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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 426
SEC. 5.6
USER INTERFACES: KEYBOARD, MOUSE, MONITOR
395
a key release. It is up to the driver to keep track of the status of each key (up or
down). So all the hardware does is give press and release interrupts. Software does
the rest.
When the
A
key is struck, for example, the scan code (30) is put in an I/O reg-
ister.
It is up to the driver to determine whether it is lowercase, uppercase, CTRL-
A, ALT-A, CTRL-ALT-A, or some other combination. Since the drivercan tell
which keys have been struck but not yet released (e.g., SHIFT), it has enough
information to do the job.
For example, the key sequence
DEPRESS SHIFT, DEPRESS A, RELEASE A, RELEASE SHIFT
indicates an uppercase A.
However, the key sequence
DEPRESS SHIFT, DEPRESS A, RELEASE SHIFT, RELEASE A
also indicates an uppercase A.
Although this keyboard interface puts the full bur-
den on the software, it is extremely flexible. For example, user programs may be
interested in whether a digit just typed came from the top row of keys or the
numeric keypad on the side.
In principle, the driver can provide this information.
Two possible philosophies can be adopted for the driver. In the first one, the
driver’s job is just to accept input and pass it upward unmodified. A program read-
ing from the keyboard gets a raw sequence of ASCII codes.
(Giving user programs
the scan codes is too primitive, as well as being highly keyboard dependent.)
This philosophy is well suited to the needs of sophisticated screen editors such
as
emacs
, which allow the user to bind an arbitrary action to any character or se-
quence of characters.
It does, however, mean that if the user types
dste
instead of
date
and then corrects the error by typing three backspaces and
ate
, followed by a
carriage return, the user program will be given all 11 ASCII codes typed, as fol-
lows:
dste
←←←
a t e CR
Not all programs want this much detail. Often they just want the corrected
input, not the exact sequence of how it was produced. This observation leads to the
second philosophy: the driver handles all the intraline editing and just delivers cor-
rected lines to the user programs. The first philosophy is character oriented; the
second one is line oriented.
Originally they were referred to as
raw mode
and
cooked mode
, respectively. The POSIX standard uses the less-picturesque term
canonical mode
to describe line-oriented mode.
Noncanonical mode
is equiv-
alent to raw mode, although many details of the behavior can be changed.
POSIX-
compatible systems provide several library functions that support selecting either
mode and changing many parameters.
If the keyboard is in canonical (cooked) mode, characters must be stored until
an entire line has been accumulated, because the user may subsequently decide to
erase part of it. Even if the keyboard is in raw mode, the program may not yet have
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