IBM AT 5170 Technical Reference page 181

addresses of any RS-232C adapters installed in the system.
Locations hex 408 to 40F contain the base addresses of any
printer adapters.
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 to be in a
different area, that area must be set by the application.
The following figure shows the reserved memory locations.
Address Mode
Function
400-4Al ROM BIOS
See BIOS list i ng
4A2-4EF Reserved
4FO-4FF Reserved as intra-application
communication area for any appl icat ion
500-5FF Reserved for DOS and BASIC
500 DOS Print screen status flag store
O=Print screen not active or successful
print screen operation
I=Print screen in progress
255=Error encountered during print
screen operation
504 DOS Single drive mode status byte
510-511
BASIC
BASIC's segment address store
512-515
BASIC Clock interrupt vector segment:offset store
516-519
BASIC
Break key interrupt vector segment :offset
store
51A-51D BASIC Disk error interrupt vector segment :offset
store
Reserved Memory Locations
The following is the BASIC workspace for DEF SEG (default
workspace) .
Offset
Length
2E
2 Line number of current line being executed
347
2
Li ne number of last error
30
2
Offset into segment of start of program text
358 2 Offset into segment of start of variables
(end of program text 1-1)
6A
1 Keyboard buffer contents
O=No characters in buffer
I=Characters in buffer
4E 1 Character color in graphics mode*
Basic Workspace Variables
*Set to 1, 2, or 3 to get text in colors 1-3. Do not set to O. The
default is 3.
System BIOS 5-9