MS-DOS Issues
*************

This section briefly describes the peculiarities of using Emacs under the MS-DOS ``operating system'' (also known as ``MS-DOG'').  If you build Emacs for MS-DOS, the binary will also run on Windows 3, Windows NT, Windows 95, or OS-2 as a DOS application; the information in this chapter applies for all of those systems, if you use an Emacs that was built for MS-DOS.

Note that it is possible to build Emacs specifically for Windows NT or Windows 95.  If you do that, most of this chapter does not apply; instead, you get behavior much closer to what is documented in the rest of the manual, including support for long file names, multiple frames, scroll bars, mouse menus, and subprocesses.  However, the section on text files and binary files does still apply.  There are also two sections at the end of this chapter which apply specifically for Windows NT and 95.

Keyboard and Mouse on MS-DOS
============================

The PC keyboard maps use the left Alt key as the META key.  You have two choices for emulating the SUPER and HYPER keys: either the right CONTROL key or the right ALT key by setting the variables dos-hyper-key and dos-super-key to 1 or 2 respectively.

The variable dos-keypad-mode is a flag variable which controls what key codes are returned by keys in the numeric keypad.  There is no dedicated LFD key; use C-j instead.  You can also define the kp-enter key to act as LFD, by putting the following line into your _emacs file:

     ;; Make the Enter key from the Numeric keypad act as LFD.
     (define-key function-key-map [kp-enter] [?\C-j])

The key which is called DEL in Emacs (because that's how it is designated on most workstations) is known as BS (backspace) on a PC.  That is why the PC-specific terminal initialization remaps the BS key to act as DEL; the DEL key is remapped to act as C-d for the same reasons.

Emacs on MS-DOS supports a mouse (on the default terminal only). The mouse commands work as documented, including those that use menus and the menu bar (see Menu Bars).  Scroll bars don't work in MS-DOS Emacs.  PC mice usually have only two buttons; these act as Mouse-1 and Mouse-2, but if you press both of them together, that has the effect of Mouse-3.

The variable dos-display-scancodes, when non-nil, directs Emacs to display the ASCII value and the keyboard scan code of each keystroke; this feature serves as a complement to the view-lossage command, for debugging.

Display on MS-DOS
=================

Display on MS-DOS cannot use multiple fonts, but it does support multiple faces, each of which can specify a foreground and a background color.  Therefore, you can get the full functionality of Emacs packages which use fonts (such as font-lock, Enriched Text mode, and others) by defining the relevant faces to use different colors.  Use the list-colors-display and list-faces-display commands (see Modifying Faces) to see what colors and faces are available and what they look like.

Multiple frames (see Frames) are supported on MS-DOS, but they all overlap, so you only see a single frame at any given moment.  That single visible frame occupies the entire screen.  When you run Emacs under Windows version 3, you can make the visible frame smaller than the full screen, but Emacs still cannot display more than a single frame at a time.

The mode4350 command switches the display to 43 or 50 lines, depending on your hardware; the mode25 command switches to the default 80x25 screen size.

By default, Emacs only knows how to set screen sizes of 80 columns by 25 or 43/50 rows.  However, if your video adapter has special video modes that will switch the display to other sizes, you can have Emacs support those too.  When you ask Emacs to switch the frame to N rows by M cols dimensions, it checks if there is a variable called screen-dimensions-NxM, and if so, uses its value (which must be an integer) as the video mode to switch to.  (Emacs switches to that video mode by calling the BIOS Set Video Mode function with the value of screen-dimensions-NxM in the AL register.)  For example, suppose your adapter will switch to 66x80 dimensions when put into video mode 85.  Then you can make Emacs support this screen size by putting the following into your _emacs file:

     (setq screen-dimensions-66x80 85)

Since Emacs on MS-DOS can only set the frame size to specific supported dimensions, it cannot honor every possible frame resizing request.  When an unsupported size is requested, Emacs chooses the next larger supported size beyond the specified size.  For example, if you ask for 36x80 frame, you will get 50x80 instead.

The variables screen-dimensions-NxM are used only when they exactly match the specified size; the search for the next larger supported size ignores them.  In the above example, even if your VGA supports 44x80 dimensions and you define a variable screen-dimensions-44x80 with a suitable value, you will still get 50x80 screen when you ask for a 36x80 frame.  If you want to get the 44x80 size in this case, you can do it by setting the variable named screen-dimensions-36x80 with the same video mode value as screen-dimensions-44x80.

Changing frame dimensions on MS-DOS has the effect of changing all the other frames to the new dimensions.

File Names on MS-DOS
====================

MS-DOS normally uses a backslash, \, to separate name units within a file name, instead of the slash used on other systems.  Emacs on MS-DOS permits use of either slash or backslash, and also knows about drive letters in file names.

