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<sect><heading>PC memory utilization<label id="memoryuse"></heading>

<p><em>Contributed by &a.joerg;.<newline>
  16 Apr 1995.</em>

<em>A short description of how FreeBSD uses the memory on the i386 
platform</em>

The boot sector will be loaded at <tt>0:0x7c00</tt>, and relocates itself
immediately to <tt>0x7c0:0</tt>.  (This is nothing magic, just an adjustment
for the <tt>%cs</tt> selector, done by an <tt>ljmp</tt>.)

It then loads the first 15 sectors at <tt>0x10000</tt> (segment BOOTSEG in the
biosboot Makefile), and sets up the stack to work below <tt>0x1fff0</tt>.
After this, it jumps to the entry of boot2 within that code.  I.e., it
jumps over itself and the (dummy) partition table, and it is going to
adjust the %cs selector---we are still in 16-bit mode there.

boot2 asks for the boot file, and examines the <tt>a.out</tt> header.  It masks
the file entry point (usually <tt>0xf0100000</tt>) by <tt>0x00ffffff</tt>, and loads the
file there.  Hence the usual load point is 1 MB (<tt>0x00100000</tt>).  During
load, the boot code toggles back and forth between real and protected
mode, to use the BIOS in real mode.

The boot code itself uses segment selectors <tt>0x18</tt> and <tt>0x20</tt> for <tt>%cs</tt> and
<tt>%ds/%es</tt> in protected mode, and <tt>0x28</tt> to jump back into real mode.  The
kernel is finally started with <tt>%cs</tt> <tt>0x08</tt> and <tt>%ds/%es/%ss</tt> <tt>0x10</tt>, which
refer to dummy descriptors covering the entire address space.

The kernel will be started at its load point.  Since it has been linked
for another (high) address, it will have to execute PIC until the page
table and page directory stuff is setup properly, at which point
paging will be enabled and the kernel will finally run at the address
for which it was linked.


<em>Contributed by &a.davidg;.<newline>
  16 Apr 1995.</em>

The physical pages immediately following the kernel BSS contain
proc0's page directory, page tables, and upages. Some time later
when the VM system is initialized, the physical memory between
<tt>0x1000-0x9ffff</tt> and the physical memory after the kernel
(text+data+bss+proc0 stuff+other misc) is made available in the
form of general VM pages and added to the global free page list.


