6 Jan
2015
6 Jan
'15
11:20 p.m.
On 2015-01-06 22:59, random832(a)fastmail.us wrote:
On Tue, Jan 6, 2015, at 15:20, Johnny Billquist
wrote:
Nope. A process cannot read or write to instruction space, nor can it
execute from data space.
It's inherent in the MMU. All references related to the PC will be done
from I-space, while everything else will be done through D-space.
So the MMU have two sets of page registers. One set maps I-space, and
another maps D-space. Of course, you can have them overlap, in which
case you get the traditional appearance of older models.
The versions of Unix I am aware of push arguments on the stack. But of
course, the kernel can remap memory, and so can of course read the
instruction space. But the user program itself would not be able to
write anything after the trap instruction.
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
>Later model PDP-11 processors had a hardware
feature called split I/D
>space. This meant that you could have one 64Kb virtual memory space for
>instructions, and one 64Kb virtual memory space for data.
Was it possible to
read/write to the instruction space, or execute the
data space? From what I've seen, the calling convention for PDP-11 Unix
system calls read their arguments from directly after the trap
instruction (which would mean that the C wrappers for the system calls
would have to write their arguments there, even if assembly programs
could have them hardcoded.)