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Emacs and MS-DOS

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: choose either the right CTRL key or the right ALT key by setting the variables dos-hyper-key and dos-super-key to 1 or 2 respectively. If neither dos-super-key nor dos-hyper-key is 1, then by default the right ALT key is also mapped to the META key. However, if the MS-DOS international keyboard support program `KEYB.COM' is installed, Emacs will not map the right ALT to META, since it is used for accessing characters like ~ and @ on non-US keyboard layouts; in this case, you may only use the left ALT as META key.

The variable dos-keypad-mode is a flag variable that controls what key codes are returned by keys in the numeric keypad. You can also define the keypad ENTER key to act like C-j, by putting the following line into your `_emacs' file:

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

The key that 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 built for MS-DOS recognizes C-BREAK as a quit character, just like C-g. This is because Emacs cannot detect that you have typed C-g until it is ready for more input. As a consequence, you cannot use C-g to stop a running command (see section Quitting and Aborting). By contrast, C-BREAK is detected as soon as you type it (as C-g is on other systems), so it can be used to stop a running command and for emergency escape (see section Emergency Escape).

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 section The Menu Bar). 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.

Emacs built for MS-DOS supports clipboard operations when it runs on Windows. Commands that put text on the kill ring, or yank text from the ring, check the Windows clipboard first, just as Emacs does on X Windows (see section Mouse Commands for Editing). Only the primary selection and the cut buffer are supported by MS-DOS Emacs on Windows; the secondary selection always appears as empty.

Due to the way clipboard access is implemented by Windows, the length of text you can put into the clipboard is limited by the amount of free DOS memory that is available to Emacs. Usually, up to 620KB of text can be put into the clipboard, but this limit depends on the system configuration and is lower if you run Emacs as a subprocess of another program. If the killed text does not fit, Emacs prints a message saying so, and does not put the text into the clipboard.

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 that 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 command (see section Setting Frame Parameters) and the list-faces-display command (see section Using Multiple Typefaces) to see what colors and faces are available and what they look like.

Multiple frames (see section Frames and X Windows) 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 from MS-Windows DOS box, 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, 28, 35, 40, 43 or 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 columns 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 40x80 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 38x80 dimensions and you define a variable screen-dimensions-38x80 with a suitable value, you will still get 40x80 screen when you ask for a 36x80 frame. If you want to get the 38x80 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-38x80.

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 section The Init File, `~/.emacs') is called `_emacs' on MS-DOS. Excess characters before or after the period are generally ignored by MS-DOS itself; thus, if you visit the 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 section Single or Numbered Backups) 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 9X, 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. Unfortunately, Windows NT doesn't allow DOS programs to access long file names, so Emacs built for MS-DOS will only see their short 8+3 aliases.

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.

Emacs on MS-DOS handles the directory name `/dev' specially, because of a feature in the emulator libraries of DJGPP that pretends I/O devices have names in that directory. We recommend that you avoid using an actual directory named `/dev' on any disk.

Text Files and Binary Files

GNU Emacs uses newline characters to separate text lines. This is the convention used on Unix, on which GNU Emacs was developed, and on GNU systems since they are modeled on Unix.

MS-DOS and MS-Windows normally use carriage-return linefeed, a two-character sequence, to separate text lines. (Linefeed is the same character as newline.) Therefore, convenient editing of typical files with Emacs requires conversion of these end-of-line (EOL) sequences. And that is what Emacs normally does: it converts carriage-return linefeed into newline when reading files, and converts newline into carriage-return linefeed when writing files. The same mechanism that handles conversion of international character codes does this conversion also (see section Coding Systems).

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

Some kinds of files should not be converted, because their contents are not really text. Therefore, Emacs on MS-DOS distinguishes certain files as binary files, and reads and writes them verbatim. (This distinction is not part of MS-DOS; it is made by Emacs only.) These include executable programs, compressed archives, etc. Emacs uses the file name to decide whether to treat a file as binary: the variable file-name-buffer-file-type-alist defines the file-name patterns that indicate binary files. Note that if a file name matches one of the patterns for binary files in file-name-buffer-file-type-alist, Emacs uses the no-conversion coding system (see section Coding Systems) which turns off all coding-system conversions, not only the EOL conversion.

In addition, if Emacs recognizes from a file's contents that it uses newline rather than carriage-return linefeed as its line separator, it does not perform conversion when reading or writing that file. Thus, you can read and edit files from Unix or GNU systems on MS-DOS with no special effort, and they will be left with their Unix-style EOLs.

You can visit a file and specify whether to treat a file as text or binary using the commands find-file-text and find-file-binary. End-of-line conversion is part of the general coding system conversion mechanism, so another way to control whether to treat a file as text or binary is with the commands for specifying a coding system (see section Specifying a Coding System). For example, C-x RET c undecided-unix RET C-x C-f foobar.txt visits the file `foobar.txt' without converting the EOLs.

The mode line indicates whether end-of-line translation was used for the current buffer. Normally a colon appears after the coding system letter near the beginning of the mode line. If MS-DOS end-of-line translation is in use for the buffer, this character changes to a backslash.

When you use NFS or Samba to access file systems that reside on computers using Unix or GNU systems, Emacs should not perform end-of-line translation on any files in these file systems--not even when you create a new file. To request this, designate these file systems as untranslated file systems by calling the function add-untranslated-filesystem. It takes one argument: the file system name, including a drive letter and optionally a directory. For example,

(add-untranslated-filesystem "Z:")

designates drive Z as an untranslated file system, and

(add-untranslated-filesystem "Z:\\foo")

designates directory `\foo' on drive Z as an untranslated file system.

Most often you would use add-untranslated-filesystem in your `_emacs' file, or in `site-start.el' so that all the users at your site get the benefit of it.

To countermand the effect of add-untranslated-filesystem, use the function remove-untranslated-filesystem. This function takes one argument, which should be a string just like the one that was used previously with add-untranslated-filesystem.

Printing and MS-DOS

Printing commands, such as lpr-buffer (see section Hardcopy Output) and ps-print-buffer (see section Postscript Hardcopy) 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 (sent to the system null device).

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 (for example, 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 section Postscript Hardcopy) 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 (such as 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"

(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, searching files with M-x grep and displaying differences between files with M-x diff do work, by running the inferior processes synchronously. This means you cannot do any more editing until the inferior process finishes.

By contrast, Emacs compiled as native Windows application does support asynchronous subprocesses. See section Subprocesses on Windows 95 and NT.

Printing commands, such as lpr-buffer (see section Hardcopy Output) and ps-print-buffer (see section Postscript Hardcopy), work in MS-DOS by sending the output to one of the printer ports. See section 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. Pressing C-c or C-BREAK might sometimes help in these cases.

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 NT

Emacs compiled as a native Windows application (as opposed to the DOS version) includes full support for asynchronous subprocesses. In the Windows version, synchronous and asynchronous subprocesses 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 artifact 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 compiled as a native Windows application 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 w32-pass-alt-to-system to a non-nil value.

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