DOS Memory in a Windows World

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DOS Memory in a Windows World

• Chapter 12
• Mike Fuszner

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Hexadecimal

• 8086 has a 20-wire address bus.
• These wires may or may not have voltage on them.
• A wire with voltage is represented by a 1 and a
  wire with no voltage is represented by a 0.
• Each combination of charged and uncharged wires
  represents one memory location.
• Each memory location should be 8-bits long.
• Hexadecimal (Hex) is a shorthand description of
  the state of wires of the 8086 address bus.

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Hexadecimal

• Memory space vs. Memory
• RAM and ROM chips are used to store bytes of code
  at certain memory locations.
• Memory addresses are not physical locations.
• Two sequential memory addresses may not be
  physically next to each other.

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The DOS Memory Map
Full address map for an 8086 CPU.
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Assigning Addresses

• The concept of allowing RAM as well as other
  devices to share the same memory space is called
  Memory Mapped I/O
• Addresses ranging from 00000 to 9FFFF are
  dedicated for RAM to run programs
• Chips other than RAM use the remaining addresses
  ranging from A0000 to FFFFF

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Conventional Memory

• Conventional area
• The area from 0 to 640 KB is called the
  conventional memory.
• It contains all the memory addresses set aside
  for the RAM to run programs.
• It has 655,360 memory locations.

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Reserved Memory

• Reserved area
• It is a complex compilation of different ROMs and
  RAMs that use memory

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Conventional Reserved Memory
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System BIOS

• The system BIOS is the most important device in
  the reserved area, and is located in memory
  locations from F0000 to FFFFF.

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Video Memory Addresses
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Optional ROMs
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Video Card ROM

• Every computer carries an optional ROM for the
  video card, so the first segment in the Optional
  ROMS area is already assigned

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640K Limit

• 640K became the standard for several years
• Programs became more complex and needed more
  memory, but you could no longer just add memory
  chips once you reached the 640K limit

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Expanded Memory

• The expanded memory specification (EMS) provides
  a way of adding more memory to the computer
  without exceeding the 1 MB limit.
• Expanded memory was originally an expansion card
  full of RAM chips.
• The chips on the card are electronically divided
  into 16 KB chunks called pages.

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Expanded Memory

• A card consists of 4 to 512 pages.
• Each page is labeled electronically with the page
  number.
• A device driver and an application is required to
  access this card of RAM chips.

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Expanded Memory

• The device driver, also known as the EMM.SYS,
  electronically readdresses the chips on the
  expanded memory board.
• The EMS page frame is a 64 KB unused area, in the
  reserved area, created by combining the memory
  cards first four 16 KB pages and addresses.

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LIM 3.2 Standard Expanded Memory

• EMS is the domain of DOS programs.
• It enables programs to use large amounts of data
  without breaking the 640 KB barrier.
• The first 4 16 KB pages are loaded into RAM.
• When other data was needed memory was swapped
• Data could be kept in expanded memory but
  programs still ran in conventional memory

LIM Lotus, Intel, Microsoft
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Shadowing

• Shadowing is the process of copying the BIOS
  routine from the ROM to the RAM at the same
  address, and accessing the routine from the RAM.
• This provides a much faster response from the
  BIOS routine.
• Shadowing can be enabled through the CMOS setup.
• It is safe and easy.

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A20 Wire

• 8086 used a 20-bit address bus created by using
  two 16-bit registers (CS and IP).
• The CPU placed a value in CS and one in IP
• The CPU then shifted the CS value to the left 4
  bits and then added the value to the IP value to
  create a 20-bit address (e.g. 124C changed to
  124C0,then add to 0AE8 12FA8)
• Why? For backward compatibility.
• What if CSFFFF and IPFFFF? Then CSIP10FFEF,
  or 6 digits. 6 hex digits 6x424 bits. A
  24-wire bus would be neededjust what the 286
  processor had.
• But to make the 286 100 compatible with the
  8086, Microsoft masked off the 21st wire (the A20
  wire)

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The High Memory Area

• The high memory area (HMA) is a tiny memory area
  available only to 286 or higher processors.
• The RAM chip must populate addresses above FFFFF
  to use this area (the first 64KB of extended
  memory)

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Extended Memory

• All memory above 1 MB is called extended memory
  (XMS).
• DOS cannot directly use XMS.
• DOS extenders are programs that exceed DOS
  capabilities in order to access XMS.
• Limulators are programs that electronically make
  XMS act like EMS.
• Why buy a 700 EMS card when a 40 limulation
  program could turn XMS into EMS? Hence, EMS cards
  became obsolete!

