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CMSC 412 - S98 (lect 02))
Announcements
lProgramming Assignment #1 is on the web page
lReading for next week:
–Chapter 3 (sections 3.2 to 3.9)
lProgramming Assignment #0
–files are available on the web page
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Study of Complex Programs in Operating Systems | CMSC 412, Study notes of Operating Systems
Material Type: Notes; Professor: Hollingsworth; Class: Operating Systems; Subject: Computer Science; University: University of Maryland; Term: Unknown 1989;
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Announcements
l Programming Assignment #1 is on the web page
l Reading for next week:
- Chapter 3 (sections 3.2 to 3.9)
l Programming Assignment
- files are available on the web page
Why Study Operating Systems?
l They are large and complex programs
- good software engineering examples
l There is no perfect OS
- too many types of users
- real-time, desktop, server, etc...
- many different models and abstractions are possible
- OS researchers have been termed abstraction merchants
l Many levels of abstraction
- hardware details: where the bits really go and when
- high level concepts: deadlock, synchronization
Computer Systems
l Computers have many different devices
- I/O Devices
- Memory
- volatile storage
- Processor(s)
Processor Memory
Mem. Controller
I/O Bus Controller
Memory Bus I/O Bus
SCSI Adapter Display Adapter SCSI Bus
Disk Drives Tape Drive^ Optical Drive
Network Adapter
Network
I/O Systems
l Many different types of devices
- disks
- networks
- displays
- mouse
- keyboard
- tapes
l Each have a different expectation for performance
- bandwidth
- rate at which data can be moved
- latency
- time from request to first data back
Issues In Busses
l Performance
- increase the data bus width
- have separate address and data busses
- block transfers
- move multiple words in a single request
l Who controls the bus?
- one or more bus masters
- a bus master is a device that can initiate a bus request
- need to arbitrate who is the bus master
- assign priority to different devices
- use a protocol to select the highest priority item
- daisy chained
- central control
Disks
l Several types:
- Hard Disks - rigid surface with magnetic coating
- Floppy disks - flexible surface with magnetic coating
- Optical (read only, write once, multi-write)
l Hard Disk Drives:
- collection of platters
- platters contain concentric rings called tracks
- tracks are divided into fixed sized units called sectors
- a cylinder is a collection of all tracks equal distant from the center of disk
- Current Performance:
- capacity: megabytes to tens of gigabytes
- throughput: sustained < 10 megabytes/sec
- latency: mili-seconds
I/O Interrupts
l Interrupt defined
- indication of an event
- can be caused by hardware devices
- indicates data present or hardware free
- can be caused by software
- system call (or trap)
- CPU stops what it is doing and executes a handler function
- saves state about what was happening
- returns where it left off when the interrupt is done
l Need to know what device interrupted
- could ask each device (slow!)
- instead use an interrupt vector
- array of pointers to functions to handle a specific interrupt
I/O Operations
l Synchronous I/O
- program traps into the OS
- request is made to the device
- processor waits for the device
- request is completed
- processor returns to application process
l Asynchronous I/O
- request is made to the device
- processor records request
- processor continues program
- could be a different one
- request is completed and device interrupts
- processor records that request is done
- program execution continues