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This is a specialised pc kernel that acts as a bootstrap loader for
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386 and amd64 kernels, and replaces the old 9pxeload, 9load and
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9loadusb. It relies on the pc port (in /sys/src/9/pc) for most of its
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non-portable source files, notably the disk and ethernet drivers, so
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you'll want an up-to-date system (see replica/pull) if installing this
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by hand. Support for user mode (such as system calls) is almost
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entirely absent. There is some duplication of code with the pc
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kernel, but we have tried to keep it to a minimum.
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Caveats:
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9boot doesn't scan disks for partition tables since it doesn't contain
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the disk drivers, but `readparts=' in plan9.ini (see
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/sys/src/9/boot/parts.c) will make the loaded kernel read them early
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on. 9load does read partition tables.
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9loadusb seems to hang for two minutes when attempting to read via the
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bios on some bioses (e.g., vmware's), then reports a disk timeout on
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the INT 13 extended read operation.
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How these new bootstraps differ from the old 9load and 9pxeload.
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The old 9load was a single process derived from (but separately
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evolved from) an old PC kernel, so it needed modified device drivers
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(primarily for ethernet and disk controllers), which was an ongoing
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maintenance nuisance, and not all ethernet controllers had bootstrap
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drivers. The new bootstraps are just specialised Plan 9 kernels that
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are loaded into the first 640KB of RAM, so they use unmodified Plan 9
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drivers. There isn't enough room below 512K to include useful support
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for user mode and system calls, but the new bootstraps do implement
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kernel processes. The 512K limit is imposed by PXE or PBS loading of
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the new bootstraps into the first 512K or 640K.
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The old 9load was capable of loading from disks, floppies, USB disks
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or ethernet (via PXE). Again due to space limitations, we've had to
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focus the new bootstraps more sharply. 9boot loads via PXE and
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nothing else, which is our usual mode of operation. 9load loads from
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non-USB disks (using BIOS INT 13 calls) but not floppies, which ought
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to be obsolete by now. If you must boot from floppy and are doing so
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now and your BIOS won't boot from USB, do this to arrange to boot from
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USB disk instead:
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a: && cp /386/9loadusb /n/a:/9load
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9loadusb loads from USB disks only. BIOSes seem to be easily confused
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by intermixing direct I/O and BIOS calls, thus we keep 9load and
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9loadusb distinct.
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The new bootstraps can load amd64 as well as 386 kernels, and generate
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Gnu Multiboot tables for their benefit.
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The new 9load will look on all available disks for FAT file systems.
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If no bootfile is specified in a plan9.ini, it will examine each file
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system to see if it contains a single Plan 9 kernel (9pc* or 9k8*),
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and if so, will boot it.
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The new 9boot contains no disk drivers, so it can't read partition
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tables and populate #ec/sd??part for the kernel's benefit, so if you
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need to access a disk partition early in the kernel's execution (e.g.,
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you have an nvram partition), you'll want to add
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readparts=
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to the /cfg/pxe file for any such machines.
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The new bootstraps should run on more machines than the old ones. We
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discovered new ways to enable the A20 address line and try them all
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until success. This may fix various odd memory corruption problems
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seen in the past. We also discovered that BIOS calls may enable
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interrupts, so we disable them again immediately upon return. This
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may prevent mysterious resets seen with the old bootstraps.
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CD booting changes
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pbsraw can be up to 2k, so we can print and be nice, uses things
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written up by mk9660. In any case, it reads a contiguous file and is
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468 bytes long (we dropped the 9fat support here), so it could be used
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to load from any raw partition supporting lba access.
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There are corresponding changes to mk9660 to annotate the PBS. A new
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parameter, -x loader, names a loader that has to be in the root
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directory. In conjuction with -B, can be used to boot directly from
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the CD.
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9load was changed to deal with a 9fat image file in the root directory
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of a CD image. It has to be called bootdisk.img and can be of any
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size (as long as it is contiguous).
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Memory Map
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This is how the bootstraps use memory:
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31K ---------------------
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start of pxe decomp + compressed 9boot
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64K ---------------------
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start of pbs decomp + compressed 9boot
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decompresses to 9MB
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...
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512K ---------------------
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pxe loader from ROM
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640K ---------------------
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UMB
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1MB ---------------------
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kernel
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9MB ---------------------
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9boot after decomp.
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decompresses kernel.gz
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from 13MB to 1MB.
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13MB ---------------------
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(kernel.gz)
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15MB ---------------------
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no-man's land
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16MB ---------------------
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malloc arena for 9boot
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...
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chinese puzzle constraints:
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BIOS calls only work from bottom 1MB (640K, really).
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with paging on, we need to have created page tables for the memory we use.
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a20 has to be enabled before we try to use memory above 1MB.
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