AT&T 6300 Programmer's Manual page 633

Advanced Features
Invoking the CALL statement causes the
following to occur:
• For each variable specified in the statement,
the two-byte offset of the variable's location
within the GWBASIC segment is pushed onto
the stack.
• The GWBASIC return address code segment
(CS) and offset (IP) are pushed onto the Stack.
• Control is transferred to the machine
language routine using the segment address,
which is given in the last DEF SEG statement
and the offset given in numvar.
B-7