Subject:
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Re: Modifying the BrickOS source code
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Newsgroups:
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lugnet.robotics.rcx.legos
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Date:
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Tue, 15 Jul 2003 11:35:10 GMT
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Viewed:
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2669 times
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Mike,
IIRC, mm_start is the beginning of the heap. When the system or a user
program requests memory (new in C++ or malloc/calloc in C) the memory is
taken from this block (from mm_start to the end of memory)
What might be confusing is how the memory manager detects the end of
memory. If your memory pointer is currently > mm_start, then you
increment the pointer and it is now < mm_start then you have reached the
end of memory (and wrapped to the beginning)
However, it is worth noting, mm_start is the end of the Kernel code. So
mm_start is also used to determine if a piece of code is part of the
kernel or a user program. User programs are located above mm_start
since the kernel will allocate memory for them when they are downloaded
(dll)
Summary:
1) Kernel code is < mm_start
2) Kernel malloc and calloc > mm_start
3) User code is > mm_start (memory allocated by kernel using malloc or
calloc)
4) User malloc and calloc > mm_start
5) Memory Manager detects end of memory when ptr wraps from > mm_start
to < mm_start
Hope that helps.
// Joe
Michael Martelli wrote:
> Thanks to all with the help, I finally got my new News setup info (my original
> login wasnt working so I had to re-sign up but it took about 2 weeks to hear
> from lugnet, so Charlie posted some stuff for me).
>
> I am looking at mm.c for my project and I see the variable mm_start and see that
> it is defined in the mm.h file but I am not 100% sure of what it defines.
>
> Is it:
> a) the end location of the memory -- RAM (i.e. Limit + Base registers in an OS
> like Windows or Umix)
> b) or is it the where the user portion of RAM starts? (like the Base register in
> a normal OS)
>
> I am not sure and everytime I look more I think it is the other one, and not the
> one I currently think it is
>
> Thanks a lot and hope I do not annoy you by asking these question that may seem
> easy but I cannot seem to understand this variable, even after looking over the
> BrickOS site.
>
> Mike
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Message has 1 Reply:
 | | Re: Modifying the BrickOS source code
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| (...) Thanks for the description Joe, but in the above paragraph you seem to contradict yourself, you say that mm_start is the end of kernel mode, but alos that user programs are located above mm_start I do not understand that part, did you mean (...) (21 years ago, 15-Jul-03, to lugnet.robotics.rcx.legos)
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Message is in Reply To:
| | Re: Modifying the BrickOS source code
|
| Thanks to all with the help, I finally got my new News setup info (my original login wasnt working so I had to re-sign up but it took about 2 weeks to hear from lugnet, so Charlie posted some stuff for me). I am looking at mm.c for my project and I (...) (21 years ago, 15-Jul-03, to lugnet.robotics.rcx.legos)
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