✎ soc:2010:cooldavid:notes:expandheap

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==== Expand gPXE heap ====

The gPXE's code can be executed on both real-mode and
protected-mode because of the GDT's base address was filled with
the starting point of the code. Which makes all the function,
global variable's pointer valid between the transition.
Also, the limit of GDT entry was set to 4G, so there is no need
to concern if we are going to across the segment limit.

Since the heap is not used by real-mode code, and is used at
late stages of gPXE. I think it might be possible to put the
heap out side of the .bss section, and put it at the very end of
the image. After we put the heap at the very end of the image,
we can make sure that no data might be needed for real-mode
codes would put beyond 1MB limit.

Currently I left 128KB for stack to grow, and put the heap right
behind it. The C code which runs in protected-mode would work
fine, since it's already using 32 bit addressing, and the
segment limit is way larger than image size.

For the real-mode code, as long as the heap is not used. All the
data needed should be within 1MB limit.

[[http://git.etherboot.org/?p=people/cooldavid/gpxe.git;a=commitdiff;h=09be316b2b1538860037233b41a43dfb3bb1f3d8|Commit-diff]]

=== Original heap location ===
[[Original heap location|{{http://bbs.cooldavid.org/pic/origheap.png}}]]

=== Modified heap location ===
[[Modified heap location|{{http://bbs.cooldavid.org/pic/hugeheap.png}}]]

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