Textual clarifications (and fix an off-by-one error) based on feedback
mostly from Jeremy Fitzhardinge.
Signed-off-by: H. Peter Anvin <[email protected]>
---
Documentation/i386/boot.txt | 32 ++++++++++++++++++++++----------
1 files changed, 22 insertions(+), 10 deletions(-)
diff --git a/Documentation/i386/boot.txt b/Documentation/i386/boot.txt
index 66fa67f..35985b3 100644
--- a/Documentation/i386/boot.txt
+++ b/Documentation/i386/boot.txt
@@ -2,7 +2,7 @@
----------------------------
H. Peter Anvin <[email protected]>
- Last update 2007-05-16
+ Last update 2007-05-23
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
@@ -202,6 +202,8 @@ All general purpose boot loaders should write the fields marked
nonstandard address should fill in the fields marked (reloc); other
boot loaders can ignore those fields.
+The byte order of all fields is littleendian (this is x86, after all.)
+
Field name: setup_secs
Type: read
Offset/size: 0x1f1/1
@@ -280,14 +282,16 @@ Type: read
Offset/size: 0x206/2
Protocol: 2.00+
- Contains the boot protocol version, e.g. 0x0204 for version 2.04.
+ Contains the boot protocol version, in (major << 8)+minor format,
+ e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
+ 10.17.
Field name: readmode_swtch
Type: modify (optional)
Offset/size: 0x208/4
Protocol: 2.00+
- Boot loader hook (see separate chapter.)
+ Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
Field name: start_sys
Type: read
@@ -304,10 +308,17 @@ Protocol: 2.00+
If set to a nonzero value, contains a pointer to a NUL-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.
+ 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 15 or higher, as:
+
+ 0x1c00 < 15*0x200 (= 0x1e00) but
+ 0x1c00 >= 14*0x200 (= 0x1c00)
+
+ 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
Field name: type_of_loader
Type: write (obligatory)
@@ -377,7 +388,7 @@ Protocol: 2.00+
This field can be modified for two purposes:
- 1. as a boot loader hook (see separate chapter.)
+ 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
2. if a bootloader which does not install a hook loads a
relocatable kernel at a nonstandard address it will have to modify
@@ -715,7 +726,7 @@ switched off, especially if the loaded kernel has the floppy driver as
a demand-loaded module!
-**** ADVANCED BOOT TIME HOOKS
+**** ADVANCED BOOT LOADER HOOKS
If the boot loader runs in a particularly hostile environment (such as
LOADLIN, which runs under DOS) it may be impossible to follow the
@@ -740,4 +751,5 @@ IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
set them up to BOOT_DS (0x18) yourself.
After completing your hook, you should jump to the address
- that was in this field before your boot loader overwrote it.
+ that was in this field before your boot loader overwrote it
+ (relocated, if appropriate.)
--
1.5.2
-
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