Interrupt 1
FH
- Graphics Character Extensions
When operating in the graphics modes
of
the Color/Graphics Monitor
Adapter (320 x 200
or
640 x 200), the read/write character interface
will form the character from the
ASCII
code point, using a set of dot
patterns. The dot patterns for the first 128 code points are
contained
in ROM.
To
access the other 128 code points, this
~
vector must be established
to
point at a table
of
up to I K bytes,
where each code point is represented by 8 bytes
of
graphic
information.
At
power on this vector is initialized to 0:0, and it
is
the responsibility of the user to change this vector
if
the
additional code points are required.
Other Read/Write Memory Usage
The IBM
ROM
BIOS routines use 256 bytes
of
memory starting
at absolute
400
to
4FF.
Locations 400-407 contain the base
addresses
of
any RS232 cards attached to the system,
O's
ifnone
attached. These locations, in order, represent the 0 to 3 values used as
the parameter to the RS232 BIOS routine. Locations
408-40F provide the same function, but for the
PRINTER.
Memory locations
300-3FF
are used as a stack area during the
power on initialization, and the bootstrap, when control passed
to it from power on.
If
the user desires the stack in a different area,
~
it must be set by the application.
Note: Use the Interrupt Vector Listing
as
an aid to locate these topics
in the
ROM
BIOS listing, Appendix
"A".
BIOS
Programming Tip
When programming with BIOS you should keep
in
mind that
if
an error
is reported
by
the diskette code, to reset the diskette adapter and retry
the operation. A specified number of retrys should be required on reads
to ensure the problem is not due to motor start-up.
3-6
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