[TUHS] Emacs and undump

Johnny Billquist bqt at update.uu.se
Mon Feb 27 20:24:47 AEST 2017

On 2017-02-27 08:26, Lars Brinkhoff <lars at nocrew.org> wrote:
> Tim Bradshaw wrote:
>>> David wrote:
>>> I remember that GNU Emacs launched the first time and then dumped
>>> itself out as a core file. Each subsequent launch would then ‘undump’
>>> itself back into memory. All this because launching emacs the first
>>> time required compiling all that lisp code.
>> It still works like that.  Indeed that's the conventional way that
>> Lisp systems tend to work for delivering applications
> Emacs came from ITS, and many Lisps derive from Maclisp which also came
> from ITS.  In ITS, it was common for applications to be dumped into a
> loadable core image, even if they were written in assembly language.

Not only i ITS. This is how things work in OS/8, for example. I believe 
it is also how things work in TOPS-10 and quite possible also in TOPS-20.
Not sure about RT-11, but I wouldn't be surprised if that's the way 
there too.

Essentially, the linker leaves the image in memory. It does not write it 
to a file. And then, the command decode have a command for dumping your 
memory to disk, as a runable image. There is some information kept 
around that the linker sets up, which means you don't normally have to 
tell the command decoder which parts of memory to save, or what the 
start address is, and so on. But you can also give that information in 
your save command.

One of the nice things of this approach is that you can load an image 
into memory, and then use the debugger to look around in it, change it, 
or run it. And if the program exists, it is still in memory, including 
all data, which means you can check the state of everything at exit 
time. And of course, if you want to, you can load a program, patch 
around in it, in memory, and then run it. And, of course, you can load a 
program, run some part of it, and dump it to disk at that stage, so all 
initializations have been done.

Your memory is always around, and is not tied to a process that comes 
and goes.

Of course, the back side of that is that you can't really run several 
programs at once.

But it's not hard to see that RMS and GNU Emacs (coming from these 
systems) wanted the same thing again. It do have some points.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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