andrew@orca.UUCP (Andrew Klossner) (12/28/84)
This has nothing to do with Unix, but traffic is light over Christmas ...
"IEFBR14, if written as the obvious, single-instruction program
" BR 14", does NOT work. Reason: It fails to set a proper
value for its return code, which means that it may fail
mysteriously. An entertaining rumor has it that this bug
actually existed in IEFBR14 once. But don't get too cocky--the
same bug also exists in the UNIX world, since many C programs
terminate by falling out the bottom of the main program and
most C startup routines simply take whatever garbage return
value this produces as the exit status of the program.]"
Under OS/360, the location for the exit status is register 15. The
calling convention for all procedures (including the main program) is
that register 15 be loaded with the program's start address. This
helps the procedure get started in finding itself, since a program can
be loaded anywhere in memory and there is no Unix-style relocation
going on. A program cannot use absolute addressing, and so all
addressing is done through an index register. (Some programs use the
supplied value in register 15; otherwise use the instruction "BALR
reg,0" -- Branch And Link Register (subroutine call), but destination 0
means don't branch -- to load the current PC into reg.)
The result was that IEFBR14 returned its start address as its status.
Since a program is never loaded to location 0, the status was never 0,
and so the program never succeeded.
-- Andrew Klossner (decvax!tektronix!orca!andrew) [UUCP]
(orca!andrew.tektronix@csnet-relay) [ARPA]