On 03/05/2012 06:30 AM, Mark Pizzolato - Info Comm wrote:
Comments in the pdp11_cpu code suggest that Bob added Idle support.
I'm not a pdp11 user so I don't have direct experience there, but I
suspect he probably didn't test with 2.11BSD. Please suggest what
might be appropriate for this operating system....
The 2.11BSD idle routine sets priority to zero, issues a wait
instruction, then restores the original priority. (**1)
So the appropriate thing would be for the CPU to just do nothing until
interrupted.
(and as I write that last sentence, I see your points more clearly than
ever. (Even though the "wait" is supposed to do "nothing" until
interrupted, the device interrupting it is not atomic from it's own
viewpoint. It has a lot of work to do to in preparation for generating
the interrupt! . . . so it is actually that the simulated device(s)
needs a way to idle efficiently until it has work to do) )
Once again, on a tangent . . .
I think that PDP-11 implementors might find 2.11BSD to be a useful tool,
if they were to start using it, as it exercises the PDP-11 in ways that
other systems either don't exercise, or it does so without source code,
the latter making it harder to ascertain whether a problem is in the
kernel or in the "hardware".
One example might be that networking runs in supervisor mode(**2), in
contrast some with other PDP-11 operating systems which use the mode to
support larger user programs.
That may involve much different usage patterns than, say RSX-11M when a
user mode program utilizes a supervisor mode library written and
compiled by the programmer at the same time as the main program
As another example, both the kernel and any user mode program may each
have up to 15 overlays. Programs too large for split I/D can be built
with overlays & there is now an optimizing overlay makefile generator,
"mkovmake", which helps to make overlays be nearly transparent to user
programs.(**3) Here again, if there was some obscure bug with
simulating a segment register, having the 2.11BSD source can make it
easier to distinguish what the PDP11 is actually doing with that
register from what the kernel code intends for it to do.
- Michael
Notes:
(**1) "Just for amusement" and for the benefit of models with a front
panel, the idle routine also rotates a bit pattern on the front panel
lights once every few times it is called. A comment in the code reads:
" /* If the system is very active the display will appear blurred. */
". SIMH coderd shouldn't care about that, which is why I moved it to
down to a note..
(**2) Moving the network from kernel space to supervisor freed up a lot
of room for making the system look more like 4.3BSD. So much stuff
ranging from kernel internals and aspects of the filesystem to the
system call API to the C library and user mode programs has been brought
over or ported from 4.3BSD, that, except for the limitations imposed by
the word size, it really does look like a 4.3BSD systrm. Programs
cross-port very easily between 4.3BSD and 2.9 BSD. The only real coding
issues occur when the coder assumes int to be 32 bits. The usual
problems of this type occur when man pages are ignored (for example, the
coder uses passes an int rather than an off_t as the second argument to
lseek() )
(**3) As an example of"near transparency":
2.11BSD comes with the source to the game "rogue". When I tried to
build it, I got a "text overflow" message from ld. After a very little
digging, I discovered "mkovmake". I ran it, following the mkovmake man
page, and was given a new Ovmakefile that built a binary with eight
overlays.
I started it up, and immediately ran into what I thought was a Kobold
(letter K) and found that it was now called a "Kestrel".
size rogue
text data bss dec hex
25856 18384 10556 54796 d60c total text: 88192
<<-------------
overlays: 8192,7936,7936,8192,7552,7488,6976,8064
Note that this is a pdp11 binary with an 88192 text size.
Just for the hell of it, here's the kernel, using a total 108288 of text:
size /unix
text data bss dec hex
55424 6340 41960 103724 1952c total text: 108288
overlays: 7744,7488,7872,7296,2240,7104,4992,5696,2432
That is without networking. The kernel separately loads /netnix to
supervisor space:
size /netnix
text data bss dec hex
60864 2362 38448 101674 18d2a
_______________________________________________
Simh mailing list
[email protected]
http://mailman.trailing-edge.com/mailman/listinfo/simh
