[TUHS] Fwd: Choice of Unix for 11/03 and 11/23+ Systems

[Noel Chiappa]

    > From: Paul Riley

    > Can you clarify something for me regarding memory? I understand the
    > bottom area of memory in a Unix system is for the Kernel and it's stuff,
    > and that the top 8kB is set aside for device I/O

Well, technically, the top 8KB of _address space_, not of memory - it's mostly
used for device registers, etc. Here:

  http://gunkies.org/wiki/Unix_V6_kernel_memory_layout

is a bit more detail on how the memory is laid out.

    > The LSI-11 board has 4kW of RAM on it, and I have already a 16KW
    > board. If I want to further expand the RAM, and say I buy another 16kW
    > board, that makes an arithmetic sum of 32kW for the boards, making 36kW
    > total. Can the 4kW of on-board RAM be disabled, and only the 32kW on
    > the boards be used?

Yeah, if you look at LSI-11 documentation, there are jumpers that allow
configuration of the on-board memory. Depending on the etch revision; for my F
revision, jumper W11 (at the top, towards the handle edge, in the middle of
that edge; just below the W1/W2 jumper pair) should be out to disable the
on-board memory.

Or you could configure the two 32KB boards to be at 020000 and 0120000; there
will be 72KB of memory total on the QBUS, but the LSI-11 CPU (no memory
management) will only be able to 'see' the bottom 56KB.


    > Is it ok for the installed RA mto overlap the 8kW at the high memory
    > area?

Yeah, what the CPU sees as the I/O page (at 0160000-0177776 in its address
space) is actually at 0760000-0777776 on the QBUS (on a Q18 QBUS); the CPU
automagically translates the 0160000-0177776 addresses up. On a PDP-11
with memory management, the MMU has to be set up to do that. E.g. in V6,
in:

  https://minnie.tuhs.org//cgi-bin/utree.pl?file=V6/usr/sys/conf/m40.s

there is the following code:

  / initialize io segment

  mov $IO,(r0)+
  mov $77406,(r1)+		/ rw 4k

to set the I/O page in kernel address space to point to the I/O page on the
bus.

	Noel