Tagged pointer

In computer science, a tagged pointer is a pointer (concretely a memory address) with additional data associated with it, such as an indirection bit or reference count. This additional data is often "folded" into the pointer, meaning stored inline in the data representing the address, taking advantage of certain properties of memory addressing. The name comes from "tagged architecture" systems, which reserved bits at the hardware level to indicate the significance of each word; the additional data is called a "tag" or "tags", though strictly speaking "tag" refers to data specifying a type, not other data; however, the usage "tagged pointer" is ubiquitous. There are various techniques for folding tags into a pointer. Most architectures are byte-addressable (the smallest addressable unit is a byte), but certain types of data will often be aligned to the size of the data, often a word or multiple thereof. This discrepancy leaves a few of the least significant bits of the pointer unused, which can be used for tags – most often as a bit field (each bit a separate tag) – as long as code that uses the pointer masks out these bits before accessing memory. E.g., on a 32-bit architecture (for both addresses and word size), a word is 32 bits = 4 bytes, so word-aligned addresses are always a multiple of 4, hence end in 00, leaving the last 2 bits available; while on a 64-bit architecture, a word is 64 bits = 8 bytes, so word-aligned addresses end in 000, leaving the last 3 bits available. In cases where data is aligned at a multiple of word size, further bits are available. In case of word-addressable architectures, word-aligned data does not leave any bits available, as there is no discrepancy between alignment and addressing, but data aligned at a multiple of word size does. Conversely, in some operating systems, virtual addresses are narrower than the overall architecture width, which leaves the most significant bits available for tags; this can be combined with the previous technique in case of aligned addresses.