth of a second). This is broadly comparable with the speed at which the CPU will need to access data. Main memory is expensive compared to external memory so it has limited capacity. The capacity available for a given price is increasing all the time. For example many home Personal Computers now have a capacity of 16 megabytes (million bytes), while 64 megabytes is commonplace on commercial workstations. The CPU will normally transfer data to and from the main memory in groups of two, four or eight bytes, even if the operation it is undertaking only requires a single byte.





MAGNETIC DISK a memory device, such as a floppy disk, a hard disk, or a removable cartridge, that is covered with a magnetic coating on which digital information is stored in the form of microscopically small, magnetized needles.






MAGNETIC TAPE has been used for data storage for over 50 years. In this time, many advances in tape formulation, packaging, and data density have been made. Modern magnetic tape is most commonly packaged in cartridges and cassettes. The device that performs actual writing or reading of data is a tape drive. Autoloaders and tape libraries are frequently used to automate cartridge handling.
When storing large amounts of data, tape can be substantially less expensive than disk or other data storage options. Tape storage has always been used with large computer systems. Modern usage is primarily as a high capacity medium for backups and archives. As of 2008, the highest capacity tape cartridges (Sun StorageTek T10000B, IBM TS1130) can store 1 TB of data without using compression.



HARDWARE PROTECTION

Dual-Mode Operation

-Sharing system resources requires operating system to ensure
that an incorrect program cannot cause other programs to
execute incorrectly.

-Provide hardware support to differentiate between at least two
modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) –
execution done on behalf of operating system.

-Mode bit added to computer hardware to indicate the current
mode: monitor (0) or user (1).

-When an interrupt or fault occurs hardware switches to monitor mode.

interrupt/fault

monitor user

set user mode

-Privileged instructions can be issued only in monitor mode.

I/O Protection

-All I/Oinstructions are privileged instructions.

-Must ensure that a user program could never gain control of
the computer in monitor mode (i.e., a user program that, as
part of its execution, stores a new address in the interrupt
vector).