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Saint Louis University

Computer Science 115
Introduction to Computer Science
Michael Goldwasser

Fall 2004
Dept. of Math & Computer Science

Assignment 05

Architecture, Low-Level Programming

Contents:

Overview

Topic: Computer Architecture, Low-Level Programming with the Super Simple CPU from Meyer's book.
Related Reading: Ch. 5, Notes on the Super Simple CPU (abbreviated SSCPU)
Due: 8pm Monday, 11 October 2004

Internet Requirements

For this assignment, we will rely at times on the Simple Super CPU software demonstrated in class. You will need to have an Internet connection to run this software.

Practice Problems (Answers appear at end of this file)

  1. Exactly how many bytes are in 128MB?

  2. Exercise 3 of Lab 5 (p. 65 of Meyer)

  3. What SSCPU instructions do NOT involve the accumulator?

  4. Translate the following machine code to assembler.  Check using the SSCPU.

    0100000000000111
    0101000000000101
    0100000000000011
    0001000000000101
    1111000000000000
     

  5. Play computer with the program above.  Assume the remaining memory cells are initialized to 0.  Keep columns for the instruction number, its corresponing opcode mnemonic and operand in decimal, the accumulator value after the instruction executes, and the value of memory cell 5 after the instruction executes:

    # Instruction  Accumulator Mem 5

  6. Translate the following assembler program into machine language.  Check with the SSCPU. 

    GO  INP
    SUB VAL
    JZR BYE
    JMP GO
BYE STP
VAL DAT 21

  7. Play computer with the program below, assuming 2 is the input.  Keep columns for the instruction number (or label), its corresponing opcode mnemonic and operand in decimal, the accumulator value after the instruction executes, and the value of the data N and P after the instruction executes: 

    0   LDI 3
1   STO P
2   INP
3   STO N       
TOP JZR BYE       ; this is line 4
5   LOD P
6   ADD P       
7   STO P
8   LOD N
9   SUB ONE       
10  STO N
11  JMP TOP
BYE STP
ONE DAT 1
N   DAT 0
P   DAT 0
    # Instruction  Accumulator N P

Problems to be Submitted (20 points)
  1. (4 points)

    What is the largest number of 16-bit memory cells a SSCPU could use without having to change the instruction formats at all? Explain your answer.

  2. (4 points)

    Suppose the assembly program below is run and the sequence of instruction numbers executed is  0, 1, 2, 4.  What number was input in the first instruction?  Explain your answer. 

    GO  INP
    SUB VAL
    JZR BYE
    JMP GO
BYE STP
VAL DAT 21

  3. (2 points)

    Write a sequence of machine code instructions to copy the data at memory location 13 to the output.

  4. (6 points)

    Write an assembler program to input a number and output that number, multiplied by five. For example if 10 binary were input, 1010 binary would be output just before the program stops.

    Of course, the SSCPU instruction set does not have an operation for multiply. So you will need to rely on repeated use of the ADD instruction. This might be done with or without a loop.

    You may assume the program is reloaded, reinitializing all of memory, every time it is run.

  5. (4 points)

    At the end of Ch. 7 there are a series of "Thought Questions" (p. 233). Choose one of questions #3, #4 or #5 to answer. The length of your answer should be appropriate for the question, however I envision answers in the range of 1/2-page to 1-page.

Overall, please type your answers to all of the problems in a single document to be submitted electronically. Please see details about the submission process.

Extra Credit (4 points)
  1. (2 points)

    The SSCPU has no instructions for multiplication, division or finding a remainder from division.  These operations can be done using the existing instructions, by doing addition or subtraction in a loop.  Write an assembly-language program which inputs positive numbers D and N and outputs the remainder of the dividing of D into N.  For example if 5 and 15 were input, the output would be 0.  If 7 and 19 were input, the output would be 5.  (Of course all inputs and outputs are actually in binary.)  Hint:  Do this by repeated subtraction.  If the remainder is 0, you will eventually end up with 0.  If the remainder is not 0, you will probably need to subtract until the result is negative.  In this case add D at the end to get the actual remainder.  For the second example:  19-7=12, 12-7 = 5, 5-7 = -2, so add 7 back to get the remainder 5

  2. (2 points)

    Write an assembly-language program which inputs a positive number N and outputs 1 + 2 + 3 + ... + N, the sum of all the integers from 1 to N.  For example, if 101  binary (5 decimal) were input, the output should be 1 + 2+ 3 + 4 + 5 = 15 decimal or 1111 binary.  Hint: part of the challenge will be fitting your program into the limited memory of SSCPU. Try adding in the order from N down to 1.

Practice Problem Solutions

  1. 128 x 220 = 128 x 1048576 = 134,217,728 bytes

  2. part 6) The first, LDI, loads the accumulator with the data in the operand, 10, while LOD loads data from memory location 10, which in this case is 7.

    part 7) Many machinecodes do have a store immediate, but in this CPU there would have to be a special operand format where the 12 bits for the operand contains both the memory address and the immediate data.  Since 4 bits are needed for the memory address, that would leave only 8 bits for immediate data.  The effect of immediate storing can be obtained from a sequence of two instructions, one loading the immediate value, and the second storing in to the desired memory address.

  3. STP, JMP

  4. 0  LDI  7
    1  STO  5
    2  LDI  3
    3  ADD  5
    4  STP  0

  5. # Instruction Accumulator Mem 5
    0   LDI 7 7 0
    1   STO 5 7 7
    2   LDI 3 3 7
    3   ADD 5 10 7
    4   STP 10 7

  6. 0110000000000000
    0010000000000101
    1010000000000100
    1000000000000000
    1111000000000000
    0000000000010101

  7. # Instruction Accumulator N P
     0
     1
     2
     3
     4
     5
     6
     7
     8
     9
    10
    11
     4
     5
     6
     7
     8
     9
    10
    11
     4
    12    		 LDI  3
    STO 15
    INP 0 
    STO 14
    JZR 12
    LOD 15
    ADD 15
    STO 15
    LOD 14
    SUB 13
    STO 14
    JMP 4 
    JZR 12
    LOD 15
    ADD 15
    STO 15
    LOD 14
    SUB 13
    STO 14
    JMP 4 
    JZR 12
    STP 0     		  3
     3
     2
     2
     2
     3
     6
     6
     2
     1
     1
     1
     1
     6
    12
    12
     1
     0
     0
     0
     0
     0    		 0
    0
    0
    2
    2
    2
    2
    2
    2
    2
    1
    1
    1
    1
    1
    1
    1
    1
    0
    0
    0
    0    		  0
     3
     3
     3
     3
     3
     3
     6
     6
     6
     6
     6
     6
     6
     6
    12
    12
    12
    12
    12
    12
    12

CSA-115, Fall 2004
Michael Goldwasser
goldwamh at our university domain

Last modified: Wednesday, 13 October 2004
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