[TUHS] pdp11 UNIX memory allocation

[Johnny Billquist]

On 2015-01-06 22:59, random832 at fastmail.us wrote:
> On Tue, Jan 6, 2015, at 15:20, Johnny Billquist wrote:
>> >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.)

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 at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol