🚀 go-pugleaf

Kazimir Kylheku (c2a192@ugrad.cs.ubc.ca) wrote:
: >
: >I can think of a way to do that without actual double indirection, but it
: >chews up an awful lot of address space and requires an MMU. Perhaps you
: >could suggest a better way? You are given a Motorola 68000 motherboard and
: >512K of RAM. Go.
:
: I wouldn't do that at all. If a program requires that the size of an allocated
: unit change, I would have it go through a function that's similar to the
: standard C realloc().
:

Yes, and the standard C realloc() may move the object, invalidating any
pointer that references it.

:
: There is another way; it's called a segmented-paged MMU like the 386.
: When you allocate an object, you can reference it through a segment:offset
: address. If you need to move the object, you just move it somewhere else
: in the virtual address space, and change the base address stored in the
: segment descriptor table.
:

This isn't really "another way", it's good old double indirection again,
although with hardware support:

	segment:offset --> linear virtual --> linear physical

:
: Mind you, the lame 386 implementation gives
: you a very narrow limit on how many segment descriptors you can have...
:

Right.  You can't really use separate segments for storing every single one
of your objects.  By the way, do you know of an architecture that has both
hardware segments and paging, and does not have the x86 limitations?
--
Jerry J. Shekhel
jerry@cybercom.net