IBM XT 5160 Technical Reference page 154

Interrupt Hex 1F - Graphics Character Extensions
When operating in the graphics modes of the IBM
Color/Graphics Monitor Adapter (320 by 200 or 640 by 200),
the read/write character interface forms 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 second 128 code points, this vector must be established to
point at a table of up to 1K bytes, where each code point is
represented by eight bytes of graphic information. At power-on,
this vector is initialized to 000:0, and it is the responsibility of the
user to change this vector if additional code points are required.
Interrupt Hex 40 - Reserved
When an IBM Fixed Disk Adapter is installed, the BIOS routines
use interrupt hex 30 to revector the diskette pointer.
Interrupt Hex 41 - Fixed Disk Parameters
This vector points to a data region containing the parameters
required for the fixed disk drive. The power-on routines initialize
the vector to point to the parameters contained in the ROM disk
routine. These default parameters represent the specified values
for any IBM fixed disk drives attached to the system. Changing
this parameter block may be necessary to reflect the specifications
of the other fixed disk drives attached.
Other Read/Write Memory Usage
The IBM BIOS routines use 256 bytes of memory from absolute
hex 400 to hex 4FF. Locations hex 400 to hex 407 contain the
base addresses of any RS-232C cards attached to the system.
Locations hex 408 to hex 40F contain the base addresses of the
Printer Adapter.
Memory locations hex 300 to hex 3FF are used as a stack area
during the power-on initialization, and bootstrap when control is
passed to it from power-on. If the user desires the stack in a
different area, the area must be set by the application.
System BIOS 5-7