Re: modifying constants in Fortran and elsewhere

> Our esteemed moderator wrote:


(snip)


> I was there, I actually used this stuff. Re abuse of the upper byte, as the size of
> OS/360 exploded way past what they expected, programmers were under pressure to make
> every bit and byte count, hence overloading the high byte. -John]


Funny how history repeats itself. (As the saying goes.)


In this case, it was only the one bit, and with 31 bit addressing, it could
still be used. Though the compile doesn't actually check for the end of
the argument list.


Much of OS/360, at all levels, uses the high byte of addresses.
Most important, in the DCB used for much I/O. With the transition to 31 bit
and 64 bit addressing, much of the old control blocks are still there.
The DCB is in user address space, and so not easily replaced.


Much of that is still true in OS/390 and z/OS, the 31 bit and 64 bit OS.


And then years later, Apple creates the Macintosh, and (original) MacOS.
And with 24 bit addressing on the 68000, uses the high byte of addresses.


Then with the 68020, there were programs known to be 32 bit clean,
and those that were not.


I suspect that all the hard learned lessons from the mainframe days were
relearned in the microcomputer days.

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