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Memory & Data

WebAssembly exposes a contiguous byte array called linear memory. You interact with it via typed load/store instructions and can initialize it with data segments.

Declaring and exporting memory

Section titled “Declaring and exporting memory”
(module
  (memory (export "memory") 1 4)  ;; min 1 page (64 KiB), max 4
)

Loads and stores

Section titled “Loads and stores”

Typed operations read/write values at byte offsets. Narrow variants sign/zero-extend on load and truncate on store.

(module
  (memory 1)
  (func (param $ptr i32) (result i32)
    (i32.store8 (local.get $ptr) (i32.const 0x7F))
    (i32.load8_u (local.get $ptr)))
)

Common ops:

  • Loads: i32.load, i64.load, f32.load, f64.load, i32.load8_s/u, i32.load16_s/u, …
  • Stores: i32.store, i64.store, f32.store, f64.store, i32.store8, i32.store16, …

memory.size and memory.grow

Section titled “memory.size and memory.grow”
(module
  (memory 1)
  (func (result i32)
    (memory.size))
  (func (param $pages i32) (result i32)
    (memory.grow (local.get $pages)))  ;; returns old size, or -1 on failure
)

After growth the underlying ArrayBuffer can change — recreate typed array views on the host.

Data segments

Section titled “Data segments”
(module
  (memory 1)
  (data (i32.const 0) "Hi")
)

Instruction Reference

Section titled “Instruction Reference”