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From: philip@pescadero.Stanford.EDU (Philip Machanick)
Newsgroups: comp.sys.mac.misc
Subject: Re: Mac & Win: Questions about memory management
Keywords: Mac, Win, MS Windows, memory management
Message-ID: <1990Jul15.003910.15391@Neon.Stanford.EDU>
Date: 15 Jul 90 00:39:10 GMT
References: <3102@gmdzi.UUCP>
Sender: news@Neon.Stanford.EDU (USENET News System)
Reply-To: philip@pescadero.stanford.edu
Organization: Computer Science Department, Stanford University
Lines: 55

In article <3102@gmdzi.UUCP>, strobl@gmdzi.UUCP (Wolfgang Strobl) writes:
> [I am posting this on behalf of Ken, so please reply to him, not me!]
[request for Windows/Mac discussion]
> To get the ball rolling, I have some questions about Mac memory management
> at System 6 and 7.
>  
> In Windows, applications do not (should not) hold pointers to data in the
> global heap.  They instead hold handles which can be dereferenced to find
> the pointer.  This allows the OS to move blocks to produce a larger contig-
> uous space when it needs one.
>  
> If I understand the Mac model (sys 6 w/MultiFinder), the applications are
> given a workspace or contiguous memory and that workspace does not move at
> all during the life of the invocation.
>  
> Is my understanding of the Mac way correct?  What if an app needs more
> memory, can it request that its partition be increased in size?  Is there
> any way to unfragment memory?  Does system 7 change this?  Would a PMMU
> help?

I don't know about System 7 (but have a strong feeling there's no radical
change). Because of its origin as a single application at a time system,
the Mac allocates a fixed partition for each application. This partition
contains the application's heap (called a "zone"). It's possible to create
a additional zones, if you have complex memory mangement requirements.
In addition, there is a system heap, and free space not yet allocated.
It's possible for a program to ask for a chunk out of this free space
(e.g., as temporary space for a buffer).

The heap contains two kinds of data objects: pointer-referenced and
handle-referenced. Handles - as in Windows - allow defragmentation.
However, this is _within_ a specific heap. Global memory can be fragmented;
however this is only a problem if you frequently launch/quit applications.
The only solution if you run out of space to launch a new application (if
there's enough free space total, but not in biggest free block) is to kill
(and maybe relaunch) applications.

In practice, this works reasonably well for users; memory management
is a big headache for programmers, especially beginners who are used
to more forgiving systems.

Adding a PMMU does not radically change the picture, because all programs
are still running in the same (potentially fragmentable) address space.

>                                                             Kenneth S.
Blackney
>      Phone: (215) 895-1505                                    Drexel
University
>   Internet: Ken_Blackney@DUPR.OCS.Drexel.EDU                 Computing
Services
>     Bitnet: BLACKNEY@DUPR.BITNET                         Philadelphia,
PA 19104

Philip Machanick
philip@pescadero.stanford.edu