Messages in this thread | | | Date | Thu, 17 May 2007 16:21:47 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] Further update of the i386 boot documentation |
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Jeremy Fitzhardinge wrote: > 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).
Next time I make a revision I'll put that in; since we're talking about x86 here it's, as you say, obvious that it's littleendian. As such, I think it can wait.
>> + >> +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?
Given that we have already established littleendian byte order, it's the same thing.
>> + >> +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?
Valid objection. It probably should be considered as (major, minor) bytes, but we haven't had any releases where it hasn't also been valid BCD. I would prefer separate bytes myself, so 2.10 = 0x20a instead of 2.10 = 0x210.
>> +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?
Fair enough...
>> +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"?
Uh? That seems more than a little silly. Yes, I guess formally speaking we're talking about "NUL-terminated", but the term "null-terminated" has over 800,000 hits on Google -- 10 times as many as "NUL-terminated" -- and is hardly an ambiguous term ("NUL-terminated" is ugly, and "zero-terminated" is ambiguous.)
>> + 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.
Probably makes sense.
-hpa
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