bit - screen graphics, mouse
bind -a #b /dev
ushort BGSHORT(uchar *p)
ulong BGLONG(uchar *p)
void BPSHORT(uchar *p, ushort v)
void BPLONG(uchar *p, ulong v)
The bit device provides the files bitblt, mouse, mousectl, and screen on machines with a
bitmapped screen and a mouse. The device is exclusive use.
The bit device provides, through the bitblt file, access to bitmaps, fonts, and subfonts
in its private storage, as described in graphics(2). Each object is identified by a
short, its id. The bitmap with id zero is special: it represents the visible display.
The subfont with id zero is also special: it is initialized to a default subfont that is
always available. There is no default font. There is also a cursor associated with the
screen; it is always displayed at the current mouse position. A process can write mes-
sages to bitblt to allocate and free bitmaps, fonts, and subfonts, read or write portions
of the bitmaps, and draw line segments, textures, and character strings in the bitmaps.
All graphics requests are clipped to their bitmaps. Some messages return a response to be
recovered by reading bitblt.
The format of messages written to bitblt is a single lower case letter followed by binary
parameters; multibyte integers are transmitted with the low order byte first. The BPSHORT
and BPLONG macros place correctly formatted two- and four-byte integers into a character
buffer. Some messages return a response formatted the same way; it usually starts with
the upper case version of the request character. BGSHORT and BGLONG retrieve values from
a character buffer. Points are two four-byte numbers: x, y. Rectangles are four four-
byte numbers: min x, min y, max x, and max y.
The following requests are accepted by the bitblt file. The numbers in brackets give the
length in bytes of the parameters.
a ldepth rect
Allocate a bitmap. Ldepth is the log base 2 of the number of bits per pixel. Rect
is a Rectangle giving the extent of the bitmap. The bitmap is cleared to all zeros.
The id of the allocated bitmap is returned on a subsequent read from bitblt, return-
ing the three bytes: followed by the id.
b dstid dstpt srcid srcrect code
Bit-block transfer (bitblt) from a rectangle in the bitmap identified by srcid to a
congruent rectangle at Point dstpt in the bitmap identified by dstid. The rectangle
is clipped against both source and destination bitmaps. See bitblt(2).
c [ pt clr set ]
Switch mouse cursor. See the description of Cursors in graphics(2) for the meaning
of the pt (the offset), set, and clr arguments. If only is provided -- that is, if
the message is one byte long -- the cursor changes to the default, typically an
e id pt value code n pts[n*2]
Join the n+1 points pt and pts with n segments, exactly as for the l operator. The
pts are specified by pairs of signed bytes holding offsets from the previous point in
Free the resources associated with the allocated bitmap identified by id.
Free the resources associated with the allocated subfont identified by id, including
its bitmap. If the subfont is cached, the associated data may be recoverable even
after it has been freed; see below.
Free the resources associated with the allocated font identified by id.
Initialize the device. The next operation on bitblt should be a read(2). A read of
length 34 returns information about the display:
I ldepth rect cliprect.
If the read count is large enough, the above information is followed by the header
and character information of the default Subfont, in the format expected by rdsub-
fontfile (see subfalloc(2) and font(6)). `Large enough' is 36 + 6n, where n is the
number of characters in the subfont. The ids of the screen bitmap and default sub-
font are both zero.
j q0 q1
Check to see whether a subfont with tags q0 and q1 is in the cache. If it is not,
the write of the j message will draw an error. If it is, the next read of bitblt
followed by the subfont information in the same format as returned by an init mes-
sage; the subfont will then be available for use.
k n height ascent bitmapid q0 q1 info[6*(n+1)]
Allocate subfont. The parameters are as described in subfalloc(2), with info in
external subfont file format. Bitmapid identifies a previously allocated bitmap con-
taining the character images. Q0 and q1 are used as labels for the subfont in the
cache; if all ones, the subfont will not be cached and hence shared with other appli-
cations. The id of the allocated subfont is recovered by reading from bitblt the
three bytes: followed by the id. Henceforth, the bitmap with id bitmapid is unavail-
able to the application; in effect, it has been freed.
