## Department of Electrical and Computer Engineering

### The University of Texas at Austin

EE 360N, Spring 2007
Problem Set 3
Due: 5 March 2007, before class
Yale N. Patt, Instructor
Chang Joo Lee, Rustam Miftakhutdinov, Poorna Samanta, TAs

## Instructions

You are encouraged to work on the problem set in groups and turn in one problem set for the entire group. Remember to put all your names on the solution sheet. Also remember to put the name of the TA in whose discussion section you would like the problem set returned to you.

## Questions

Please note that Problem 7 was updated! (03/04/07)

Please include your solutions to problems 6 and 7 from Problem Set 2.

1. A bit-serial transmitter-receiver system operates at 1 GHz. It appends an even parity bit after every eight bits processed, as described in class. Therefore, for each byte of data, it transmits a nine bit message. Since the system is bit-serial, one message bit is transmitted each cycle. Assume that the probability of a bit being flipped while it is being transmitted is 10-7. In your solution, treat bit flips as statistically independent events.

1. What is the probability that a transmitted nine bit message will have one or more flipped bits? Hint: what is the probability that a transmitted nine bit message will have zero flipped bits?

2. If the parity check logic in the receiver detects an error in a message, how many bits may have been flipped in that message? (1, 2, 3, 4, 5, 6, 7, 8, 9 – circle all that apply).

3. If the parity check logic in the receiver does not detect an error in a message, how many bits may have been flipped in that message? (1, 2, 3, 4, 5, 6, 7, 8, 9 – circle all that apply).

4. What is the probability that a transmitted nine bit message will have exactly:

• 1 bit flipped
• 2 bits flipped
• 3 bits flipped

Notice that the probability of exactly three bits being flipped is negligible compared to the probabilities of one or two bits being flipped. Thus, for the rest of this problem, you may neglect the probabilities of three or more bits being flipped in one message.

5. On average, how many detected bit errors per second will occur in the system?
6. On average, how many undetected bit errors per second will occur in the system?

Note: this course is not about probability theory, and the undergraduate probability course (EE351K) is not a prerequisite. Thus, if you have difficulty solving this problem, please see one of the TA's.

2. We discussed in class the various ways to build a rotator out of muxes. In this question, we ask you to compare two designs of a 32-bit left rotator: 1) one level of 32-to-1 muxes, and 2) three levels (4-to-1 muxes, 2-to-1 muxes, 4-to-1 muxes). In particular, we ask you to calculate the number of AND, OR, and NOT gates necessary for each of those designs.

As you know, one way to implement a 2N-to-1 mux is the simple NOT-AND-OR approach (shown below).

To make grading easier, please use this mux implementation in your calculations. Note: please provide separate counts for each type of gate (AND, OR and NOT) and the number of inputs. For example, your answer should not be “655 AND gates” if it happened to be “400 2-input AND gates, 200 3-input AND gates, and 55 4-input AND gates.”

After doing the calculations, which design do you think costs less hardware?

3. If the latency of a DRAM memory bank is 37 cycles, into how many banks would you interleave this memory in order to fully hide this latency when making sequential memory accesses?

4. An ISA supports an 8-bit, byte-addressable virtual address space. The corresponding physical memory has only 128 bytes. Each page contains 16 bytes. A simple, one-level translation scheme is used and the page table resides in physical memory. The initial contents of the frames of physical memory are shown below.

Frame NumberFrame Contents
0 empty
1 Page 13
2 Page 5
3 Page 2
4 empty
5 Page 0
6 empty
7 Page Table

A three-entry Translation Lookaside Buffer that uses LRU replacement is added to this system. Initially, this TLB contains the entries for pages 0, 2, and 13. For the following sequence of references, put a circle around those that generate a TLB hit and put a rectangle around those that generate a page fault. What is the hit rate of the TLB for this sequence of references? (Note: LRU policy is used to select pages for replacement in physical memory.)

References (to pages): 0, 13, 5, 2, 14, 14, 13, 6, 6, 13, 15, 14, 15, 13, 4, 3.

