Flashback a few decades to the days when this steel mill is getting an automation upgrade -- which means for the first time there will be real-time tracking of the manufacturing process, according to a pilot fish on the project.
"Terminals were added to the various control areas within the mill to track the metal as it moved from ingot to coil," says fish. "We also communicated with the corporate mainframe for key data, as well as certain process-control systems on the floor.
"We were running on MicroPDP-11 16-bit minicomputers where each process was limited to 64K, some of which was used for memory-mapped I/O or for sharing data between the individual programs in the system."
So even though each machine has a whopping 4MB of real memory, some programs must be written using overlays, with segments of memory getting swapped in and out of that 64K block.
Result: It can take between 30 and 60 minutes to compile and link a new version of a program when it's being developed. And of course it can take multiple 30-to-60-minute cycles to get the overlay structure right after a change to the code.
That doesn't sit well with the corporate manager responsible for the project, who's annoyed that the compile/link cycle takes so long.
"He was a mechanical engineer who may have had one computer course -- freshman Fortran, likely using punch cards," fish says. "He didn't believe that we could not just use the 4MB of memory in the system."
What's worse, he thinks he has evidence that he's right. The PDP-11 line is made by Digital Equipment Corp., which also offers pricier 32-bit computers under the name VAX (for "virtual address extension") -- and the VAXen can run programs that are hundreds of megabytes in size, which would cut out all that overlay overhead.
Fish soon gets very tired of hearing the manager repeat the VAX marketing slogan: "Anything you can do on a PDP-11, you can do on a VAX."
In the end, fish finally brings copies of the PDP-11 and VAX hardware manuals into the office for a long meeting with the manager.
"I spent the better part of the day showing him, from the documentation, how the architecture was different," says fish, "and that while a VAX would do anything a PDP-11 did -- you didn't even have to recompile executables -- a PDP-11 certainly could not do everything a VAX did. The key was in the virtual address extensions.
"He was also fond of saying 'Seems to me it's no big deal.' Sure -- all it takes is time or money. Your choice."
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