## Homework #5

### CMSC 611, Fall 1998

Assigned: Tuesday, November 17, 1998

Due Tuesday, November 24, 1998 (in class)

1. Problem 5.1 from the text.
2. You just purchased a new computer, and want to know whether there's enough extra main memory bandwidth to add a new peripheral. Your measurements of the computer have found the following information:
• Hit rate for the D-cache is 98%, and the I-cache has a 99.5% hit rate.
• Block size for the D-cache is 1 word, and for the I-cache is 2 words.
• The processor runs at 250 MHz and has a native CPI of 1.2 without memory accesses.
• 15% of all instructions are loads, and 5% are stores.
• The bus can only support single word operations (i.e., two memory accesses require two complete bus cycles and memory latencies).
• The memory can handle 10 million references per second (i.e., memory cycle time is 100 ns).
1. What is the memory utilization if the cache is write-through?
2. What is the memory utilization if the cache is write-back? Assume that 40% of the data blocks are dirty.
3. Problem 5.5 from the text.
4. You want to build a system that can run at 250 native MIPS (including memory stalls). You’re given a 500 MHz CPU with a CPI of 0.4 excluding memory stalls. The CPU has a split onboard L1 cache, with 8 byte data blocks and 32 byte instruction blocks. The instruction cache has a hit rate of 99%, while the data cache has a hit rate of 95%. 40% of the misses in the writeback data cache require the replacement of a dirty block. In this system, 20% of the instructions are loads and 10% are stores.
This system can have 1, 2, or 4 banks of main memory, each 64 bits wide. There is a single bus connecting the banks to the caches; while the banks can all fetch or store data in parallel, only one bank can transfer data on a given bus cycle. The bus has separate addresses and data lines. Costs for system components of various speeds are as follows:
Memory access time
Cost (per bank)
100 ns
\$50
80 ns \$75
60 ns \$100
Bus speed Cost
50 MHz \$50
66.6 MHz \$100
125 MHz \$200
250 MHz \$500

1. If cost were no object, how fast could the system be made to run?
2. How fast is the cheapest memory system that will meet your design goal of 250 native MIPS?
5. Problem 5.8 from the text.
6. Problem 5.14 (part a only) from the text.
7. Problem 5.21 from the text.

Last updated 17 Nov 1998 by Ethan Miller (elm@csee.umbc.edu)