Runtime object header

In .NET every object carries a header word just before the object reference, at a negative offset, in addition to the MethodTable pointer at offset 0. The runtime uses it for bookkeeping that has no field of its own: the cached identity hash code (what GetHashCode returns for a plain object) and the thin lock that Monitor.Enter (the lock statement) takes on an uncontended object.

The GC never reads the hash code or the lock, but it must leave room for the word: the runtime writes at objRef - 4, so writable bytes must exist before every object, including the very first object of a segment and any object that directly follows recycled free space. CoreLib reserves the word's three high bits for the collector and its compare-exchange loops leave them as they are; OrionGC uses one of them, bit 30 (.NET's BIT_SBLK_FINALIZER_RUN), as the flag GC.SuppressFinalize sets, see Finalization.

In OrionGC that is why each segment reserves 8 bytes before its first object, and why every free block keeps its last 8 bytes out of allocation. This header is distinct from the GCObject layout that starts at the object reference itself. See Segments and Free lists.