MS-DOS: "Just Say No!"  (OS/2: "The Nightmare Continues...")

history -- a billion dollar lottery win for Bill Gates

  MS-DOS only runs on Intel 8088 and x86 architectures

  produced by Microsoft and Bill Gates for IBM's first PC, an 8088
  with 64KB of memory and a 160 KB floppy disk

  MS-DOS design limited programs to 640KB of the 8088 1MB address
  space (rest was ROM and VRAM)

  PC architecture had no hardware memory protection, meaning user
  programs could "take over" the hardware, e.g. change interrupt
  vectors

  BIOS ROM contains terminal and disk device drivers

  MS-DOS designed as a single user system (no uid's)

  MS-DOS has used only the higher speed of the x86 chips (not the
  user/kernel mode, not the protection rings, not the multitasking,
  not the 16MB of memory)

  IBM and Microsoft fought big battle over OS/2; Microsoft abandoned
  it for Windows and Windows-NT

versions, Fig 8-1

  version 6 of MS-DOS out after textbook was published

overview

  user interface, command format, Fig 8-2

  shell

    no background processes

    drive e.g. A: is part of file path names

    shell expands wildcard * and ? for internal commands but
    does not for external commands

    replace *.c *.old is not "nearly impossible" in UNIX, viz. using csh

      foreach i (*.c)
      mv $i $i:r.old
      end

fundamental concepts

  processes

    parent and child cannot run in parallel (command interpreter waits
    for commands to finish)

    first 256 bytes of a process are Program Segment Prefix containing
    program size, file descriptor table, cntl-C handler address, pointer
    to parent PSP

    child inherits open files and file positions

    terminate-and-stay-resident programs, what a concept! (Fig 8-5)

      when a process exits, do not reclaim its memory

      the process can install some of its own interrupt handlers
      before exiting

      when "hotkey" is struck, code inside the dead process is executed

  memory management, 8088 memory architecture

    registers in 8088, Fig 8-6

    segment registers CS, DS, SS, ES hold 16-bit addresses

    programs have 4 64KB segments

    segment registers are shifted left 4 bits before being added
    to program addresses or pointer registers, giving a 20-bit
    memory address, Fig 8-7(a)

    high memory, upper memory, extended memory, expanded memory, what could
    be next? (Fig 8-7(b))

    conventional memory: 0KB            -> 640KB
    upper memory area:   640KB          -> 1MB
      (ROM, VRAM)
    high memory area:    1MB            -> 1MB+(64KB-16B)
      (recognized by 286, MS-DOS can be put here)
    extended memory:     1MB+(64KB-16B) -> 16MB
      (RAM disks and buffer cache)

    memory models, Fig 8-8

    manual software overlays, Fig 8-9

    bank switching (expanded memory) used on 8088, Fig 8-10 (answer to #9,
    p. 142 on top p. 339)

    memory management techniques summary, Fig 8-11

  file system, Fig 8-12

  input/output

    character special files con, com1, lpt1, nul

    cooked and raw modes

    stdin, stdout, stderr

    user installable device drivers, Fig 8-13

system calls, Fig 8-14

  process management, no separate fork and exec
  memory management
  files and directories
  input and output

implementation: BIOS, kernel, command interpreter

  BIOS supplied by manufacturer and contains device drivers

  kernel handles machine-independent process management, memory management,
  file system, system call interface, traps to BIOS

  after booting, memory looks like Fig 8-15

  processes, PSP's are chained by backward pointer to parent (no process table)

  memory management, search arenas for free one big enough, Fig 8-16

  file system

    hard disk layout, Fig 8-18

    p. 352 boot stuff doesn't differentiate between primary and secondary boot
    sectors; see p. 347-348 for more clues

    FAT layout, Fig 8-19

    directory entry layout, Fig 8-20 (hard links are not possible)

  input/output

    character special and block special files

    driver layout, Fig 8-21