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73 lines
3.0 KiB
Plaintext
73 lines
3.0 KiB
Plaintext
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~From: J Wunsch <j@uriah.heep.sax.de>
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~Message-Id: <199504160843.KAA16160@uriah.heep.sax.de>
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~Subject: Memory usage (Was Re: Memory init pattern)
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~To: freebsd-hackers@FreeBSD.org (FreeBSD hackers)
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~Date: Sun, 16 Apr 1995 10:43:29 +0200 (MET DST)
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[Audience extended to -hackers, since it's a general topic.]
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As Frank Durda IV wrote:
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>
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> By the way, I have seen no description of how FreeBSD uses PC memory, ie
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> what 0-640K gets used for, does the kernel load there or higher,
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> is the kernel relocated, etc. Is there a paper on this?
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Since i've just digged through the boot code, i can tell you what's
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going there. :) [Someone going to collect this sort of messages
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and making a kernel hackers manual?]
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The boot sector will be loaded at 0:0x7c00, and relocates itself
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immediately to 0x7c0:0. (This is nothing magic, just an adjustment
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for the %cs selector, done by an ljmp.)
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It then loads the first 15 sectors at 0x10000 (segment BOOTSEG in the
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biosboot Makefile), and sets up the stack to work below 0x1fff0.
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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's 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 a.out header. It masks
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the file entry point (usually 0xf0100000) by 0x00ffffff, and loads the
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file there. Hence the usual load point is 1 MB (0x00100000). 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 0x18 and 0x20 for %cs and
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%ds/%es in protected mode, and 0x28 to jump back into real mode. The
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kernel is finally started with %cs 0x08 and %ds/%es/%ss 0x10, which
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refer to dummy descriptors covering the whole address space.
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The kernel will be started at its load point. Since it's 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 finally runs at the address
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where it has been linked to.
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[... -- no longer valid]
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The later memory usage (once paging is enabled) could better be
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explained by the VM folks.
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--
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cheers, J"org
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joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/
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Never trust an operating system you don't have sources for. ;-)
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~Message-Id: <199504160955.CAA00143@corbin.Root.COM>
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~To: freebsd-hackers@FreeBSD.org (FreeBSD hackers)
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~Subject: Re: Memory usage (Was Re: Memory init pattern)
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~From: David Greenman <davidg@Root.COM>
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~Date: Sun, 16 Apr 1995 02:55:50 -0700
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...
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The physical pages immediately following the kernel BSS contain proc0's page
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directory, page tables, and upages. Some time later when the VM system is
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initialized, the physical memory between 0x1000-0x9ffff and the physical memory
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after the kernel (text+data+bss+proc0 stuff+other misc) is made available in
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the form of general VM pages and added to the global free page list.
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Does this answer the question?
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-DG
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