On 2017-12-16 16:21, Timothe Litt wrote:
On 16-Dec-17 07:35, Johnny Billquist wrote:
Top-posting to make it simple, while still keeping the original text.
While it is true that device access might be done using relative
addressing, that would in general be a bad software design. Things
like device CSRs should never be accessed as relative addresses, but
should be absolute. Any sane programmer should know that. And it is
not any extra effort in doing this on a PDP-11, so the programmer have
no real excuse for not doing it right. (But I know that people
sometimes still do things wrong anyway.)
While this case is moot, dumping peripheral ROMs has come up before, so
correct information may be useful to someone in the future.
Indeed.
I'm pretty sure that (among other things), I'm knowledgeable, sane and a
programmer. I was not writing about "the general case". This thread
was specifically about ROMs embedded in a peripheral.
Not trying to make any statements about you here.
This is a
different environment from the "general" case, and one where your
comments are off-base. This is the exception to *never*. It has
nothing to do with laziness or 'wrong'; I assumed neither. And PIC is
only one aspect of the problem as presented.
It is true that in a typical OS driver, a CSR base is specified by an
absolute (if possibly mapped) address, with code like
mov #173100, r0 ; or
mov csrbase(R5), r0 ; where r5 points to some data structure, and
csrbase contains an absolute I/O address
bit #100, 4(r0)
This ensures that the *code* can be moved; *the device is at some fixed
address* determined by SYSGEN, autoconfiguration, or the like. So the
references are register deferred and/or indexed relative to an
"absolute" address. The code should be PIC, as most (later) OSs want to
be able to dynamically load drivers. I'd agree that in this case, PIC
with an absolute device address is the preferred (and sometimes
required) implementation.
In the specific case of a device ROM, CSR access would indeed be
properly done with relative addressing. There isn't much (sane)
choice. Calling it 'bad software design' is inappropriate, and
overlooks the environmental constraints and functional requirements of a
device ROM.
Fair enough. If you actually have a device which includes both CSR and
code, that are moved together through I/O space, then yes you'd want to
use relative addressing for the CSR.
However, such a device would be wrong to start with. And I've never seen
anyone do that, but sure, it could be done.
But with the limited space in the I/O page, you would not want to have
the same code replicated all over the space.
So in the end no, I do not believe that it would ever be the right thing
to have relative addressing for CSRs. Your example is valid in that such
a design would make sense to have relative addressing, but such a device
is already a wrong thing to start with, and should not be.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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