[retrac]

Usually, tagged memory at a hardware level, goes along with support for tagged operations in the processor.

In the LISP machines for example, you had an add instruction. Which would happily work correctly on pointers, floats, and integers depending on the data type, at the machine code level. Offers safety and also makes the the compiler simpler.

But where this really shines is in things like, well, lists, since the tags can distinguish atoms from pairs and values like nil, fairly complex list-walking operations can be done in hardware, and pretty quickly too. It also makes hardware implementation of garbage collection possible.

This is just my intuition, but I suspect, these days, it all works out to about the same thing in the end. You use some of the cache for code that implements pointer tagging, or you can sacrifice some die area away from cache for hardwired logic doing the same thing. It probably is in the same ballpark of complexity and speed.

[gumby]

In addition to what retrac wrote, these tag bits would apply to immediates as well, not just pointers.