Data-Blocks comprise only descriptors, or they contain immediate data and raw bits interpreted by the system. GC must skip the latter when scanning the heap, so it does not look at a word of raw bits and interpret it as a pointer descriptor. These data-blocks require headers for GC as well as for operations that need to know how to interpret the raw bits. When GC is scanning, and it sees a header-word, then it can determine how to skip that data-block if necessary. Header-Words are tagged as other-immediates. See sec-other-immediates and sec-data-blocks-and-header for comments on distinguishing header-words from other-immediate data. This distinction is necessary since we scan through data-blocks containing only descriptors just as we scan through the heap looking for header-words introducing data-blocks.
Data-Blocks containing only descriptors do not require header-words for GC since the entire data-block can be scanned by GC a word at a time, taking whatever action is necessary or appropriate for the data in that slot. For example, a cons is referenced by a descriptor with a specific tag, and the system always knows the size of this data-block. When GC encounters a pointer to a cons, it can transport it into the new space, and when scanning, it can simply scan the two words manifesting the cons interpreting each word as a descriptor. Actually there is no cons tag, but a list tag, so we make sure the cons is not nil when appropriate. A header may still be desired if the pointer to the data-block does not contain enough information to adequately maintain the data-block. An example of this is a simple-vector containing only descriptor slots, and we attach a header-word because the descriptor pointing to the vector lacks necessary information – the type of the vector’s elements, its length, etc.
There is no need for a major tag for GC forwarding pointers. Since the tag bits are in the low end of the word, a range check on the start and end of old space tells you if you need to move the thing. This is all GC overhead.