IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

Interrupt 1 FH - Graphics Character Extensions
When operating in the graphics modes ofthe 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.
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