CMPE 401 - Computer Interfacing

Assignment #2

1. In the context of the RS-232 serial communciations, briefly explain what is meant by a "null modem". Many point-to-point wireline communications standards (e.g. RS-232 and USB) are asymmetrical with respect to the connector types used at either end of the communications medium. In such standards, when is it necessary to have null modems? How could one design a new point-to-point communications standard that avoids the need for null modem connections?

2. Consult the on-line documentation or the course textbook, and then briefly explain how the T1 and T0 trace mode bits in the status register work. In your answer be sure to highlight the difference between the two trace modes. Why were both modes provided in the CPU32 microprocessor?

3. Consult the on-line documentation or the course textbook, and then briefly describe the three CMPU32 return instructions, namely, RTS (return from subroutine), RTR (return and restore CCR), and RTD (return and de-allocate). What previous stack operations should have been performed before (not necessarily immediately before) each of these return instructions is executed?

4. In the lectures we discussed the various reasons why communications interfaces are layered. In you own words, summarize those advantages. Why do you suppose that it is generally best to access an interface at the highest possible layer? What would be a good reason for accessing a communications interface at a lower than usual layer?

5. The Internetworking Protocol (IP) has the right to split up (i.e. to segment) TCP datagrams into smaller packets as required. What major steps would be required to take one TCP segment, split it up, and then produce two shorter (and yet still legal) IP datagrams? Then explain what the major steps would be in re-assembling two IP datagrams into one longer IP datagram that contains the same data as in the original two datagrams. Be sure to mention any situation that might prevent make datagram re-assembly impossible.

6. Briefly explain how the architecture of the lightweight IP (lwIP) stack promotes portability across both processors and operating systems.