It comes from a time of machines executing instructions thousands of times slower than they do now. Literally – thousands. Memory access was about as fast as an instruction execution, so the amount of compute you can justify per unit of data was hundreds of times less than it is now. They did however have virtual memory systems with on demand page fetching.
Also, that machine was being time shared with a dozen or more users.
Launching emacs or TeX on this machine might take tens of seconds without access to unexec(), but only 3 seconds for the freeze dried version.
unexec() was easier at the time. There were no shared libraries, no address space layout randomization. One memory region grew up from the bottom, one down from the top. There was no mmap() jamming mysterious stuff in the middle. Just copy the bottom, copy the top, do magic to adjust the stack for your unexec() call, and write the thing out as an executable.
(Yeah, I excised unexec() from BibTeX back in the ‘80s to port it to a 68k Mac for a coworker, then later implemented unexec() for a Motorola 88k based multilevel secure SysV system in the early ‘90s because launching emacs was driving me insane. I prefer our shiny new future of stupidly fast computers.)
Because linkers are too easy?
It really looks like some overly clever college student's weird trick that somehow managed to survive for decades in an established product.
It comes from a time of machines executing instructions thousands of times slower than they do now. Literally – thousands. Memory access was about as fast as an instruction execution, so the amount of compute you can justify per unit of data was hundreds of times less than it is now. They did however have virtual memory systems with on demand page fetching.
Also, that machine was being time shared with a dozen or more users.
Launching emacs or TeX on this machine might take tens of seconds without access to unexec(), but only 3 seconds for the freeze dried version.
unexec() was easier at the time. There were no shared libraries, no address space layout randomization. One memory region grew up from the bottom, one down from the top. There was no mmap() jamming mysterious stuff in the middle. Just copy the bottom, copy the top, do magic to adjust the stack for your unexec() call, and write the thing out as an executable.
(Yeah, I excised unexec() from BibTeX back in the ‘80s to port it to a 68k Mac for a coworker, then later implemented unexec() for a Motorola 88k based multilevel secure SysV system in the early ‘90s because launching emacs was driving me insane. I prefer our shiny new future of stupidly fast computers.)