Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.forth > #25396

Relocatable Blocks

Show all headers | View raw

Most modern operating systems make used of one or more heaps to collect
transient data items.  This is done by means of handles, i.e. indirect
addresses, which point to relocatable blocks for access on the heap.  This
allows the size of a block to be changed, moving the block when necessary,
but the handle remains the same.

Relocatable blocks are very useful for structures whose size is not known
beforehand, particularly for stacks and queues and certain types of object
structures.  In Mas OS 9 only windows records are kept in non-relocatable
blocks; the other data structures are held in relocatable blocks.

For embedded systems, relocatable blocks can be accomplished by placing the
block in the Forth data space and moving it explicitly when the size is
changed but, again, it is necessary to use indirect addressing for
reference.

I suggest that the equivalent definitions for ALLOCATE, FREE and RESIZE be
defined in the standard.

NEW-HANDLE ( u -- hndl ior)
Attempt to allocate a relocatable block of size u address units on the heap.
The initial content of the allocated space is undefined.  Attempts to create
a free block of the requested size, including compacting the heap zone, if
necessary.
   If the allocation succeeds, ior is returned as zero.  If it has not been
possible to allocate a block of the required size, ior is the
implementation-defined error result code.

FREE-HANDLE ( hndl -- ior)
Dispose of a relocatable block, given its handle.  Returns a zero if the
operation was performed correctly, returns a non-zero if there has been an
error.

RESIZE-HANDLE ( hndl u -- ior)
Changes the logical size of the relocatable block, given a handle and
required size.  Returns zero ior if successful.  If it has not been possible
to allocate a block of the required size, ior is the implementation-defined
result code.

I have used relocatable blocks in my Forth dictionary, along the lines
outlined in Crafting a Compiler (Fischer, el al).  Particularly, allowing
the dictionary to be case sensitive or not, as required.

An addition, it allows words not use in a running system to be discarded.
For my system, there are a fair number of word that are not relevant to
application code - various Mac OS Traps, a number of subroutines and a large
number of OS data structures - which can be discarded in a precompiled
system.

There are compiler directives for this: GLOBAL causes definition headers to
be marked as visible in a running system up to the next LOCAL; LOCAL (not to
be confused with LOCALS| ) which sets a flag that marks subsequent
definition headers as used only during compilation up to the next GLOBAL;
INVISIBLE discards all LOCAL headers from the dictionary (but not their
code!).  For example, I have a dictionary of 2480 words but in a running
system this drops to 1120 for a fully pre-compiled ANS system (plus other
items), from a minimal kernel after applying INVISIBLE.

It is important to note that the purpose here is not to reduce memory usage
but instead to simplify the dictionary and avoiding cluttering in it up with
words not needed in a working system (dictionary entropy/pollution).

This means that LOCAL headers are not included on WORDS.  I also use a
definition that only gives that TASK-WORDS for a compiled application.
Also, there is switch that allows WORDS to work either in chronological or
alphabetical order.

Ian van Breda

Back to comp.lang.forth | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

Relocatable Blocks Ian van Breda <igvb@btopenworld.com> - 2013-08-25 20:21 +0100
  Re: Relocatable Blocks Paul Rubin <no.email@nospam.invalid> - 2013-08-25 14:04 -0700
    Re: Relocatable Blocks Ian van Breda <igvb@btopenworld.com> - 2013-08-29 19:55 +0100
  Re: Relocatable Blocks albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-08-26 11:34 +0000
    Re: Relocatable Blocks Hans Bezemer <the.beez.speaks@gmail.com> - 2013-08-26 14:58 +0200
      Re: Relocatable Blocks Hans Bezemer <the.beez.speaks@gmail.com> - 2013-08-26 15:30 +0200
      Re: Relocatable Blocks anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-08-26 14:33 +0000
        Re: Relocatable Blocks Ian van Breda <igvb@btopenworld.com> - 2013-08-29 20:22 +0100
  Re: Relocatable Blocks Mark Wills <markrobertwills@yahoo.co.uk> - 2013-08-26 07:08 -0700

csiph-web