l id pt1 pt2 value code
Draw a line segment from Point pt1 to Point pt2, using code for the drawing function,
and value as the source pixel. See segment in bitblt(2). Id identifies the destina-
Read the colormap associated with the bitmap with the specified id. The next read of
bitblt will return 12*2^n bytes of colormap data where n is the number of bits per
pixel in the bitmap.
n height ascent ldepth ncache
Allocate a font with the given height, ascent, and ldepth. The id of the allocated
font is recovered by reading from bitblt the three bytes: followed by the id. The
initial cache associated with the font will have ncache character entries of zero
p id pt value code
Change the pixel at Point pt using code for the drawing function, and value as the
source pixel. See point in bitblt(2).
q id rect
Set the clipping rectangle for the bitmap with specified id to the given rectangle,
which will itself be clipped to the bitmap's image rectangle.
r id miny maxy
Read rows ymin, ymin+1, ... ymax-1 of the bitmap with the given bitmap id. See the
description of rdbitmap in balloc(2). A subsequent read of bitblt will return the
requested rows of pixels. Note: in this case, the response does not begin with an to
simplify the reading of large bitmaps. Also, the reply may be too large to fit in a
single 9P message (see read(5)), so multiple reads may be necessary; each read will
return only complete rows.
s id pt fontid code n indices[2*n]
Draw using code code in the bitmap identified by id the text string specified by the
n cache indices in font fontid, starting with the upper left corner at pt.
t dstid rect srcid code
Texture the given rectangle in the bitmap identified by dstid by overlaying a tiling
of the bitmap identified by srcid (aligning (0,0) in the two bitmaps), and using code
as a drawing code for bitblt; see texture in bitblt(2).
v id ncache width
Reset, resize, and clear the cache for font id; the maximum width of the ncache char-
acters the cache may hold is set to width. Must be done before the first load of a
cache slot. If the cache cannot be resized, the write of this message will fail but
the cache will be unaffected.
w id miny maxy data[n]
Replace rows ymin, ymin+1, ... ymax-1 of the bitmap with the given bitmap id with
the values in data. See the description of wrbitmap in balloc(2).
x x y
Move the cursor so its origin is at (x,y).
y id cacheindex subfontid subfontindex
Load the description and image of character subfontindex in subfont subfontid into
slot cacheindex of font id.
z id map[m]
Replace the colormap associated with bitmap id with map, which contains m=12*2^n
bytes of colormap data (see rgbpix(2) for the format).
A read of the mouse file returns the mouse status: its position and button state. The
read blocks until the state has changed since the last read. The read returns 14 bytes:
m buttons x y msec
where x and y are the mouse coordinates in the screen bitmap, msec is a time stamp, in
units of milliseconds, and buttons has set the 1, 2, and 4 bits when the mouse's left,
middle, and right buttons, respectively, are down.
Writing to the mousectl file configures and controls the mouse. The messages are:
serial n sets serial port n to be the mouse port.
ps2 sets the PS2 port to be the mouse port.
accelerated turns on mouse acceleration.
linear turns off mouse acceleration
res n sets mouse resolution to a setting between 0 and 3 inclusive.
swap swaps the left and right buttons on the mouse.
Which messages are implemented is machine-dependent.
The screen file contains the screen bitmap in the format described in bitmap(6).
Most messages to bitblt can return errors; these can be detected by a system call error on
the write(see read(2)) of the data containing the erroneous message. The most common
error is a failure to allocate because of insufficient free resources. Most other errors
occur only when the protocol is mishandled by the application. Errstr(2) will report
Because each message must fit in a single 9P message, subfonts are limited to about 1300
Can only change the color map of bitmap 0.