ACT apricot - Page 259

SCREEN DRIVER
SYSTEMS INTEREST
1.
CRTC initialization.
The Motorola MC6845 CRTC chip contains 18 internal
registers which have to be initialized before any screen I/O
can take place. Information on the register functions and
the actual data which has to be placed in them to perform
various operations
can be found in the CRTC description in
the Hardware section. Once this information is established,
the port addresses used are 68H for the Address register, and
6AH for the control registers.
2. Low level display
routines. ,
These routine runs closely in conjunction with the cursor
management code. First of all, the current cursor position is
established,
the cursor switched off, and the character to be
printed on the screen is moved into the position in the
screen memory that the cursor occupied'. Then the attribute
byte (see later) is set. Various tests are then made before the
cursor is restored, for example, does the screen need to be
scrolled?
When any actions have been taken, a short delay
(which
CAN be interrupted) is executed. This is called the
cursor TIMEOUT and is used to stop the cursor appearing
when large volumes of text (such as a directory) are being
output. When the timeout has expired, the cursor position
is updated, and the cursor is switched back on.
3.
Cursor management routines.
A record must always be kept of where the cursor is on
the screen, in order that the character display routines know
where to put the next character, and certain console
functions such as report cursor posi tion can work. The
cursor is managed by a set of routines which can move it
around the screen. The initial cursor mode is known, so the
actual position on the screen has to be calculated. Once this
is done, the cursor position is sent to the CRTC which then
moves its internal cursor reference accordingly.