Due date: See Class Webpage

Observe the Emory Honor Code when doing this (and all other) homework.

0. Preparations

• To do this project, you must obtain a copy of the prepared files with EGTAPI

  Use project code hw9

  You will have create a hw9 folder containing these program files:

  hw9.s Skeleton assembler program for HW9

1. Purpose

This homework continues with the ARM programming and re-inforces:

Accessing data structures: retrieve values from arrays and linked lists Arithmetic operations (+, - and * operations)

In depth understanding about (1) how a computer accesses variables (especially structured varaibles) in memory and (2) using registers in computations forms the basis of assembler programming.

To get you going with programming in ARM, I have provided the skeletal program hw9.s. In the preparation step, you have made a copy of this file into your CS255 project directory.

The skeletal program hw9.s has 10 sections, labeled as Q1, Q2, ..., Q10 corresponding to questions 1 through 10 (see below).

Write the assembler instructions the each answer in the corresponding section of the hw9.s file.

The variables definition section of the skeletal program hw9.s contains the following "high level language" variable:

byte ans_b; // Variables containing answers to the questions short ans_s int ans_i byte b = 2; // Variables used as index to arrays short s = 3; int i = 4; // 3 arrays byte A[] = {11, -22, 33, -44, 55, -66, 77, -88, 99, -123}; short B[] = {111, -222, 333, -444, 555, -666, 777, -888, 999, -5432} int C[] = {1111, -2222, 3333, -4444, 5555, -6666, 7777, -8888, 9999, -9876} // One linked list class list { int value1; // 4 bytes short value2; // 2 bytes short value3; // 2 bytes list next; } list head; // The linked list has been set up

These variables are defined in assembler as follows:

************************************** * Variables used to display output ************************************** ans_b: .skip 1 * In Java: byte ans_b; ans_w: .skip 2 * In Java: short ans_b; ans_l: .skip 4 * In Java: int ans_b; **************************************** * Indices used to select array elements **************************************** i: .byte 2 * In Java: byte i = 2; j: .2byte 3 * In Java: short w = 3; k: .4byte 4 * In Java: int k = 4; **************************************** * 3 arrays of 10 elements **************************************** A: .byte 11, -22, 33, -44, 55, -66, 77, -88, 99, -123 B: .2byte 111, -222, 333, -444, 555, -666, 777, -888, 999, -5432 C: .4byte 1111, -2222, 3333, -4444, 5555, -6666, 7777, -8888, 9999, -9876 Comment: A is an array of 10 bytes, Comment: B is an array of 10 shorts and Comment: C is an array of 10 integers. ******************************************************** * One linked list starting at head * * head ->[1234,89,-11]->[2345,78,66]->[-3456,67,44] * ->[4567,56,77]->[5678,45,44] ******************************************************** head .4byte list1 Comment: head points to the first element of a linked list. The linked list has been set up already. Each linked list element has the following structure: class list { int value1; // 4 bytes short value2 // 2 bytes short value3 // 2 bytes list next; // points to the next list element }

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The homework assignment questions will ask you to:

Retrieve the value from elements in one of the arrays or in the linked list Then store the retrieved value into one of the variables ans_b, ans_w or ans_l

Be very careful and use the correct operand size.

2. Questions

Write ARM assembler instructions to do the following:

Q1: ans_s = B[5]; (ans_s: -666) Q2: ans_b = A[3]; (ans_b: -44) Q3: ans_i = C[j]; (ans_i: -4444) Q4: ans_i = B[k - j]; (ans_i: -222) Q5: ans_s = C[i + j]; (ans_s: -6666) Q6: ans_i = A[k] - B[k]; (ans_i: 55 - 555 = -500) Q7: ans_s = B[A[i] - 30]; (ans_s: B[33 - 30] = B[3] = -444) Q8: ans_s = B[18]; (ans_s: 5555, can you explain why ?) Q9: ans_b = head.value2; (ans_b: 89) Q10: ans_s = head.next.next.value3; (ans_s: 44)

3. Hint on Debugging

The EGTAPI tool is designed to help you debug an assembler program with ease....

The following is a general guideline for debugging assembler programs:

First you must run EGTAPI and get to the part of the program containing a bug To accomplish this: Use the Run button to get to the breakpoint right before the problem spot in the program. (Do not run the program upto the problem spot, it will be too late !!!) When you have reached the place just before before the program spot with a bug, you must use the Single Step button to execute one instruction at a time !!! Check the outcome of the execution: Check the value in the registers or variables and verify whether the result is the one that want to achieve. (This of course assumes that you know what the correct asnwer should be....)

(More information on debug: read the Egtapi documentation)

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A concrete Example:

Suppose see (in the variable display window of Egtapi) that your answer for question Q6 is wrong.... Start EGTAPI and (re)-load your program if you have not already done so... Press Run on EGTAPI until you have reach the start of your solution for Q6. That means you should stop pressing "Run" when EGTAPI has reached the label Q6 !!! When you have reached the first instruction for your assembler code for Q6, you must use the Step button to execute your code one instruction at a time (the program will stop after executing one instruction). Egtapi will display the outcome of the execution of each instruction You can now find the bug (= the instruction(s) that did something wrong) by: verifying the result of the instruction against the expected result that you want the instruction(s) to do If the result is NOT what you expected, you must have used the wrong instruction. Try to figure out what went wrong and use the correct instruction or instructions to achieve the correct result. (Like I said before, you would need to know what you want the instruction to achieve first before you can find your bugs.... So, an essential part of "debugging" requires that you know what you want to achieve...).

4. Turn in

• Use EGTAPI to turn in your hw9.s program file with assignment code hw9

5. Extension request

• Use EGTAPI to request an extension with the assignment code hw9

  You must make the extenion request before the deadline of the homework

  Your request will be successful if you have not exceeded the maximum number of "free" (no-questions-asked) requests allowed