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SubjectRe: [PATCH] Further update of the i386 boot documentation
H. Peter Anvin wrote:
> +Field name: boot_flag
> +Type: read
> +Offset/size: 0x1fe/2
> +Protocol: ALL
> +
> + Contains 0xAA55. This is the closest thing old Linux kernels have
> + to a magic number.
>

Endianess? I guess a blanket statement saying that all constants are
stored little-endian enough (which is obvious, but its always good to be
explicit).

> +
> +Field name: jump
> +Type: read
> +Offset/size: 0x200/2
> +Protocol: 2.00+
> +
> + Contains an x86 jump instruction, 0xEB followed by a signed offset
> + relative to byte 0x202. This can be used to determine the size of
> + the header.
> +
> +Field name: header
> +Type: read
> +Offset/size: 0x202/4
> +Protocol: 2.00+
> +
> + Contains the magic number "HdrS" (0x53726448).
>

This is a bit confusing from an endian perspective. Does the "HdrS"
notation mean that it is a byte array containing 'H', 'd', 'r', 'S', as
the string syntax suggests? Or is that the ascii interpretation of each
byte of a 4-byte value read as a little-endian encoding from that location?

> +
> +Field name: version
> +Type: read
> +Offset/size: 0x206/2
> +Protocol: 2.00+
> +
> + Contains the boot protocol version, e.g. 0x0204 for version 2.04.
>

So the version is in BCD?

> +
> +Field name: readmode_swtch
> +Type: modify (optional)
> +Offset/size: 0x208/4
> +Protocol: 2.00+
> +
> + Boot loader hook (see separate chapter.)
>

Chapter? Is there a more specific reference you could make?

> +
> +Field name: start_sys
> +Type: read
> +Offset/size: 0x20c/4
> +Protocol: 2.00+
> +
> + The load low segment (0x1000). Obsolete.
> +
> +Field name: kernel_version
> +Type: read
> +Offset/size: 0x20e/2
> +Protocol: 2.00+
> +
> + If set to a nonzero value, contains a pointer to a null-terminated
>

"nil-terminated"? "\0-terminated"?

> + human-readable kernel version number string, less 0x200. This can
> + be used to display the kernel version to the user. This value
> + should be less than (0x200*setup_sects). For example, if this value
> + is set to 0x1c00, the kernel version number string can be found at
> + offset 0x1e00 in the kernel file. This is a valid value if and only
> + if the "setup_sects" field contains the value 14 or higher.
>
How about something like:

This example is only valid if "setup_sects" is greater than

((0x1e00 - 0x200) / 0x200) = 14.

Just to make the calculation explicit.

J
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