On MS-DOS, file names are case-insensitive and limited to eight characters, plus optionally a period and three more characters.  Emacs knows enough about these limitations to handle file names that were meant for other operating systems.  For instance, leading dots . in file names are invalid in MS-DOS, so Emacs transparently converts them to underscores _; thus your default init file (see Init File) is called _emacs on MS-DOS.  Excess characters before or after the period are generally ignored by MS-DOS itself, so if you, e.g., visit a file LongFileName.EvenLongerExtension, you will silently get longfile.eve; but Emacs will still display the long file name on the mode line.  Other than that, it's up to you to specify file names which are valid under MS-DOS; the transparent conversion as described above only works on file names built into Emacs.

The above restrictions on the file names on MS-DOS make it almost impossible to construct the name of a backup file (see Backup Names) without losing some of the original file name characters.  For example, the name of a backup file for docs.txt is docs.tx~ even if single backup is used.

If you run Emacs as a DOS application under Windows 95 or NT, you can turn on support for long file names.  If you do that, Emacs doesn't truncate file names or convert them to lower case; instead, it uses the file names that you specify, verbatim.  To enable long file name support, set the environment variable LFN to y before starting Emacs.

MS-DOS has no notion of home directory, so Emacs on MS-DOS pretends that the directory where it is installed is the value of HOME environment variable.  That is, if your Emacs binary, emacs.exe, is in the directory c:/utils/emacs/bin, then Emacs acts as if HOME were set to c:/utils/emacs.  In particular, that is where Emacs looks for the init file _emacs. With this in mind, you can use ~ in file names as an alias for the home directory, as you would in Unix.  You can also set HOME variable in the environment before starting Emacs; its value will then override the above default behavior.

Text Files and Binary Files
===========================

Emacs on MS-DOS distinguishes between text and binary files.  This distinction is not part of MS-DOS; it is made by Emacs only.  Emacs treats files of human-readable text (including program source code) as text files, and treats executable programs, compressed archives, etc., as binary files.  Emacs uses the file name to decide whether to treat a file as text or binary: the variable file-name-buffer-file-type-alist defines the file name patterns which denote binary files.

Emacs reads and writes binary files verbatim.  Text files use a two character sequence to end a line: carriage-return (control-m) followed by newline (control-j).  When you visit a text file, Emacs strips off these control-m characters; when you write a text file to disk, Emacs puts them back in.  Thus, the text appears within Emacs with just a newline character at the end of each line.

You can tell whether Emacs considers the visited file as text or binary based on the mode line (see Mode Line).  Text files have a T: marker prefixed to the major mode name; binary files have a B: prefix.

One consequence of this special format-conversion of text files is that character positions as reported by Emacs (see Position Info) do not agree with the file size information known to the operating system.

Printing and MS-DOS
===================

Printing commands, such as lpr-buffer (see Hardcopy) and ps-print-buffer (see Postscript) can work in MS-DOS by sending the output to one of the printer ports, if a Unix-style lpr program is unavailable.  A few DOS-specific variables control how this works.

If you want to use your local printer, printing on it in the usual DOS manner, then set the Lisp variable dos-printer to the name of the printer port---for example. "PRN", the usual local printer port (that's the default), or "LPT2" or "COM1" for a serial printer.  You can also set dos-printer to a file name, in which case ``printed'' output is actually appended to that file.  If you set dos-printer to "NUL", printed output is silently discarded.

If you set dos-printer to a file name, it's best to use an absolute file name.  Emacs changes the working directory according to the default directory of the current buffer, so if the file name in dos-printer is relative, you will end up with several such files, each one in the directory of the buffer from which the printing was done.

The commands print-buffer and print-region call the pr program, or use special switches to the lpr program, to produce headers on each printed page.  MS-DOS doesn't normally have these programs, so by default, the variable lpr-headers-switches is set so that the requests to print page headers are silently ignored. Thus, print-buffer and print-region produce the same output as lpr-buffer and lpr-region, respectively.  If you do have a suitable pr program (e.g., from GNU Textutils), set lpr-headers-switches to nil; Emacs will then call pr to produce the page headers, and print the resulting output as specified by dos-printer.

Finally, if you do have an lpr work-alike, you can set print-region-function to nil.  Then Emacs uses lpr for printing, as on other systems.  (If the name of the program isn't lpr, set the lpr-command variable to specify where to find it.)

