Assignment 12, due May 1

On every assignment, write your name legibly as it appears on your University ID card! Homework is due on paper at the start of class on the day indicated (usually Friday). Exceptions will be made only by advance arrangement (excepting "acts of God"). Late work must be turned in to the TA's mailbox (ask the CS receptionist in 14 MLH for help). Never push homework under someone's door!

1. Background: The code to virtualize the LOADCC instruciton distributed in the file mp6.a includes a block of code to update the processor status word, ending with the comment savearea.pswsv = psw | cc.

   a) How does this instruction get the new value of the condition codes that the instruction is supposed to set? If you read the manual for LOADCC, you will see that the condition code setting is complicated, with one bit that tells you whether the loaded value has any NUL bytes in it, yet the code doesn't seem to check for this at all. (0.3 points)

   b) The code to update the condition codes could update just the least significant 4 bits of the PSW, but it updates many more bits. How many bits of the saved PSW does it the code actually update? You can answer this from inspecting the code. (0.3 points)

   c) The extra bits of the PSW that get updated (see part b) contain a field of the PSW other than the condition codes. What is the name of this field? (You may need to review Chapter 1 of the Hawk manual.) (0.2 points)

   d) What is the use of the field of the processor status word discussed in part c)? (You may need to review Chapter 9 of the class notes.) (0.2 points)

2. Background: Look back at the (rather archaic) keyboard interface defined in Chapter 12, and assume the interrupt enable bit is wired as suggested near the end of Chapter 13.

   Furthermore, assume that the keyboard is connected to trap vector entry C0₁₆.

   In addition, assume that the three least-significant bits of the level field of the processor status word are decoded as a 3-bit integer. If this field of the level is nnn, this interrupts are enabled via trap-vector entries 1nnn and lower. For example, if the level field is 0010, then interrupts using trap-vector entries 1000, and 1001 and 1010 are enabled, but 1011 and up are disabled.

   a) Suppose someone hits a key on the keyboard. What combination of values in the keyboard device control register and the processor status word will permit the keyboard to interrupt the CPU? (0.5 points)

   b) When control enters the interrupt service routine for the keyboard, the level field of the PSW is set to 0000. Within the interrupt service routine for a slow device like the keyboard, it might be important to allow higher priority devices to interrupt the interrupt service routine. What instruction sequence should the interrupt service routine contain in order to enable higher priority interrupts? (Hint, it is an LIW followed by a CPUSET, what you need to work out are the arguments.) (0.5 points)

3. Background: Suppose you are interested in the performance of algorithm X, an algorithm you just invented and named. One thing that worries you is the speed of the algorithm, so you devise a test. You write a program to measure the how long the algorithm takes (t) as a function of the size of the data (s). Your theoretical work suggests that a plot of the how long the algorithm takes t vs s ought to be a straight line.

   Like essentially all theoretical computer scientists, your theoretical work is based on the assumption that the memory access time is a constant.

   You take data covering a huge range of sizes. For values of s below approximately 16000, your t vs s curve is straight. Above this, the t vs s curve bends upward, until s exceeds 128000. Above this, the curve seems straight again, but the slope is steeper.

   a) Is your theoretical analysis of Algorithm X wrong, or did your theory overlook something. Explain. (0.4 points)

   b) Estimate the size of the cache on this machine? (0.3 points)

   c) Suggest a way to estimate the ratio of the memory speeds of the cache and the main memory on this machine, using your t vs s curves. (0.3 points)