----- Original Message -----
From: "Tony Thigpen" <[email protected]>
Sent: Saturday, March 11, 2017 11:52 PM
I am working on some REALLY old code. Some of the code has dates back in
1967! The oldest date found is 5/9/67.
This code is still running daily. That's as good as 50 years later. The
only reason we are touching the code is because we are migrating this
application from z/VSE to z/OS.
I don't know the original operating system, but I expect it was DOS/360.
But, it could have been an early OS/360.
Looks like DOS-style code.
Anyway, I am seeing some code that am interested to know about,
historically.
For instance, it appears that USING could not support multiple
registers. I see this code sequence in the 'start' macro:
USING *,3 ESTABLISH REG 3
USING *+4095,4 AND REG 4
USING *+2*4095,5 AND REG 5 AS BASE REGS
Also, it appears that R15 was not standardized to have the programs
entry address because they used:
&NAME BALR 3,0 INITALIZE REG 3
LA 3,0(3)
BCTR 3,0
BCTR 3,0
DOS machines did not usually have heaps of memory, so it is surprising
to see wasted bytes like the above.
LA 4,4095(3) AND REG 4
LA 5,4095(4) AND REG 5
(FYI: The above code snippets were contiguous.)
Also, when they needed to call a subroutine, it appears they did not yet
have =A(xxx) constants, because they had to manually adjust the address:
L 15,=V(UPSI) LOAD REG 15 WITH UPSI ROUTINE ADDRESS
AR 15,3 ADD RELOCATION FACTIR FROM REG 3
BAL 14,4(15) BRANCH TO UPSI RETURN IN REG 14
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