/* * Memory layout: * * 0x00000 -> 0x07BFF    our stack        (to  31k) * 0x07A00 -> 0x07BFF    typical MBR loc        (at  30k5) * 0x07C00 -> 0x07DFF    our code        (at  31k) * 0x07E00 -> ...        /boot inode block    (at  31k5) * 0x07E00 -> ...        (indirect block if nec) * 0x40000 -> ...    /boot            (at 256k) * * The BIOS loads the MBR at physical address 0x07C00.  It then relocates * itself to (typically) 0x07A00. * * The MBR then loads us at physical address 0x07C00. * * We use a long jmp to normalise our address to seg:offset 07C0:0000. * (In real mode on x86, segment registers contain a base address in * paragraphs (16 bytes).  0000:00010 is the same as 0001:0000.) * * We set the stack to start at 0000:7BFC (grows down on i386) * * We then read the inode for /boot into memory just above us at * 07E0:0000, and run through the direct block table (and the first * indirect block table, if necessary). * * We load /boot at seg:offset 4000:0000. * * Previous versions limited the size of /boot to 64k (loaded in a single * segment).  This version does not have this limitation. */

/* * The data passed by installboot is: * * inodeblk    uint32    the filesystem block that holds /boot‘s inode * inodedbl    uint32    the memory offset to the beginning of the *            direct block list (di_db[]).  (This is the *            offset within the block + $INODEOFF, which is *            where we load the block to.) * fs_bsize_p    uint16    the filesystem block size _in paragraphs_ *            (i.e. fs_bsize / 16) * fs_bsize_s    uint16    the number of disk sectors in a filesystem *            block (i.e. fs_bsize / d_secsize). Directly written *            into the LBA command block, at lba_count. *            XXX LIMITED TO 127 BY PHOENIX EDD SPEC. * fsbtodb    uint8    shift count to convert filesystem blocks to *            disk blocks (sectors).  Note that this is NOT *            log2 fs_bsize, since fragmentation allows *            the trailing part of a file to use part of a *            filesystem block.  In other words, filesystem *            block numbers can point into the middle of *            filesystem blocks. * p_offset    uint32    the starting disk block (sector) of the *            filesystem * nblocks    uint16    the number of filesystem blocks to read. *            While this can be calculated as *            howmany(di_size, fs_bsize) it takes us too *            many code bytes to do it. * * All of these are patched directly into the code where they are used * (once only, each), to save space. * * One more symbol is exported, in anticipation of a "-c" flag in * installboot to force CHS reads: * * force_chs    uint8    set to the value 1 to force biosboot to use CHS *            reads (this will of course cause the boot sequence *            to fail if /boot is above 8 GB). */