remove sbrk from MVP, add mmap&friends to AstSemantics

AstSemantics.md

@@ -136,10 +136,7 @@ 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
-*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)).
-The semantics of out-of-bounds accesses are discussed
+*out-of-bounds*. The semantics of out-of-bounds accesses are discussed
 [below](AstSemantics.md#out-of-bounds).
 
 The use of infinite-precision in the effective address computation means that
@@ -154,6 +151,30 @@ by [available resources](Nondeterminism.md)). In the future, to support
 [>4GiB linear memory](FutureFeatures.md#heaps-bigger-than-4gib), support for
 indices with type `int64` will be added.
 
+### Adjusting memory size and permissions: `mmap`
+
+In the MVP the size of linear memory is fixed: The initial size of linear
+memory will remain unchanged for the life of that WebAssembly module. Later,
+we will support a limited form of `mmap` which can:
+
+  * Allocate pages of memory, in order to increase or decrease
+    the amount of available memory to the WebAssembly module.
+  * Adjust the permissions of a page of memory, for example to make
+    small effective addresses close to `0` behave as if they are
+    [out-of-bounds](AstSemantics.md#out-of-bounds)
+    (see [discussion](https://github.com/WebAssembly/design/issues/204)).
+
+In addition to the `mmap` operation, applications will also have access to
+
+ * `munmap`, to unmap `mmap`ed pages, and
+ * `mmap_get_page_size`, to detect the system page size (this may not
+   provide accurate information about the underlying system, but it
+   should be good enough for purposes of calling `mmap`).
+
+In the future,
+[further `mmap` functionality](FutureFeatures.md#finer-grained-control-over-memory)
+may be added.
+
 ### Alignment
 
 Each linear memory access operation also has an immediate positive integer power

MVP.md

@@ -97,7 +97,6 @@ isomorphic to the [binary format](BinaryEncoding.md).
   necessary for two WebAssembly modules to share the same linear memory.
 * Modules can specify memory size and initialization data (`data`, `rodata`,
   `bss`) in the [memory-initialization section](MVP.md#module-structure).
-* Modules can specify whether memory is growable (via `sbrk`).
 * Modules can optionally export memory, allowing it to be aliased by the
   embedder, such as JavaScript:
   * JavaScript sees the exported memory as an `ArrayBuffer`.