System Computer Overview
Chapter 1
Operating System
Exploits the hardware resources of one or more processors Provides a set of services to system users Manages secondary memory and I/O devices
Basic Elements
Processor Main Memory referred to as real memory or primary memory volatile I/O modules secondary memory devices communications equipment terminals System bus communication among processors, memory, and I/ O modules
Top-Level Components
Processor Registers
User-visible registers
Enable programmer to minimize mainmemory references by optimizing register use
Control and status registers
Used by processor to control operating of the processor Used by operating-system routines to control the execution of programs
User-Visible Registers
May be referenced by machine language Available to all programs - application programs and system programs Types of registers
Data Address
Index Segment pointer Stack pointer
User-Visible Registers
Address Registers
Index
involves adding an index to a base value to get an address
Segment pointer
when memory is divided into segments, memory is referenced by a segment and an offset
Stack pointer
points to top of stack
Control and Status Registers
Program Counter (PC)
Contains the address of an instruction to be fetched
Instruction Register (IR)
Contains the instruction most recently fetched
Program Status Word (PSW)
condition codes Interrupt enable/disable Supervisor/user mode
Control and Status Registers
Condition Codes or Flags
Bits set by the processor hardware as a result of operations Can be accessed by a program but not altered Examples
positive result negative result zero Overflow
Instruction Cycle
Instruction Fetch and Execute
The processor fetches the instruction from memory Program counter (PC) holds address of the instruction to be fetched next Program counter is incremented after each fetch
Instruction Register
Fetched instruction is placed in the instruction register Types of instructions
Processor-memory
transfer data between processor and memory
Processor-I/O
data transferred to or from a peripheral device
Data processing
arithmetic or logic operation on data
Control
alter sequence of execution
Example of Program Execution
Direct Memory Access (DMA)
I/O exchanges occur directly with memory Processor grants I/O module authority to read from or write to memory Relieves the processor responsibility for the exchange Processor is free to do other things
Interrupts
An interruption of the normal sequence of execution Improves processing efficiency Allows the processor to execute other instructions while an I/O operation is in progress A suspension of a process caused by an event external to that process and performed in such a way that the process can be resumed
Classes of Interrupts
Program
arithmetic overflow division by zero execute illegal instruction reference outside user's memory space
Timer I/O Hardware failure
Interrupt Handler
A program that determines nature of the interrupt and performs whatever actions are needed Control is transferred to this program Generally part of the operating system
Interrupt Cycle
Interrupt Cycle
Processor checks for interrupts If no interrupts fetch next the instruction for the current program If an interrupt is pending, suspend execution of the current program, and execute the interrupt handler
Multiple Interrupts
Disable interrupts while an interrupt is being processed
Processor ignores any new interrupt request signals
Multiple Interrupts Sequential Order
Disable interrupts so processor can complete task Interrupts remain pending until the processor enables interrupts After interrupt handler routine completes, the processor checks for additional interrupts
Multiple Interrupts Priorities
Higher priority interrupts cause lowerpriority interrupts to wait Causes a lower-priority interrupt handler to be interrupted Example when input arrives from communication line, it needs to be absorbed quickly to make room for more input
Multiprogramming
Processor has more than one program to execute The sequence the programs are executed depend on their relative priority and whether they are waiting for I/O After an interrupt handler completes, control may not return to the program that was executing at the time of the interrupt
Memory Hierarchy
Going Down the Hierarchy
Decreasing cost per bit Increasing capacity Increasing access time Decreasing frequency of access of the memory by the processor
locality of reference
Disk Cache
A portion of main memory used as a buffer to temporarily to hold data for the disk Disk writes are clustered Some data written out may be referenced again. The data are retrieved rapidly from the software cache instead of slowly from disk
Cache Memory
Invisible to operating system Increase the speed of memory Processor speed is faster than memory speed
Cache Memory
Cache Memory
Contains a portion of main memory Processor first checks cache If not found in cache, the block of memory containing the needed information is moved to the cache
Cache/Main Memory System
Cache Design
Cache size
small caches have a significant impact on perfo...