Contributed by Jörg Wunsch <joerg@FreeBSD.org>. 16 Apr 1995.
A short description of how FreeBSD uses memory on the i386 platform
The boot sector will be loaded at 0:0x7c00, and relocates itself immediately to 0x7c0:0. (This is nothing magic, just an adjustment for the %cs selector, done by an ljmp.)
It then loads the first 15 sectors at 0x10000 (segment BOOTSEG in the biosboot Makefile), and sets up the stack to work below 0x1fff0. 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 a.out header. It masks the file entry point (usually 0xf0100000) by 0x00ffffff, and loads the file there. Hence the usual load point is 1 MB (0x00100000). 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 0x18 and 0x20 for %cs and %ds/%es in protected mode, and 0x28 to jump back into real mode. The kernel is finally started with %cs 0x08 and %ds/%es/%ss 0x10, 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.
Contributed by David Greenman <dg@FreeBSD.org>. 16 Apr 1995.
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 0x1000-0x9ffff 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.