24 Mar
2016
24 Mar
'16
11:17 a.m.
On 2016-03-24 03:00, "Ron Natalie"<ron(a)ronnatalie.com> wrote:
Ok. Color me stupid, but I don't get it. I totally do not understand how
this locks anything out.
It is the normal behavior of any instruction that interrupts are not
recognized until the next instruction fetch. This is how the microcode
works, and it is also pretty much the same in any processor today.
Except for instructions that take a long time, and which can be
interrupted in the middle, the context preserved, and the instruction
restarted and continued, instructions are normally atomic. You cannot
get interrupts in the middle of an instruction.
Second, I cannot understand how filling the memory with SPL instructions
(or any other instruction) can lock out the CPU. As noted, they are
individual instructions. You still get a fetch between each instruction,
at which point, interrupts will be recognized.
Now, if you instead talked about actually raising the CPU to SPL 7, then
I agree that no interrupts will happen. But that is because you
essentially disabled interrupts.
The front panel still works though. It is not handled like an interrupt,
but it is true that it do interact with the processor states, and
normally if you pull HALT, it will only halt when it's going to fetch
the next instruction. You can, of course, also set the front panel
switch for single microcode instruction, at which point the CPU will
halt at the next microcode instruction instead, and you can single step
the microcode as well.
The one CPU I know you can sunset is the KA10 (PDP-10). I'm sure there
are others, but I have never seen how this could be done on a PDP-11, so
I'm most curious about this, and if you can provide more details I would
be most interested. As I also happen to know where a PDP-11/70 is
standing, I intend to test this out next I get close to it.
As for the KA10 (I think it was the KA10, but it might have been the
PDP-6), the problem is related to the indirect addressing feature. Since
memory is 36 bits, but addresses only 18, you have plenty of bits to
play with. And one of them is the indirect bit. And if you refer to a
memory location that also have the indirect bit set, you get another
memory access to get the actual content. The fun thing happens if you
set the indirect bit, and give your own address. This is then an
infinite memory reference. And the KA10 can not be broken out of that
lookup. The only solution is to pull the power plug.
The CPU is essentially stuck in one state, just tightly reading memory,
and then repeating reading memory. Later PDP-10 models have an explicit
check in the microcode in this loop to be able to break out of this.
Sorry for the offtopic content. :-)
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
>Closest I've ever been murdered was when
I "accidentally" filled the local
>11/70 with an uninterruptible instruction sequence."
SPL instruction.
The PDP-11 was odd that while SPL was a "privileged"
instruction, rather that trapping if you did it in user mode, it just
"ignored" it.
Well, what it ignored was the actual change of the processor level. What
it still implemented was the side effect was that interrupts were locked out
until the next instruction fetch.
If you filled your instruction space up with SPLs you could lock up the
computer so that even the HALT key didn't work (you had to do a bus RESET).