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Apr 18, 2016
Merged

Add current_memory operator #648

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85196de
Add current_memory
rossberg Apr 8, 2016
93457e0
Adjust wording
rossberg Apr 8, 2016
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23 changes: 14 additions & 9 deletions AstSemantics.md
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Expand Up @@ -82,19 +82,20 @@ operators.

The main storage of a WebAssembly instance, called the *linear memory*, is a
contiguous, byte-addressable range of memory spanning from offset `0` and
extending for `memory_size` bytes which can be dynamically grown by
[`grow_memory`](AstSemantics.md#resizing). The linear memory can be considered
extending up to a varying *memory size*.
This size always is a multiple of the WebAssembly page size,
which is 64KiB on all engines (though large page support may be added in the
[future](FutureFeatures.md#large-page-support)).
The initial state of linear memory is specified by the
[module](Modules.md#linear-memory-section), and it can be dynamically grown by
the [`grow_memory`](AstSemantics.md#resizing) operator.

The linear memory can be considered
to be an untyped array of bytes, and it is unspecified how embedders map this
array into their process' own [virtual memory][]. The linear memory is
sandboxed; it does not alias the execution engine's internal data structures,
the execution stack, local variables, or other process memory.

The initial state of linear memory is specified by the
[module](Modules.md#linear-memory-section). The initial and maximum memory size
are required to be a multiple of the WebAssembly page size, which is 64KiB on
all engines (though large page support may be added in the
[future](FutureFeatures.md#large-page-support)).

[virtual memory]: https://en.wikipedia.org/wiki/Virtual_memory

In the MVP, linear memory is not shared between threads of execution. Separate
Expand Down Expand Up @@ -152,7 +153,7 @@ interpreted as an unsigned byte index.

Linear memory operators access the bytes starting at the effective address and
extend for the number of bytes implied by the storage size. If any of the
accessed bytes are beyond `memory_size`, the access is considered
accessed bytes are beyond the current memory size, the access is considered
*out-of-bounds*.

The use of infinite-precision in the effective address computation means that
Expand Down Expand Up @@ -215,6 +216,10 @@ reserve the space up front or if enabling the reserved memory fails.
When there is no maximum memory size declared, `grow_memory` is expected
to perform a system allocation which may fail.

The current size of the linear memory can be queried by the following operator:

* `current_memory` : return the current memory size in units of pages.

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)
Expand Down