UNIX and MVS

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UNIX (and MVS)

• CIS 250 Operating Systems
• Lecture 12

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Today

• Talk about UNIX, say a few words about MVS
• Semester is almost over! Christmas is here!

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UNIX Family

• Also includes Linux
• Older operating systemfirst variants appeared
  30 years ago
• Many different variants floating around now, so
  not everything said here is true for all variants
• ATT, BSD, AIX, HP-UX, etc

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Target

• Portability
• Professional computer usersmulti-user
  environment
• Single processor environment (traditional UNIX)
  or multiprocessor (modern)
• Keep it simple stupid!

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Other Basics

• Once written in assembler, then rewritten into C
• UNIX and C histories tied together
• Open Source, even from early days
• Borrows ideas, naming from MULTICS
• Linux developed off of the MINIX offshoot
• Threw out critical microkernel idea

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Memory Management

• Most variants use demand pagingsome old versions
  use total swapping
• Page replacement in part decided on by two-handed
  clock method (some variants use LRU)
• Some details of two-handed clock
• Virtual memory supported in most variants

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Processor Management

• Round robin scheduling supports multi-user
  environment
• Traditional UNIX used process priorities to
  synchronize processes (hence lack of support for
  multiple CPUs)
• Forking used for process creation
• Threads not in all variantsintroduced as part of
  Mach
• Later developed as part of POSIX library
  standards
• Solaris and Linux support POSIX threads

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Process Generation

• We talked about how shell spins off new process
  to handle commands
• Each process has a user structure
• Text, data and stack segments
• Child process shares parents data and stack
  segments!
• Cheating done copy-on-write

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Device Management

• External devices treated as a special class of
  file
• Introduction of device drivers gave UNIX unusual
  flexibility in dealing with new devices

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File Management

• Three classes of file Directories, files,
  special (hardware)
• Old versions had name limits, most modern ones
  dont
• We talked a bit about I-nodes already
• Symbolic links (hard and soft)
• NFS (Sun) mounting of directories

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Common UNIX Directories
dev (device special files)
bin (binaries of UNIX system)
lib (library files, eg C programming lang.)
usr (shared applications)
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MVS

• Now called OS/390
• Designed for multiprogrammed industrial computing
  in a mostly batch environment
• Runs on mainframes
• MVS handles timesharing, multiple users
• Backwardly compatible hardware gave this OS a
  very long life

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Memory and Processor

• MVS uses segmented/page memory scheme
• Multiple virtual memory spaces supported
• Processor management functions broken up
  differently from what you read earlier
• No support for threads (as far as I know)

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Other

• All forms of file access are supported, including
  a few variants we havent talked about
• Very strong security features