New subject: pdp11 UNIX memory allocation.

7 Jan 2015

...
  From: Clem Cole &lt;clemc(a)ccc.com&gt; 
...
  Depends the processor. For the 11/45 class processors,
you had a 17th
 address bit, which was the I/D choice. For the 11/40 class you shared
 the instructions and data space. 
To be exact, the 23, 24, 34, 35/40 and 60 all had a single shared space.
(I have no idea why DEC didn't put it in the 60 - probably helped kill that

otherwise intersting machine, with its UCS, early...). The 44, 45/50/55, 70,
73, 83/84, and 93/94 had split.

...
  From: random832(a)fastmail.us 
...
  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.) 
Here's the code for a typical 'wrapper' (this is V6, not sure if V7
changed
the trap stuff):

  _lseek:
	jsr	r5,csv
	mov	4(r5),r0
	mov	6(r5),0f
	mov	8(r5),0f+2
	mov	10.(r5),0f+4
	sys	indir; 9f
	bec	1f
	jmp	cerror
  1:
	jmp	cret

  .data
  9:
	sys	lseek; 0:..; ..; ..

Note the switch to data space for storing the arguments (at the 0: label
hidden in the line of data), and the 'indirect' system call.

...
  From: Ronald Natalie &lt;ron(a)ronnatalie.com&gt;

...
  Some access at the kernel level can be done with MFPI
and MPFD
 instructions. 
Unless you hacked your hardware, in which case it was possible from user mode
too... :-)

I remember how freaked out we were when we tried to use MFPI to read
instruction space, and it didn't work, whereupon we consulted the 11/45
prints, only to discover that DEC had deliberately made it not work!

...
  From: Ronald Natalie &lt;ron(a)ronnatalie.com&gt;

...
  After the changes to the FS, you'd get missing
blocks and a few 0-0
 inodes (or ones where the links count was higher than the links). These
 while wasteful were not going to cause problems. 
It might be worth pointing out that due to the way pipes work, if a system
crashed with pipes open, even (especially!) with the disk perfectly sync'd,
you'll be left with 0-0 inodes. Although as you point out, those were merely
crud, not potential sourdes of file-rot.

	Noel

Re: [TUHS] pdp11 UNIX memory allocation.