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Diffstat (limited to 'docs/assembly.rst')
| -rw-r--r-- | docs/assembly.rst | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/docs/assembly.rst b/docs/assembly.rst index cf9bf840..978e71e3 100644 --- a/docs/assembly.rst +++ b/docs/assembly.rst @@ -405,6 +405,16 @@ changes during the call, and thus references to local variables will be wrong. } } +.. note:: + If you access variables of a type that spans less than 256 bits + (for example ``uint64``, ``address``, ``bytes16`` or ``byte``), + you cannot make any assumptions about bits not part of the + encoding of the type. Especially, do not assume them to be zero. + To be safe, always clear the data properly before you use it + in a context where this is important: + ``uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ }`` + To clean signed types, you can use the ``signextend`` opcode. + Labels ------ @@ -647,6 +657,11 @@ Solidity manages memory in a very simple way: There is a "free memory pointer" at position ``0x40`` in memory. If you want to allocate memory, just use the memory from that point on and update the pointer accordingly. +The first 64 bytes of memory can be used as "scratch space" for short-term +allocation. The 32 bytes after the free memory pointer (i.e. starting at ``0x60``) +is meant to be zero permanently and is used as the initial value for +empty dynamic memory arrays. + Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, 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 |