AT&T 6300 Programmer's Manual page 230

DEF SEG
Statement
7·74
If you enter a value outside the range, then an
"Illegal function call" error results. Previous
value will be retained.
If you do not separate DEF and SEG by at
least one blank, GWBASIC would interpret the
statement:
DEFSEG=150
to assign the value 150 to the variable
DEFSEG
10 DEF SEG = &HBBOO 'Set segment to
15 'Screen buffer
20 DEF SEG 'Restore segment to
25 'GWBASIC's DS
Note that in statement 10 the screen buffer is
at absolute address B8000 hex, as the last
hexadecimal digit is dropped on the DEF SEG
statement.