Midterm Exam Logistics

• Thursday, Oct. 25• Exam starts at 4:35pm and lasts 75 minutes

• Location: DRL A6

• Open-book! Open-notes!• No electronic devices!

• Review session: Weds Oct. 24, 6:15pm, Towne 315

What does “open-book” mean?

• You may use:– your textbook– your lecture notes– lecture notes from course website– sample questions and solutions– graded homeworks and solutions

What does “open-book” mean?

• You may not use:– electronic devices (no exceptions!)– the book/papers of the person sitting next to you

(even if that person is really good looking!)– secret notes that you are sharing with your

friends in the men's room trash bin

Midterm Exam Material

• Assigned reading from P&P chapters 2-10– not chapter 6

• Anything covered in lecture

• No UNIX stuff from lab!• No computer science history!

Format of the exam

• Multiple-choice definitions/concepts

• Short answer and problem solving– Like questions from book and homework

• Understanding LC-3 code– no writing LC-3 from scratch!

Preparing for the exam• Review the lecture notes• Go through the sample questions from the

study guide and last year's exam• Make sure you understand the examples

we did in class• Find students who are smarter than you

and convince them to form a study group• Use Piazza to ask questions• Don't panic!

Preparing for an open-book exam• Make sure you actually study!

• Have a sheet of definitions and terms• Create an index mapping concepts to

chapters in the textbook

• Organize all your documents so you can find things quickly

Secret of the Chris Murphy exam• The number of points a question is worth is

(roughly) equal to the number of minutes you should be spending on it

– You have 75 minutes to complete the exam– There are 60 total points on the exam

• I like questions that are easy to grade– Quantitative instead of qualitative

• Some questions have lots of words: do those later

Any questions about theFORMAT of the exam?

Topics Covered

• Computer Numbers (Chp.2)• Digital Logic and Circuits (Chp.3)• von Neumann Architecture (Chp.4)• LC-3 Instruction Set Architecture (Chp.5)• Assembly Language (Chp.7)• Input/Output (Chp.8)• Subroutines (Chp.9)• Interrupts (Chp.10)

Computer Numbers

• Unsigned binary integers– Decimal-to-binary conversion– Hexadecimal numbers– Unsigned binary arithmetic

• Signed binary integers– Sign/magnitude– Two’s complement– Overflow

Computer Numbers (cont.)

• Floating point representation

• Logical operations and bit vectors

• Character representation– ASCII– Unicode

Digital Logic

• p-type and n-type transistors

• NOT, AND, and OR gates

• Sum-of-products algorithm

Combinational Logic Circuits

• Adder (1-bit, 4-bit, n-bit)

• Decoder: n inputs, 2n outputs

• Demultiplexer: 1 input, n select lines, 2n output

• Multiplexer: 2n inputs, n select lines, 1 output

Exercises 3.13 and 3.14

• How many output lines will a five-input decoder have?

• How many output lines will a 16-input multiplexer have?

• How many select lines will this multiplexer have?

Memory

• R-S Latch: depends on “state”

• Gated D Latch: single bit of memory

• Register: some number of Gated D Latches that form a single unit of memory

von Neumann Architecture

• Memory– address space vs. addressability– MAR and MDR

• Control Unit– fetch, decode, execute– IR and PC– ALU, registers

MAR/MDR questions

• 32-bit address space• 8-bit addressability

• How many bits in MAR?• How many bits in MDR?

• How are MAR/MDR used?• ST R2, DATA• LD R3, DATA

Homework #3, Part 1

• Instructions are 24 bits long• 43 distinct operations in ISA• 16 one-byte registers• each memory address holds one byte• total addressable memory space is 64kB

• In “fetch” phase of instruction cycle, what value is added to program counter?

LC-3 Instruction Set Architecture• Instruction format– Opcode – Operands

• Encoding from assembly language to machine language (and decoding)

• Types of instructions & what they do– ALU operations– Data movement operations– Control operations

LC-3 Addressing Modes

• Immediate– LEA R1, DATA

• Direct– LD R1, DATA

• Indirect– LDI R1, DATA

• Base-offset– LEA R0, DATA– LDR R1, R0, #0

Label Address ValueDATA x1234 x1236

x1235 xABAB x1236 x5588

Assembly Language

• Assembler and symbol table• Finding and fixing bugs• What does this program do?• How can this program be improved?

• BLKW, FILL, and STRINGZ

Homework 4 Question #3

.ORIG x3000

LEA R0, PHRASE

LEA R1, DATA

LD R2, VALUE

LOOP LDR R3, R0, #0

STR R3, R1, #0

ADD R0, R0, #1

ADD R1, R1, #1

ADD R2, R2, #-1

BRp LOOP

END HALT

PHRASE .STRINGZ

“This is not a test!”

DATA .BLKW x10

VALUE .FILL x10

Input/Output

• Polling: keep checking to see if input device has data available (or if output device is ready to receive data)

• memory-mapped I/O: special addresses in memory are dedicated to I/O related actions:

– KBDR, KBSR, DDR, DSR

Traps, Interrupts, Subroutines

• How are subroutines called? How does the program know where to go back to?

• How do traps work? How does the program know where to go back to?

• How do interrupts work? How is the state saved? What happens when the handler finishes?

• Traps: GETC, IN, OUT, PUTS, HALT

Exercise 9.2

• How many trap service routines can be implemented in the LC-3?

• Why must a RET instruction be used to return from a trap routine? Why not BR?

• How many accesses to memory are made during the processing of a TRAP instruction? Assume instruction is already in the IR

Additional review questions

Exercise 3.5

Complete the truth table for

a transistor-level circuit

Exercise 3.27

Truth table for circuit

with feedback loop

Exercise 5.22

PC: x3010

What do these three lines of code do?

x3010 1110 0110 0011 1111

x3011 0110 1000 1100 0000

x3012 0110 1101 0000 0000

Address Valuex3050 x70A4x70A2 x70A3x70A3 xFFFFx70A4 x123B

Exercise 5.30

Address Data

x3100 1001 001 001 111111

x3101 0001 010 000 000 001

x3102 1001 010 010 111111

x3103 0000 010 111111100

What is known about R1 and R0 if the conditional branch redirects to x3100?

Exercise 8.15

Interrupt-driven I/O