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Elaborate on the semantics of page_size. #296

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Merged
jfbastien merged 6 commits into from
Sep 17, 2015
Merged

Elaborate on the semantics of page_size. #296

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f428a6d
Elaborate on the semantics of `page_size`.
sunfishcode Aug 13, 2015
9f89d88
Clarify that page sizes are in bytes.
sunfishcode Aug 13, 2015
f866601
Add a note that page sizes may be virtual.
sunfishcode Aug 13, 2015
02a3fe1
Drop the minimum page size.
sunfishcode Aug 13, 2015
88d6193
Remove the sentance about page_size trapping.
sunfishcode Sep 17, 2015
618fc54
Simplify the description of page_size's type and result value.
sunfishcode Sep 17, 2015
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9 changes: 8 additions & 1 deletion AstSemantics.md
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Expand Up @@ -219,14 +219,21 @@ page size. To determine page size, a nullary `page_size` operation is provided.

* `resize_memory` : grow or shrink linear memory by a given delta which
must be a multiple of `page_size`
* `page_size` : nullary constant function returning page size
* `page_size` : nullary constant function returning page size in bytes

Also as stated [above](AstSemantics.md#linear-memory), linear memory is
contiguous, meaning there are no "holes" in the linear address space. After the
MVP, there are [future features](FutureFeatures.md#finer-grained-control-over-memory)
proposed to allow setting protection and creating mappings within the
contiguous linear memory.

The result type of `page_size` is `int32` for wasm32 and `int64` for wasm64.
The result value of `page_size` is an unsigned integer which is a power of 2.

(Note that the `page_size` value need not reflect the actual internal page size
of the implementation; it just needs to be a value suitable for use with
`resize_memory`)

## Local variables

Each function has a fixed, pre-declared number of local variables which occupy a single
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