Layout in MemoryΒΆ

Solidity reserves four 32-byte slots, with specific byte ranges (inclusive of endpoints) being used as follows:

  • 0x00 - 0x3f (64 bytes): scratch space for hashing methods
  • 0x40 - 0x5f (32 bytes): currently allocated memory size (aka. free memory pointer)
  • 0x60 - 0x7f (32 bytes): zero slot

Scratch space can be used between statements (i.e. within inline assembly). The zero slot is used as initial value for dynamic memory arrays and should never be written to (the free memory pointer points to 0x80 initially).

Solidity always places new objects at the free memory pointer and memory is never freed (this might change in the future).

Elements in memory arrays in Solidity always occupy multiples of 32 bytes (this is even true for byte[], but not for bytes and string). Multi-dimensional memory arrays are pointers to memory arrays. The length of a dynamic array is stored at the first slot of the array and followed by the array elements.


There are some operations in Solidity that need a temporary memory area larger than 64 bytes and therefore will not fit into the scratch space. They will be placed where the free memory points to, but given their short lifetime, the pointer is not updated. The memory may or may not be zeroed out. Because of this, one should not expect the free memory to point to zeroed out memory.

While it may seem like a good idea to use msize to arrive at a definitely zeroed out memory area, using such a pointer non-temporarily without updating the free memory pointer can have unexpected results.