A separate variable, dos-ps-printer, defines how PostScript files should be printed.  If its value is a string, it is used as the name of the device (or file) to which PostScript output is sent, just as dos-printer is used for non-PostScript printing.  (These are two distinct variables in case you have two printers attached to two different ports, and only one of them is a PostScript printer.)  If the value of dos-ps-printer is not a string, then the variables ps-lpr-command and ps-lpr-switches (see Postscript) control how to print PostScript files.  Thus, if you have a non-PostScript printer, you can set these variables to the name and the switches appropriate for a PostScript interpreter program (e.g., Ghostscript).

For example, to use Ghostscript for printing on an Epson printer connected to LPT2 port, put this on your .emacs file:

     (setq dos-ps-printer t)  ; Anything but a string.
     (setq ps-lpr-command "c:/gs/gs386")
     (setq ps-lpr-switches '("-q" "-dNOPAUSE"
     			"-sDEVICE=epson"
     			"-r240x72"
     			"-sOutputFile=LPT2"
     			"-Ic:/gs"
     			"-"))

(This assumes that Ghostscript is installed in the "c:/gs" directory.)

Subprocesses on MS-DOS
======================

Because MS-DOS is a single-process ``operating system'', asynchronous subprocesses are not available.  In particular, Shell mode and its variants do not work.  Most Emacs features that use asynchronous subprocesses also don't work on MS-DOS, including spelling correction and GUD.  When in doubt, try and see; commands that don't work print an error message saying that asynchronous processes aren't supported.

Compilation under Emacs with M-x compile and grep with M-x grep do work, by running the inferior processes synchronously. This means you cannot do any more editing until the compilation or the grep process finishes.

Printing commands, such as lpr-buffer (see Hardcopy) and ps-print-buffer (see Postscript), work in MS-DOS by sending the output to one of the printer ports.  See Printing and MS-DOS.

When you run a subprocess synchronously on MS-DOS, make sure the program terminates and does not try to read keyboard input.  If the program does not terminate on its own, you will be unable to terminate it, because MS-DOS provides no general way to terminate a process.

Accessing files on other machines is not supported on MS-DOS.  Other network-oriented commands such as sending mail, Web browsing, remote login, etc., don't work either, unless network access is built into MS-DOS with some network redirector.

Dired on MS-DOS uses the ls-lisp package where other platforms use the system ls command.  Therefore, Dired on MS-DOS supports only some of the possible options you can mention in the dired-listing-switches variable.  The options that work are -A, -a, -c, -i, -r, -S, -s, -t, and -u.

Subprocesses on Windows 95 and Windows NT
=========================================

Subprocesses, both synchronous and asynchronous, work fine on both Windows 95 and Windows NT as long as you run only 32-bit Windows applications.  However, when you run a DOS application in a subprocess, you may encounter problems or be unable to run the application at all; and if you run two DOS applications at the same time in two subprocesses, you may have to reboot your system.

Since the standard command interpreter (and most command line utilities) on Windows 95 are DOS applications, these problems are significant when using that system.  But there's nothing we can do about them; only Microsoft can fix them.

If you run just one DOS application subprocess, the subprocess should work as expected as long as it is ``well-behaved'' and does not perform direct screen access or other unusual actions.  If you have a CPU monitor application, your machine will appear to be 100% busy even when the DOS application is idle, but this is only an artefact of the way CPU monitors measure processor load.

You must terminate the DOS application before you start any other DOS application in a different subprocess.  Emacs is unable to interrupt or terminate a DOS subprocess.  The only way you can terminate such a subprocess is by giving it a command that tells its program to exit.

If you attempt to run two DOS applications at the same time in separate subprocesses, the second one that is started will be suspended until the first one finishes, even if either or both of them are asynchronous.

If you can go to the first subprocess, and tell it to exit, the second subprocess should continue normally.  However, if the second subprocess is synchronous, Emacs itself will be hung until the first subprocess finishes.  If it will not finish without user input, then you have no choice but to reboot if you are running on Windows 95.  If you are running on Windows NT, you can use a process viewer application to kill the appropriate instance of ntvdm instead (this will terminate both DOS subprocesses).

If you have to reboot Windows 95 in this situation, do not use the Shutdown command on the Start menu; that usually hangs the system.  Instead, type CTL-ALT-DEL and then choose Shutdown.  That usually works, although it may take a few minutes to do its job.

Using the System Menu on Windows
================================

Emacs normally turns off the Windows feature that tapping the ALT key invokes the Windows menu.  The reason is that the ALT also serves as META in Emacs.  When using Emacs, users often press the META key temporarily and then change their minds; if this has the effect of bringing up the Windows menu, it alters the meaning of subsequent commands.  Many users find this frustrating.

You can reenable Windows's default handling of tapping the ALT key by setting win32-pass-alt-to-system to a non-nil value.