CSE 221 - Operating Systems - Notes on "The UNIX Time-Sharing System"

Q: What aspects of Unix as described in the 1974 paper do not survive today, or have been considerably changed?

• Powerful OS at low cost
• Significantly easier to use than most

• OS includes assembler, text editor, linking loader, debugger, comopiler, interpreter of BASIC, text formatting, fortran, macro processor

• built on dogfooding
• minimum 50kb of core memory
• written in assembly, then re-written in C.

• Lots of device drivers

• most important role is to provide the FS
• contains
  • disk files
  • directories
  • special files
• Ordinary files
  • could be data, programs, etc
• directories induce a tree structure
  • enforce a limitation on file name length
  • a file which is not a directory may appear in many places (linking)
    • even with different names
  • has . and .. concepts for directory traversal
• special files
  • every IO device corresponds to a file found under /dev
  • useful b/c file and device IO make interaction easy
  • same protection mechanisms as files

Removable FS

• include a mount system
  • provide a file name, and a device which has another FS on it.
  • no links between FS

Protection

• Users get UIDs
• 7 protection bits, but only for owner, and all other users (no group)
  • 7th bit is for change of UID on program execution.
• set-user-id bit is used to cross execution boundaries (i.e. for mkdir)

IO Calls

• no predetermination of IO size
• no distinction on “random” or “sequential” IO
• open with filep = open(name, flag)
• operate on file with read, write, and seek

Implementation of the FS

• directory gets the i-number corresponding from its table of children -> i-number (inode id)
• looks up i-number in system table to read from FS location
• FS location of a file contains
  • owner, protection bits, addresses of blocks, size, last mod time, number of links to file, dir bit, special file bit, small/large bit
• file descriptor corresponds to table mapping of read/write pointer, device, and inumber
• creation corresponds to adding a direcory link, increasing inode link count by 1
• deletion corresponds ton decremting link count, removing dir entry, and then erasing blocks in the inode’s link count is 0
• small/large file bit determines whether the file blocka ddress are stored directly, or a layer of indirection is added to store more block addresses for additional file capacity (1M)
• special file handling
  • if the inode is special, the last 7 device addr words don’t mattr. The first word is a pair of bytes identifying and internal device name.
    • two bytes specify device type and device number.
    • device type indicates which routine handles IO on the device, and subdevice number selects the device attached to the controller of one or more connected interfaces
• write IO, page is read in if necessary, and byte replaced - actual write make take place at a later time

Processes and Images

• image (program) structure divided into 3 parts
  • 0 to N program text (data)
  • N + (8k / N) (next 8k byte boundary) to M is shared writeable data segment (heap)
  • Max stack to M is the stack.
• new process only via fork()
• communicate with pipe
• execute another program with execute
• synchronize on wait()
• exit with exit()

The Shell

• command line interpreter
• use > to output command output to a file
• use < to read from a file data into a program
• piping output with |
• commands one after another with ;
• & to run in background
• shell can call itself
• has init program
• interrupt a program from delete character.
• quit signal can be invoked, dump core image.