56 lines
2.3 KiB
Plaintext
56 lines
2.3 KiB
Plaintext
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<!-- $Id:$ -->
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<!-- The FreeBSD Documentation Project -->
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<chapt><heading>PC memory utilization</heading>
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<p><em>Contributed by &a.joerg;.<newline>
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16 Apr 1995.</em>
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<bf>Question:</bf> <em>By the way, I have seen no description
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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?</em>
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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 will finally run at the address
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for which it was linked.
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The kernel still skips over the first 0x500 bytes of code, in the
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assumption this were valuable BIOS data space (back from old days
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where it has been loaded low).
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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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0x1000-0x9ffff 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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