EECE 321 Exam 2, Spring 2007

AMERICAN UNIVERSITY OF BEIRUT FACULTY OF ENGINEERING AND ARCHITECTURE DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING PROFS: M. SAGHIR L. HAMANDI EECE 321 COMPUTER ORGANIZATION SPRING 2006-2007 QUIZ 2 NAME: Hou sscon We Snen ebook APRIL 28, 2007 CLOSED BOOK (2 HRS) NON-PROGRAMMABLE CALCULATORS ARE ALLOWED PROVIDE SOLUTION ON THE ANSWER BOOKLET THE QUESTION SHEET MUST BE RETURNED WITH THE ANSWER BOOKLET READ ALL QUESTIONS ON ALL PAGES CAREFULLY. (15%) 1. Machine A has a clock cycle time of 2 ns. We run a program (800 instructions) on it and find its CPI = 1.8. Machine B has a clock cycle time of 3 ns. We run the same program on it and find its CPI = 1.5. a) Which machine is faster, and by how much? (10%) b) How long will it take the program to run on machine B? (5%) (15%) 2. A pulse width modulator (PWM) is used to control a DC motor. Assuming the PWM output (pwm_o) is 8 V when high and 0 V when low. a) What voltage is supplied to the motor if the output of the PWM, pwm_o, is shown in the below diagram? (10%) pwm_o | clk b) If the motor’s maximum speed is 10,000 RPM, what will be its RPM when a voltage of 5V is applied to it? (5%) EECE 321 Computer Organization — Quiz 2 Page 1 of 4 (20%) 3. Consider the single cycle datapath of the MIPS R2000 processor (a datapath diagram is provided for your reference). Set the control signals shown in the following table to their appropriate values in order to execute the corresponding instructions on the datapath. ALU ‘ : Control ¢ Instruction RegDst | RegWrite | ALUSre Inputs MemWrite | MemtoReg | Branch | Jump G bits) LW St1,100(S2 ae ’ m me) op 4 1 [000 | o 1 | Oo |a SW Stl, LO0(St2) gee . vs a OA | 0 \ 000 { 4 iy 6 ORI $s1,100 e H 2 - - sz, 1X | OA] blo 0 0) 3) 0 BEQ $t1,$t2, LABELI! xX 0 0 oo' x aK X \ 9 SUB St1,S22, $3 j 0 | O | ze XA| vo 0 0 The following table shows the values of the ALU control signals and the corresponding ALU operations. [ ALU control inputs | ALU operation | 000 Add | 001 Subtract | 010 And 110 Or 111 | Set-on-less-than | (30%) 4. You have been asked to design a PIC-based system for controlling a water pump. Figure 1 shows a block diagram of the system. Two sensors. Sp and S;, are used to sense the water level. If a sensor is in contact with water its value is HIGH; otherwise it is LOW. Your controller must monitor the water level continuously. Once sensor S; is no longer in contact with water, the controller must turn the water pump on. The pump must remain ON until both sensors are in contact with water again. The pump turns on when the value on signal P is HIGH; otherwise the pump is off. EECE 321 Computer Organization — Quiz 2 Page 2 of 4 a) b) (20%) PIC16F84 Figure 1: PIC-based system to control a water pump If sensors So and S; are connected to pins | and 2, and pump P is connected to pin 17, write a sequence of PICI6F84 assembly instructions to properly configure those pins. Briefly explain what your code does. A diagram of the PIC16F84 is provided for your reference. (5%) Assuming the pins have been properly configured, write a PIC]16F84 assembly loop to continuously read the values of sensors Sp and $), and to call subroutine FILLTANK when both sensors are no longer in contact with water. Briefly explain what your code does. (10%) Write the FILLTANK subroutine in PIC16F84 assembly code. The subroutine must turn the pump on and keep it on until both sensors are in contact with water again. Once the pump has been turned off, the subroutine must return to its caller. Briefly explain what your code does. (15%) 5. Please refer to the provided diagram of the PIC16F84’s INTCON register to answer the following questions: a) b) Provide a sequence of PIC16F84 instructions to enable external interrupts and timer overflow interrupts. (5%) Assuming the external and timer overflow interrupts have been enabled, and that an external interrupt has just occurred, show the value of the INTCON register at the start of the ISR (before any instructions in the ISR have been executed). (5%) Assuming the external and timer overflow interrupts have been enabled, and the timer overflow interrupt has just been serviced, show the value of the INTCON EECE 321 Computer Organization — Quiz 2 Page 3 of 4 register at the end of the ISR. immediately before executing the RETFIE instruction. (5%) d) Can we replace the RETFIE instruction at the end of an ISR with the RETURN instruction? Why or why not? (5%) (20%) 4. Orv eit a = \ Assuming A = +13.7625 and B =-5.25. a) b) Show how to represent A and B as 16-bit fixed-point numbers where the integer and fractional components are each represented using 8 bits. (6%) Show how A and B can both be represented using the single-precision, [EEE-754, floating-point format. (6%) Using floating-point arithmetic, show how the product A x B is computed. Show your work and the final result. (8%)