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Sep 29, 2015
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Tidy up references to memory type after #366 #373

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f3e144e
Tidy up references to memory type after #366
lukewagner Sep 25, 2015
271b78b
s/memory size/storage size/
lukewagner Sep 25, 2015
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28 changes: 9 additions & 19 deletions AstSemantics.md
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Expand Up @@ -53,15 +53,6 @@ Also note that there is no need for a `void` type; function signatures use
[sequences of types](Calls.md) to describe their return values, so a `void`
return type is represented as an empty sequence.

### Memory Types

*Memory types* are a superset of the local types, adding the following:

* `i8`: 8-bit integer
* `i16`: 16-bit integer

Global variables and linear memory accesses use memory types.

## Linear Memory

The main storage of a WebAssembly instance, called the *linear memory*, is a
Expand All @@ -85,8 +76,9 @@ will be possible to share linear memory between threads of execution when
### Linear Memory Accesses

Linear memory access is accomplished with explicit `load` and `store` operations.
Integer loads can specify a size which is smaller than the result type as well as a
signedness. Such integer loads perform an implicit sign- or zero-extension as specified below.
Integer loads can specify a *storage size* which is smaller than the result type as
well as a signedness which determines whether the bytes are sign- or zero-
extended into the result type.

* `i32.load8_s`: load 1 byte and sign-extend i8 to i32
* `i32.load8_u`: load 1 byte and zero-extend i8 to i32
Expand All @@ -104,8 +96,8 @@ signedness. Such integer loads perform an implicit sign- or zero-extension as sp
* `f64.load`: load 8 bytes as f64

Stores have an additional input operand which is the `value` to store to memory.
Like loads, integer stores may specify a smaller size than the
operand size and include an implicit integer wrap operation which discards the upper bits.
Like loads, integer stores may specify a smaller *storage size* than the operand
size in which case integer wrapping is implied.

* `i32.store8`: wrap i32 to i8 and store 1 byte
* `i32.store16`: wrap i32 to i16 and store 2 bytes
Expand All @@ -118,7 +110,7 @@ operand size and include an implicit integer wrap operation which discards the u
* `f64.store`: (no conversion) store 8 bytes

In addition to storing to memory, store instructions produce a value which is their
`value` input operand without conversion.
`value` input operand before wrapping.

### Addressing

Expand All @@ -128,11 +120,9 @@ index. The infinite-precision sum of the address operand's value with the byte
offset attribute's value is called the *effective address*, which is interpreted
as an unsigned byte index.

Linear memory accesses access the bytes starting at the location in the linear
memory storage indexed by the effective address, and extending for the number
of bytes implied by the memory type attribute of the access.

If any of the accessed bytes are beyond `memory_size`, the access is considered
Linear memory operations 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
*out-of-bounds*. A module may optionally define that out-of-bounds includes
small effective addresses close to `0`
(see [discussion](https://github.com/WebAssembly/design/issues/204)).
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