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The Unix Timesharing System

What is the Problem?

A very unspecific problem. Essentially they were burnt by Multics and wanted to create a simpler and more general system.

Summary

Key design goal is simplicity. Everything is hierarchical. Everything is a file.

File System Implementation

• Tree strucutre
• FS mounted on a file
• Sys table of i-numbers (i-list)
• i-node contains metadata for each file
• path names don't distinguish between files and directories
• mount table for mounted file systems
• buffering is built into kernel and transparent to user
  • write-behind (flushes to disk when buffer is full)

Storage Devices

• block devices
  • data is stored in fixed-size blocks
  • "free list" of blocks for allocation, Linked list of blocks
  • hard disks, usb drive, ssd, tape drives
  • early versions of ethernet
• character devices

Execution

• Image is an execution env (parallel container)
• Process is an instance/execution of an image
  • Program text write-protected and shared between all instances of that process
  • Separate virtual address space for each process
• Kernel
  • Mediator for accessing services/hardware/shared resources

Key Insights

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