AT&T 6300 Programmer's Manual page 636

Advanced Features
Observe the following rules when coding a
subroutine:
• The called routine must preserve segment
registers DS, ES, SS, and BP. If interrupts are
disabled in the routine, they must be enabled
before exiting. The stack must be cleaned up
on exit.
• The called program must know the number
and length of the arguments passed. The
following routine shows an easy way to
reference arguments:
push BP
mov BP,SP
add BP, (2*number of arguments)+4
Then:
argument 0 is at BP
argument 1 is at BP-2
argument n is at BP-2*n
(number of arguments=n+
1)
• Variables may be allocated either in the Code
Segment or on the stack. Be careful not to
modify the return segment and offset stored on
the stack.
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