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[MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and p
From: |
Daniel Kiper |
Subject: |
[MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section |
Date: |
Thu, 9 Jun 2016 22:30:42 +0200 |
Signed-off-by: Daniel Kiper <address@hidden>
---
doc/multiboot.texi | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/doc/multiboot.texi b/doc/multiboot.texi
index c81b2ea..bf02a1b 100644
--- a/doc/multiboot.texi
+++ b/doc/multiboot.texi
@@ -1384,6 +1384,7 @@ document, but are included for prospective operating
system and boot
loader writers.
@menu
+* C structure alignment and padding consideration::
* Notes on PC::
* BIOS device mapping techniques::
* Example OS code::
@@ -1391,6 +1392,22 @@ loader writers.
@end menu
address@hidden C structure alignment and padding consideration
address@hidden C structure alignment and padding consideration
+
+Many C compilers try to optimize memory accesses aligning structure
+members properly. Usually they reach the goal by adding some padding.
+This is very useful thing in general. However, if you try to mix assembler
+with C or use C to implement structure low level access this behavior
+may lead, at least, to quite surprising results. Hence, compiler should
+be instructed to not optimize such accesses. Usually it is done by special
+attribute added to structure definition, e.g. GCC compatible sources use
address@hidden ((__packed__))} for this purpose. However, this is not
+required if it is known that its members are properly aligned and compiler
+does not do any optimization. Very good example of this is shown below in
address@hidden file.
+
+
@node Notes on PC
@section Notes on PC
--
1.7.10.4
- [MULTIBOOT2 DOC PATCH 02/10] multiboot2: Clarify meaning of information request header tag, (continued)
- [MULTIBOOT2 DOC PATCH 02/10] multiboot2: Clarify meaning of information request header tag, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 03/10] multiboot2: Fix description of EFI boot services tag, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 05/10] multiboot2: Add description of EFI image handle tags, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 04/10] multiboot2: Add description of support for EFI boot services, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 06/10] multiboot2: Add description of support for relocatable images, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 09/10] multiboot2: Add me to authors, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section,
Daniel Kiper <=
- [MULTIBOOT2 DOC PATCH 07/10] multiboot2: Say that memory maps may not be available on EFI platforms, Daniel Kiper, 2016/06/09
- [MULTIBOOT2 DOC PATCH 10/10] multiboot2: Bump version to 2.0, Daniel Kiper, 2016/06/09