itype width of 3 limitations

[IainCRobertson]

A recent discussion on the N-Trace email list highlighted the need for a clarification related to 3-bit itype and retiring instructions in blocks.

With a 3-bit itype, inferable calls are retired using itype = 0. However, they must still be the final instruction in a block, because a retirement block is defined as a number of contiguous instructions.