50 lines
2.3 KiB
Text
50 lines
2.3 KiB
Text
<!-- $Id: memoryuse.sgml,v 1.9 1997-02-22 12:58:51 peter Exp $ -->
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<!-- The FreeBSD Documentation Project -->
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<sect><heading>PC memory utilization<label id="memoryuse"></heading>
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<p><em>Contributed by &a.joerg;.<newline>
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16 Apr 1995.</em>
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<em>A short description of how FreeBSD uses the memory on the i386
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platform</em>
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The boot sector will be loaded at <tt>0:0x7c00</tt>, and relocates itself
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immediately to <tt>0x7c0:0</tt>. (This is nothing magic, just an adjustment
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for the <tt>%cs</tt> selector, done by an <tt>ljmp</tt>.)
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It then loads the first 15 sectors at <tt>0x10000</tt> (segment BOOTSEG in the
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biosboot Makefile), and sets up the stack to work below <tt>0x1fff0</tt>.
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After this, it jumps to the entry of boot2 within that code. I.e., it
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jumps over itself and the (dummy) partition table, and it is going to
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adjust the %cs selector---we are still in 16-bit mode there.
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boot2 asks for the boot file, and examines the <tt>a.out</tt> header. It masks
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the file entry point (usually <tt>0xf0100000</tt>) by <tt>0x00ffffff</tt>, and loads the
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file there. Hence the usual load point is 1 MB (<tt>0x00100000</tt>). During
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load, the boot code toggles back and forth between real and protected
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mode, to use the BIOS in real mode.
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The boot code itself uses segment selectors <tt>0x18</tt> and <tt>0x20</tt> for <tt>%cs</tt> and
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<tt>%ds/%es</tt> in protected mode, and <tt>0x28</tt> to jump back into real mode. The
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kernel is finally started with <tt>%cs</tt> <tt>0x08</tt> and <tt>%ds/%es/%ss</tt> <tt>0x10</tt>, which
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refer to dummy descriptors covering the entire address space.
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The kernel will be started at its load point. Since it has been linked
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for another (high) address, it will have to execute PIC until the page
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table and page directory stuff is setup properly, at which point
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paging will be enabled and the kernel will finally run at the address
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for which it was linked.
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<em>Contributed by &a.davidg;.<newline>
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16 Apr 1995.</em>
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The physical pages immediately following the kernel BSS contain
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proc0's page directory, page tables, and upages. Some time later
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when the VM system is initialized, the physical memory between
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<tt>0x1000-0x9ffff</tt> and the physical memory after the kernel
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(text+data+bss+proc0 stuff+other misc) is made available in the
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form of general VM pages and added to the global free page list.
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