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From: Rich Alderson <news@alderson.users.panix.com>
Newsgroups: comp.os.vms
Subject: Re: Never Got Used To Those New-Fangled VMS Filename Extensions ...
Date: 29 Jul 2024 15:40:05 -0400
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Lawrence D'Oliveiro <ldo@nz.invalid> writes:

> On 28 Jul 2024 15:14:17 -0400, Rich Alderson wrote:
> 
>> TOPS-20 (based on TENEX) used the .EXE extension on sharable
>> executables, although TENEX used .SAV on both sharable and nonsharable
>> executables.  Tops-10 used .SAV (or .LOW and .HGH for executables with a
>> sharable segment), until late in the game when TOPS-20 style sharable
>> executables came to Tops-10.

>> I believe that RT-11 got .EXE from the larger systems.

> I think the significance of .SAV was that it was essentially a straight 
> memory dump, so loading it into memory was very simple. Whereas .EXE 
> required a bit more setup.

Before the .SAV format for executables came along, the PDP-6 and PDP-10 used an
actual memory image format called .DMP, where the first word of the image file
was mapped to word 74 of the memory space into which it was loaded and each
succeeding word of the file was filled in.

..SAV files were compressed:  An I/O word consisting of a negative length and an
address (the desired memory address - 1), followed by <count> words.  Stretches
of zero valued words were thus skipped, making the files smaller than the .DMP
equivalent.

..EXE files have a directory page at the start which describes the memory layout
of the following file pages.  Pages of all zeroes are not included, but long
stretches of zeroes in pages with nonzero contents are retained.

-- 
Rich Alderson					  news@alderson.users.panix.com
      Audendum est, et veritas investiganda; quam etiamsi non assequamur,
	  omnino tamen proprius, quam nunc sumus, ad eam perveniemus.
									--Galen