Rewrite the sections "Header graphics fields" and "Machine state".
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@ -486,86 +486,97 @@ start running the operating system.
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@node Header graphics fields
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@subsection The graphics fields of Multiboot header
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Also, information about a preferred default mode can be specified in the
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@samp{mode_type}, @samp{width}, @samp{height} and @samp{depth}
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fields. This is only a @emph{recommended} mode by the kernel. If the
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mode exists, the boot loader should set it if the user hasn't specified
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a mode. If not, it should fall back to a similar mode, if available.
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All of the graphics fields are enabled by flag bit 2. They specify the
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preferred graphics mode. Note that that is only a @emph{recommended}
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mode by the OS image. If the mode exists, the boot loader should set
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it, when the user doesn't specify a mode explicitly. Otherwise, the
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boot loader should fall back to a similar mode, if available.
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Valid numbers for @samp{mode_type} is 0 for linear graphics mode and 1
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for EGA-standard text mode. Everything else is reserved for future
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expansion. Please note that even if you set this field to indicate that
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you want a graphics mode, you might get a text mode.
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The meaning of each is as follows:
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@samp{width} and @samp{height} is specified in pixels, if graphics mode,
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or characters in EGA text mode. @samp{depth} is given in bits per pixel
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for graphics, or zero for EGA text modes.
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@table @code
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@item mode_type
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Contains @samp{0} for linear graphics mode or @samp{1} for
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EGA-standard text mode. Everything else is reserved for future
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expansion. Note that the boot loader may set a text mode, even if this
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field contains @samp{0}.
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Any, some or all of those three fields may be set to zero, indicating to
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the boot loader that no preference is given. This makes it possible for
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a kernel to just say it wants a 32-bit resolution, for example.
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@item width
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Contains the number of the columns. This is specified in pixels in a
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graphics mode, and in characters in a text mode. The value zero
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indicates that the OS image has no preference.
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@item height
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Contains the number of the lines. This is specified in pixels in a
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graphics mode, and in characters in a text mode. The value zero
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indicates that the OS image has no preference.
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@item depth
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Contains the number of bits per pixel in a graphics mode, and zero in
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a text mode. The value zero indicates that the OS image has no
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preference.
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@end table
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@node Machine state
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@section Machine state
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FIXME: Use a table like the chapter ``Header layout''.
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When the boot loader invokes the 32-bit operating system, the machine
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must have the following state:
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@itemize @bullet
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@item
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@code{CS} must be a 32-bit read/execute code segment with an offset of 0
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and a limit of 0xFFFFFFFF.
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@item
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@code{DS}, @code{ES}, @code{FS}, @code{GS}, and @code{SS} must be a
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32-bit read/write data segment with an offset of 0 and a limit of
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0xFFFFFFFF.
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@item
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The address 20 line must be usable for standard linear 32-bit addressing
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of memory (in standard @sc{pc} hardware, it is wired to 0 at bootup,
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forcing addresses in the 1-2 MB range to be mapped to the 0-1 MB range,
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3-4 is mapped to 2-3, etc.).
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@item
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Paging must be turned off.
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@item
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The processor interrupt flag must be turned off.
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@item
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@code{EAX} must contain the magic value 0x2BADB002; the presence of this
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@table @samp
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@item EAX
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Must contain the magic value @samp{0x2BADB002}; the presence of this
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value indicates to the operating system that it was loaded by a
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Multiboot-compliant boot loader (e.g. as opposed to another type of boot
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loader that the operating system can also be loaded from).
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Multiboot-compliant boot loader (e.g. as opposed to another type of
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boot loader that the operating system can also be loaded from).
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@item
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@code{EBX} must contain the 32-bit physical address of the Multiboot
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@item EBX
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Must contain the 32-bit physical address of the Multiboot
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information structure provided by the boot loader (@pxref{Boot
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information format}).
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@end itemize
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@item CS
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Must be a 32-bit read/execute code segment with an offset of @samp{0}
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and a limit of @samp{0xFFFFFFFF}. The exact value is undefined.
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@item DS
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@itemx ES
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@itemx FS
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@itemx GS
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@itemx SS
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Must be a 32-bit read/write data segment with an offset of @samp{0}
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and a limit of @samp{0xFFFFFFFF}. The exact values are all undefined.
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@item A20 gate
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Must be enabled.
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@item CR0
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Bit 31 (PG) must be cleared. Bit 0 (PE) must be set. Other bits are
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all undefined.
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@item EFLAGS
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Bit 17 (VM) must be cleared. Bit 9 (IF) must be cleared. Other bits
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are all undefined.
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@end table
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All other processor registers and flag bits are undefined. This
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includes, in particular:
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@itemize @bullet
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@item
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@code{ESP}: the 32-bit operating system must create its own stack as
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soon as it needs one.
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@table @samp
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@item ESP
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The OS image must create its own stack as soon as it needs one.
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@item
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@code{GDTR}: Even though the segment registers are set up as described
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above, the @code{GDTR} may be invalid, so the operating system must not
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load any segment registers (even just reloading the same values!) until
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it sets up its own @code{GDT}.
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@item GDTR
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Even though the segment registers are set up as described above, the
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@samp{GDTR} may be invalid, so the OS image must not load any segment
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registers (even just reloading the same values!) until it sets up its
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own @samp{GDT}.
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@item
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@code{IDTR}: The operating system must leave interrupts disabled until
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it sets up its own @code{IDT}.
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@end itemize
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@item IDTR
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The OS image must leave interrupts disabled until it sets up its own
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@code{IDT}.
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@end table
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However, other machine state should be left by the boot loader in
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@dfn{normal working order}, i.e. as initialized by the @sc{bios} (or
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@ -1154,7 +1165,7 @@ is a full Multiboot-compliant boot loader, supporting all required and
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optional features present in this specification. A public release has
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not been made, but the test release is available from:
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@url{ftp://alpha.gnu.org/gnu/hurd/src/}
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@url{ftp://alpha.gnu.org/gnu/grub}
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See the webpage @url{http://www.gnu.org/software/grub/grub.html}, for
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more information.
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