The UNIX System

History

	Bell Labs --- MULTICS ancestory (Thompson, Richie)
	Version 7 --> System III --> System V
	   \
	    \--> BSD systems

Design Principles

• Layered time-sharing system
• File system -- multi-layered tree
• Files and I/O devices treated similarly
• Demand paging
• Made by programmers for programmers
  • replacable shell
  • "make"
  • "SCCS"
• "C" based and therefore portable
• Source online
• Controlled crashes and problem prevention
• Scalable

Programmer Interface

                           users
                             |
                 shells, GUI's and commands
            (compilers, system libraries, etc.)
                             |
                   system -call Interface
                             |
                          kernal
            (signals, file sys, swapping, CPU sked,
            paging, virtual mem, disk & tape drivers)
                             |
                   kernal interface to H/W
                             |
                         hardware

Files

• Sequence of bytes
• Path names, links, root
• "standard" files
• "creat, open, read, write, close, unlink, lseek, trunc"
• Blocks and fragments (small blocks) Disk sectors usually 4K bytes
• Inodes --- user/group id's, links, type, pointers to disk blocks, time of last modification
• vnodes
• File descriptor (index to open file table)

Processes

• "fork, execve, wait"
• Process Management:

  PCB's --- stored in kernel

  
  

• CPU skeds:
  • Priority Q's --- system jobs higher than users employs aging and time accumulation to adjaust priorities. "nice"
• Memory Management:
  • Old systems --- swapping using first-fit
  • Paging --- introduced in BSD systems. LRU or Second Chance on old systems. Swapping will occur to relieve overloaded page system.
• Interprocess Communications:
  • Shared memory. Not in old sys, just in new.
  • Pipes --- file implementation.
  • Sockets --- stream socket vs datagram socket.
  • Streams --- only in System V.

Signals

Miscellaneous

LINUX

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