AT&T 6300 Programmer's Manual page 229

Syntax
address
DEF SEG
Statement
Assigns the current segment of memory.
DEF SEG [= address]
isa numeric expression returning an unsigned integer in the
range 0 to 65535. The address specified identifies the
segment address used by BLOAD, BSAVE, PEEK, POKE,
DEF USR, and CALL.
If 'address'
IS
omitted, then the segment to be
used is set to GWBASIC's data segment (i.e.,
the beginning of your user workspace in
memory). This is the initial default value.
If 'address' is specified, then it will be based
upon
a
16 byte boundary. For the BSAVE,
PEEK, POKE, or CALL statement, the value
is shifted left
4
bits to form the Code Segment
address for the subsequent call instruction.
Note: GWBASIC does not check if the
resultant segment is valid.
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