102 lines
4.0 KiB
Markdown
102 lines
4.0 KiB
Markdown
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---
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title: Alignment
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TARGET DECK: Obsidian::STEM
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FILE TAGS: c17 x86-64
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tags:
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- c17
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---
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## Overview
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For a large class of modern ISAs, storage for basic C datatypes respect **self-alignmnet**. This means `char`s can start on any byte address, `short`s on any even address, 4-byte `int`s and `float`s must start on an address divisible by 4, and `double`s must start on an address divisible by 8. Likewise pointers are also self-aligned.
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%%ANKI
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Basic
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What does self-alignment refer to?
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Back: The placement of C datatypes on an address divisible by the size of the datatype.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516929-->
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END%%
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%%ANKI
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Basic
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What addresses can a `char` be stored at?
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Back: Any address.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516932-->
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END%%
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%%ANKI
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Basic
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What addresses can a `int` be stored at?
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Back: Any address divisible by 4.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516934-->
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END%%
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%%ANKI
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Basic
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What addresses can a `short` be stored at?
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Back: Any address divisible by 2.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516936-->
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END%%
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%%ANKI
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Basic
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What addresses can a `double` be stored at?
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Back: Any address divisible by 8.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516938-->
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END%%
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%%ANKI
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Basic
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What addresses can a pointer be stored at?
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Back: Any address divisible by 8.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516941-->
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END%%
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%%ANKI
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Basic
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How does self-alignment make access faster?
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Back: It enables single instruction fetches and puts.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516943-->
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END%%
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%%ANKI
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Cloze
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With respect to memory alignment, {slop} is {waste space for padding datatypes to their alignment}.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516944-->
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END%%
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%%ANKI
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Basic
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What value is slop initialized to?
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Back: Undefined.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516946-->
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END%%
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%%ANKI
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Basic
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*Why* isn't equality for `struct`s well-defined?
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Back: The value of slop is undefined.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516947-->
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END%%
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%%ANKI
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Basic
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*Why* isn't inequality for `struct`s well-defined?
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Back: The value of slop is undefined.
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Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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<!--ID: 1730831516948-->
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END%%
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## Bibliography
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* Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
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