Subject:
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Re: Question about mm.C in BrickOS
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Newsgroups:
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lugnet.robotics.rcx.legos
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Date:
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Wed, 13 Aug 2003 21:01:50 GMT
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Viewed:
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3237 times
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In lugnet.robotics.rcx.legos, Michael Martelli wrote:
> Hey
>
> My Senior Preoject where I update BrickOS is slowly taking shape (I am actually
> trying to do some now, but time is very limited). This project I am editing the
> Memory Managment Alg used by BrickOS. the first one I am going to add is Next
> Fit, it seems to be the easiest to add since it is the same as First Fit except
> instead of starting from MM_Start when looking for free block, I need to start
> at ptr++ (where ptr is the the last free block found).
>
> It has been a while since I looked at this code or done any programming in any
> langauge other that VB.NET (for work). My OS coding background is becoming very
> blurry and I dont understand the a whole block of code very well in mm.c. Of
> the block I understand some statements and conditions but not all, if possible
> could someone please hlep me and explain what the block is doing?
>
> Here is the malloc function in mm.c (the block is below after the equal signs):
<snip>
>
> ============
> 1 if(*ptr>=size) { // big enough?
> 2 *(ptr-1)=(size_t)ctid; // set owner
> 3
> 4 // split this block?
> 5 if((*ptr-size)>=MM_SPLIT_THRESH) {
> 6 next=ptr+size+1;
> 7 *(next++)=MM_FREE;
> 8 *(next)=*ptr-size-MM_HEADER_SIZE;
> 9 mm_try_join(next);
> 10
> 11 *ptr=size;
> 12 }
> ===========
I've never looked at brickOS code, but here is my take:
By the time you hit line 2, you've found a free block that is big enough. Line
2 sets the owner of the block. Line 5 decides if the block is oversized enough
to split.
Line 6 points to the unused part of the block. Line 7 marks the unused part of
the block as free (part of making the unused part of the block a new block).
Line 8 marks the size of the new block (the unused part of the original block
found at line 2.)
If someone else knows better, please correct me.
Kevin
>
> I know what the jist of the block does but I have a foggy idea of what lines 7
> and 8 do. Any help with those 2 lines and a summary of the block would be
> great. Thanks
They split the oversized block into two blocks, by marking the remainder of the
block as free, and writing in the size.
Kevin
>
> Mike
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Message is in Reply To:
 | | Question about mm.C in BrickOS
|
| Hey My Senior Preoject where I update BrickOS is slowly taking shape (I am actually trying to do some now, but time is very limited). This project I am editing the Memory Managment Alg used by BrickOS. the first one I am going to add is Next Fit, it (...) (21 years ago, 13-Aug-03, to lugnet.robotics.rcx.legos)
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