[TUHS] pdp11 UNIX memory allocation.

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Clem Cole <clemc at ccc.com>

    > 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 at 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 <ron at ronnatalie.com>

    > 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 <ron at ronnatalie.com>

    > 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