UNIX v6

• Multi-user time-sharing operating system for PDP11
  • PDP-11 (1972):
  • 16-bit processor, 18-bit physical (40)
  • UNIBUS
  • memory-mapped I/O
  • performance: less than 1MIPS
  • register-to-register transfer: 0.9 usec
  • 56k-228k (40)
  • no paging, but some segmentation support
  • interrupts, traps
  • about $10K
  • rk disk with 2MByte of storage
  • with cabinet 11/40 is 400lbs
• UNIX v6
  • 1976; first widely available UNIX outside Bell labs
  • R&T
  • simplicity (reaction to MULTICS)
  • complete (used for real work)
  • small (43 system calls)
  • modular (composition through pipes; one had to split programs!!)
  • compactly written (2 programmers, 9,000 lines of code)
  • advanced UI (shell)
  • introduced C (derived from B)
  • distributed with source
  • V7 was sold by Microsoft for a couple years under the name Xenix
• Lion's commentary
  • surpressed because of copyright issue
  • resurfaced in 1996

Newer UNIXs have inherited many of the conceptual ideas even though they added paging, networking, graphics, etc.

You will need to read most of the source code multiple times. Your goal is to explain every line to yourself without using the commentary. Read it one or multiple times with Lion's commentary until you reach the goal.

V6 organization

The OS is organized as a (monolithic) kernel, with user programs running in their own address space. Address spaces are set up using page address register (PAR) and page descriptor register (PDR). Note that physical addresses (18 bits) are bigger than virtual addresses (16 bits)

User address spaces are limited to 64Kbyte (on pdp11/40), and the kernel one to 48Kbyte. The top 8Kbyte of the kernel address space is reserved for memory-mapped I/O. Of the 8Kbyte below that in the virtual address, 1Kbyte is used for the u structure.

Booting v6 (chapter 6)

PDP-11 assembly (8 general register, pc (r7), sp (r6), environment (r5)). r0, r1 used for results

We covered the lines 612 through 0669 using the simulator. These lines are explained by Lions in Chapter 6, but the accompanying picture may be helpful, since it depicts the end result.