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 this word, but it must leave room for it: 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.

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.