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Extended Memory

• The HIMEM.SYS device driver provides a standard
  method for using the extended memory.
• Himem.sys was written by Microsoft to provide a
  consistent way of accessing extended memory
• The DOSHIGH option moves most of the COMMAND.COM
  files from the conventional memory into the HMA.
• Upper memory blocks (UMBs) are unused memory
  addresses in the reserved area.

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Reducing Conventional Memory Use

• The goal of DOS memory management is to reduce
  the amount of conventional memory used by
  applications other than the current running
  application.
• Adding device drivers to CONFIG.SYS and TSRs to
  AUTOEXEC.BAT takes up conventional memory.
• Moving the device drivers and TSRs to the HMA and
  UMBs can reduce conventional memory usage.

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UMB Gateways/Memory Managers

• The UMB gateway is used for loading device
  drivers and TSRs into the UMB, freeing
  conventional memory.
• EMM386.EXE is a UMB gateway that comes with every
  version of DOS and Windows.
• The UMB gateway is more popularly known as the
  memory manager since it needs to inspect the
  reserved area to stay clear of ROMs, video RAM,
  and EMS page frames in the upper memory area.

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UMB Gateways/Memory Managers

• Memory managers only prepare the UMB for device
  drivers and programs.
• Loading is carried out by special commands that
  are recognized by the memory manager.
• The DEVICEHIGH command in CONFIG.SYS and
  LOADHIGH command in AUTOEXEC.BAT are used for
  loading device drivers and programs into the UMB.

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Limulation

• Limulators are programs that access XMS memory
  and convert it to EMS memory.
• EMM386.EXE is DOSs built-in limulator.
• Just add the following lines to config.sys
• Devicec\dos\himem.sys
• Devicec\dos\emm386.exe
• If you use devicec\dos\emm386.exe 1024, you
  will limit the amount of XMS memory turned into
  EMS memory
• Or use devicec\dos\emm386.exe RAM to allow
  emm386 to dynamically allocate EMS or XMS
• Changing memory on the fly is called DOS
  Protected Mode Interface (DPMI)
• Or use devicec\dos\emm386.exe NOEMS to load
  high but not limulate

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Loading High

• The DOSUMB command needs to be specified in
  CONFIG.SYS for loading high.
• EMM386 enables loading high as well as
  limulating.
• DOSHIGH loads part of DOS high
• You may combine as DOSHIGH,UMB

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Typical Startup Configurations

• Config.sys
• Devicec\dos\himem.sys
• Devicec\dos\emm386.exe RAM
• DOSHIGH,UMB
• Devicehighc\cdrom\oakcdrom.sys /DMSCD001

• Autoexec.bat
• _at_echo off
• Prompt pg
• smartdrv
• LH c\mouse\mouse.com
• LH c\dos\mscdex.exe /DMSCD001

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The MEM Command

• Command-line program used by DOS for checking
  upper memory.
• Shows the amount of conventional and UMB space
  used and available.
• /C and /P switches

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Mem /C
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Virtual Machine MS-DOS Mode

• The most common method of running a DOS
  application in Windows is via a DOS virtual
  machine, sometimes called running in a DOS box
• AltEnter to run full screen
• The MS-DOS mode is a special mode supported by
  Windows 9x to run DOS programs that cannot run in
  a virtual machine.
• Create the config.sys and autoexec.bat files you
  need
• Reboot in MS-DOS mode

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Virtual Machine

• The program information file (PIF) is a file
  created by Windows in the event of any changes
  being made to the settings in the DOS
  applications properties.

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General Tab
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Program Tab
Makes the program think that it started in this
directory.
Used to start other programs that this program
may need.
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Font Tab
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Memory Tab
You almost never need to edit anything on this
screen. The protected box prevents Windows from
moving any memory to the swap file.
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Screen Tab
Saves the windows position and restores it the
next time you start the program.
Windows video driver will emulate the trivial
video calls used in DOS programs. Reserves the
entire video area for the DOS application.
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Misc Tab
Idle sensitivity will more quickly reduce the
timeslice given to the DOS app when not in
use. Exclusive mode permits mouse to function
only in DOS app. Fast pasting should be
unchecked if you loose characters when pasting
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Problematic DOS Programs

• Some DOS programs require special drivers or
  TSRs, necessitating the need for a unique
  CONFIG.SYS or AUTOEXEC.BAT for that program.
• Windows 9x uses the MS-DOS mode, whereas Windows
  2000 provides a method to create a unique
  CONFIG.SYS and AUTOEXEC.BAT.

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Windows 2000s Method

• Advanced button on the Program tab brings up this
  screen where you can customize the config.sys and
  autoexec.bat file for the DOS program.

A nice way to slow down the computer for DOS
games.
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Windows 9x Method

• Some DOS program require direct access to
  hardware and cannot run in Windows 2000
• Advanced button in the Program tab allows Windows
  to suggest MS-DOS mode if the program tries to
  access hardware directly at startup.

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