Editing Booting (section)

== Notes ==
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{{reflist|group="nb"|refs=
<ref group="nb" name="NB_Magic_AA55">The signature at offset <code>+1FEh</code> in boot sectors is <code>55h AAh</code>, that is <code>55h</code> at offset <code>+1FEh</code> and <code>AAh</code> at offset <code>+1FFh</code>. Since [[little-endian]] representation must be assumed in the context of [[IBM PC]] compatible machines, this can be written as 16-bit word <code>AA55h</code> in programs for [[x86]] processors (note the swapped order), whereas it would have to be written as <code>55AAh</code> in programs for other CPU architectures using a [[big-endian]] representation. Since this has been mixed up numerous times in books and even in original Microsoft reference documents,<!-- one example of many: http://technet.microsoft.com/en-us/library/cc977219.aspx as of 2012-01-14 --> this article uses the offset-based byte-wise on-disk representation to avoid any possible misinterpretation.</ref>
<ref group="nb" name="NB_PC_DOS_Contiguous_files">The [[PC&nbsp;DOS&nbsp;5.0]] manual incorrectly states that the system files no longer need to be contiguous. However, for the boot process to work the system files still need to occupy the first two directory entries and the first three sectors of [[IBMBIO.COM]] still need to be stored contiguously. [[SYS (DOS command)|SYS]] continues to take care of these requirements.<!-- See Chappell "DOS Internals" for further details --></ref>
<ref group="nb" name="NB_DR-DOS_File-size">There is one exception to the rule that [[DR-DOS]] [[Volume boot record|VBR]]s will load the whole [[IBMBIO.COM]] file into memory: If the IBMBIO.COM file is larger than some 29&nbsp;KB, trying to load the whole file into memory would result in the boot loader to [[#Paul_1997_ODA3|overwrite]] the [[call stack|stack]] and [[Relocation (computing)|relocated]] [[Disk Parameter Table]] (DPT/FDPB).{{citeref|Paul|1997|A}} Therefore, a [[DR-DOS&nbsp;7.07]] VBR would only load the first 29&nbsp;KB of the file into memory, relying on another loader embedded into the first part of IBMBIO.COM to check for this condition and load the remainder of the file into memory by itself if necessary. This does not cause compatibility problems, as IBMBIO.COM's size never exceeded this limit in previous versions without this loader.{{citeref|Paul|1997|A}} Combined with a dual entry structure this also allows the system to be loaded by a [[PC&nbsp;DOS]] VBR, which would load only the first three sectors of the file into memory.</ref>
<ref group="nb" name="NB_DR-DOS_707">As an example, while the extended functionality of DR-DOS [[master boot record|MBR]]s and [[volume boot record|boot sector]]s compared to their [[MS-DOS]]/[[PC&nbsp;DOS]] counterparts could still be achieved utilizing conventional [[code optimization]] techniques in [[assembly language]] up to [[DR-DOS 7.05|7.05]]<!-- yes, 7.05, not 7.06, as 7.06 was designed to be compatible with Microsoft's MS-DOS 7.10 boot sector -->, for the addition of [[logical block addressing|LBA]], [[FAT32]] and [[NEWLDR|LOADER]] support the [[DR-DOS 7.07|7.07]] sectors had to resort to [[self-modifying code]], [[opcode]]-level programming in [[machine language]], controlled utilization of (documented) [[side effect (computer science)|side effect]]s, multi-level data/code [[instruction overlapping|overlapping]] and algorithmic [[fold (function)|fold]]ing techniques to squeeze everything into a single physical sector, as it was a requirement for [[backward compatibility|backward]]- and cross-compatibility with other operating systems in [[multi-boot]] and [[chain load]] scenarios.</ref>
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