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Display on MS-DOS cannot use font variants, like bold or italic,
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.
The section section International Support on MS-DOS, later in this chapter, describes how Emacs displays glyphs and characters which aren't supported by the native font built into the DOS display.
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.
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