ACT apricot - Page 242

GUIDE TO THE BIOS
Double word pointers are four bytes long, and conform to
the standard Intel addressing foramts. The first two bytes
are
the offset from the start of a segment that the pointer is
to reference,
and the second two bytes are the segment
address.
Tables are Jump tables, used by
the BIOS to quickly pass
control
to various parts of itself without having to make
multiple conparisons which would use memory and slow
down operations considerably.
A
constant is two bytes long, the low byte containing the
relevant data.
Care
should be taken when altering any of these values as
many of them are used frequently by the BIOS (the keyboard
pointers are accessed a
number of times on each key
depression) .
Here is
an example method of using the pointers from
Basic:
10 DEF SEG=O:CS=PEEK(&H070S)+256*PEEK(&H0707)
20 DEF SEG=PEEK(&H0708)+25S*PEEK(&H0709)
In this case, line 10 sets the variable CS to point to the
beginning of the character set table in RAM, and the current
active segment is set to the start of the segment containing
the character set.
CHARACTER FONTS 1,2 and 3
The BIOS uses a character font to define the shapes of the
characters as they appear on the screen. It consists of 256
16-word data elements - one for each character, making a
total of
8K bytes. For more details see the Character Set
section. There is always one default character set in RAM,
which starts at 0800H. After this there is 16K reserved for
another two sets if required. When the cache or graphics is .
used, however, there is only room for the default character
set.