You are asked to design a new processor with a 64?bit word. Taking advantage of advances in technology, you decide that you can allocate an extra 5 bits to each word. That is, data words in registers and memory will occupy 69 bits. The additional 5 bits will be used to describe the data. How might you allocate these additional bits (i.e., what functions would you assign to them) in the light of what you have read in the previous chapters?

Question

What will be an ideal response?

Answer 1

Ultimately, this question is asking you how you would tag data. There is, of course, no single answer. If you
were designing a processor with a new architecture, you would have to simulate its behaviour and measure
performance increase against the cost of adding new architectural features. Although we have not yet dealt
with multiprocessing and multicore processors, we would have to consider the issue of scalabilty; for example,
does any new feature become a bottleneck when the system is scaled up by, for example, parallelism? We also
need to consider the target applications; multimedia, cryptography, scientific, commercial, database, ighsecurity, real?time etc.

Students can answer this question in many ways. One possible set of tag bits might be:

Tag bit 0 Integer/multimedia or floating?point/other data
Tag bit 1,2 1 × 4?byte, 2 × 2?byte, 4 × 1?bytes SIMD
Tag bit 3 Signed or unsigned
Tag bit 4 Saturating or standard integer