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

BASIC Workspace Variables
If you do DEF SEG (Default workspace segment)
OFFSET
LENGTH
Line number of current line being executed X '2E' 2
Line number of last error X '347' 2
Offset into segment of start of program text X '30' 2
Offset into of start of variables X '358' 2
(end of program text 1-1)
Keyboard buffer contents X'SA' 1
if O-no characters in buffer
if l-characters in buffer
if you POKE & H6A, 0 you
flush any characters in buffer
Example:
100 Print PEEK (&H2E) + 256*PEEK (&H2F)
) L H
100
I
X'S4'
I
X '00'
I
3-23