 WebAssembly linear memory

WebAssembly linear memory is an indexed sequence of bytes accessed by WebAssembly memory instructions. Its runtime representation records its type and current byte sequence. The length is a multiple of the WebAssembly page size, $65536$ bytes, or $64\mathrm{KiB}$. Declared limits give an initial minimum and optionally a maximum size in pages.^[ref]

A load must fit its entire access into the current WebAssembly linear memory. For an instruction offset of zero, a four-byte i32.load in a one-page memory succeeds at address $65532$: its last accessed byte has address $65535$. At address $65533$, it would need a byte at address $65536$ and therefore traps. In general, effective address $a$ and width $w$ must satisfy $a+w\le L$, where $L$ is the current length; the check includes the instruction's immediate offset.^[ref]

This boundary example also distinguishes alignment from bounds. Ordinary unaligned loads are permitted; their alignment annotation is an optimization hint. A successful bounds check establishes that the accessed bytes lie inside this memory. It does not establish that the program selected its intended object.^[ref]