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2012ab71de
There is no fixed structure for a PXE NBP: the format is just an
opaque block of executable code that is loaded into memory verbatim
and executed by jumping to the first byte. It is consequently
impossible for external code to unambiguously identify a PXE NBP, or
to inspect any metadata about the NBP's functionality.
The first five bytes of an iPXE NBP are already fixed as being an ljmp
instruction that resets the code segment to 0x7c0 and continues
execution from the following byte. We can extend this to include a
minimal header as follows:
Offset Content
------ -------
0 ljmp instruction (0xea)
1-2 ljmp offset (and therefore length of header)
3-4 ljmp segment (0x07c0)
5+ Metadata fields
\_ 5 CPU architecture (0x32=i386, 0x64=x86_64)
\_ 6-7 Magic value (0x18ae)
This is backwards-compatible to existing binaries (which effectively
have zero bytes of metadata following the ljmp instruction), and
allows for future expansion by appending metadata fields (with the
ljmp offset used to determine the overall header length and therefore
the presence of further fields).
In this initial version of the header, define a magic value (used to
differentiate an iPXE NBP from other binaries that happen to start
with an ljmp instruction), and a single-byte value that encodes
whether this binary is built for 32-bit or 64-bit CPUs.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
iPXE network bootloader
iPXE is the leading open source network boot firmware. It provides a full PXE implementation enhanced with additional features such as:
-
boot from a web server via HTTP or HTTPS,
-
boot from an iSCSI, FCoE, or AoE SAN,
-
control the boot process with a script,
You can use iPXE to replace the existing PXE ROM on your network card, or you can chainload into iPXE to obtain the features of iPXE without the hassle of reflashing.
iPXE is free, open-source software licensed under the GNU GPL (with some portions under GPL-compatible licences).
You can download the rolling release binaries (built from the latest commit), or use the most recent stable release.
For full documentation, visit the iPXE website.