1. At the end of this sequence, what three entries are contained in the TLB?
2. What are the contents of the 8 physical frames?
5. We have been referring to the LC-3b memory as 2^16 bytes of memory, byte-addressable. This is the memory that the user sees, and may bear no relationship to the actual physical memory. Suppose that the actual physical address space is 8K bytes, and our page size is 512 bytes. What is the size of the PFN? Suppose we have a virtual memory system in which virtual memory is divided into User Space and System Space, and System Page Table remains resident in physical memory. System space includes trap vector table, interrupt vector table, operating system and supervisor stack as shown in Figure A.1 in Appendix A. The rest of the address space in Figure A.1 is user space. If each PTE contained, in addition to the PFN, a Valid bit, a modified bit, and two bits of access control, how many bits of physical memory would be required to store the System Page Table?

6. A machine with 64KB, byte addressable virtual memory and 4KB physical memory has two-level virtual address translation similar to the VAX. The page size of this machine is 256 bytes. Virtual address space is partitioned into the P0 space, P1 space, system space and reserved space. The space a virtual address belongs to is specified by the most significant two bits of the virtual address, with 00 indicating P0 space, 01 indicating P1 space, and 10 indicating system space. Assume that the PTE is 32 bits and contains only the Valid bit and the PFN in the format V0000000..000PFN.

For a single load instruction the physical memory was accessed three times, excluding instruction fetch. The first access was at location `x108` and the value read from that location (`x108`, `x109`, `x10A`, `x10B`) was `x80000004`. Hint: What does this value mean?

The second access was at location `x45C` and the third access was at location `x942`.

If SBR = `x100`, P0BR = `x8250` and P1BR = `x8350`,

1. What is the virtual address corresponding to physical address `x45C`?
2. What is 32 bit value read from location `x45C`?
3. What is the virtual address corresponding to physical address `x942`?
7. (Updated: 03/04/07)

In this problem, the PTBR does not point to the first entry in the user space page table, which actually starts at 0x3A0. This is an optimization to make it easier for the machine to index into the user space page table (as discussed in the review session).

Consider a processor that supports a 9-bit physical address space with byte addressable memory. We would like the processor to support a virtual memory system. The features of the virtual memory system are:

```    Virtual Memory Size : 4 Kbytes (12 bit address-space)
Page Size           : 32 bytes
PTBR                : 0x380
SBR                 : 0x1E0```

The virtual memory is divided into two spaces: system space and user space. System space is the first kilobyte of the virtual address space (i.e., most significant two bits of the virtual address are 00). The rest of the virtual memory is user space. The system page table remains resident in physical memory. Each PTE contains, in addition to the PFN, a Valid bit, a modified bit and 2 bits for access control. The format of the PTE is

 Valid Modified Access Control PFN

(Valid bit is the most significant bit of the PTE and the PFN is stored in the least significant bits.)

1. How many virtual pages does the system accommodate?
2. What is the size of the PFN? How big is the PTE?
3. How many bytes are required for storing the entire user space pagetable? How many pages does this correspond to?

4. Since the user space page table can occupy a significant portion of the the physical memory, this system uses a 2 level address translation scheme, by storing the user space Page Table in virtual memory (similar to VAX).

The following table shows the contents of the physical memory that you may need to do the translation:

`x1F8` `xBA`
`x1F9` `xBB`
`x1FA` `xBC`
`x1FB` `xBD`
`x1FC` `xBE`
`x1FD` `xB8`
`x1FE` `xB7`
`x1FF` `xB6`
`x118` `x81`
`x119` `x72`
`x11A` `x65`
`x11B` `x34`
`x11C` `x97`
`x11D` `x83`
`x11E` `xC6`
`x11F` `xB2`
8. The virtual address of variable X is `x3456789A`. Find the physical address of X. Assume a Virtual Memory model similar to VAX.

Remember that in VAX each Virtual Address consists of:

• 2 bits to specify the Address Space
• 21 bits to specify Virtual Page Number
• 9 bits to specify the byte on the page

You will need to know the contents of P0BR: `x8AC40000` and SBR: `x000C8000`.

You will also need to know the contents of the following physical memory locations:

```x1EBA6EF0:    x80000A72
x0022D958:    x800F5D37```