Strings

A string is an array representing a sequence of characters. To store a string of n characters in your program, you need to set aside n+1 bytes of memory. This area of memory will contain the characters in the string, plus one extra special characterthe null characterto mark the end of the string. The null character is a byte whose bits are all zeros (0x00). The actual string consists of any group of characters, which none of them can be the null character.

QUESTION 1 (25 marks)

Draw a detailed flowchart and write an ARM assembly language program to concatenate two stings (STRING1 and STRING2) and store the result in a null terminated STRING3. Assume that the length of STRING1 + the length of

STRING2 255.

Your code should be highly optimized, i.e., use as little number of instructions as possible.

You may want to define the strings as follow:

STRING1 DCB This is a test string1 ;String1

EoS1 DCB 0x00 ;end of string1

STRING2 DCB This is a test string2 ;String

EoS2 DCB 0x00 ;end of string2 STRING3 space 0xFF

QUESTION 2 (35 marks)

Draw a detailed flowchart and write an ARM assembly language program to copy a null terminated STRING1 to a null terminated STRING2, after removing any occurrences of the word the in STRING1. I.e., if STRING1 is the woman and The man said the then STRING2 would become woman and The man said . However, if STRING1 is and they took breathe then STRING2 would become

and they took breathe without any change.

Your code should be highly optimized, i.e., use as little number of instructions as possible.

You may want to define the strings as follow:

STRING1 DCB and the man said they must go ;String1

EoS DCB 0x00 ;end of string1 STRING2 space 0xFF

QUESTION 3 (40 marks)

Draw a detailed flowchart and write an ARM assembly language function (subroutine) that takes a data value stored in register r0 and returns a value in r0 as well. The function returns y = a x² + b x + c where a, b, and c are signed integer parameters built into the function (i.e., they are not passed to it) and are defined in the memory using three DCD instructions. The subroutine also performs clipping, i.e., if the output is greater than a value d, it is clipped to d, where d is another parameter defined in the memory using a DCD instruction. The input in r0 is a signed integer binary value. Apart from r0, no other registers may be modified by this subroutine, i.e., if you want to use any register as a working register, you have to store its value in a safe place first prior changing it, and to restore this value before returning from the function.

After implementing the function, write an assembly program which stores a value in r0 and calls your function. Once the control is returned back from the function, the program will double the returned value and store this doubled value in r1. Your code should be highly optimized, i.e., use as little number of instructions as possible. Example1: if a = 5, b = 6, c = 7, d = 90, and r0 = 3, then the returned value in r0 should be 70 and the value in r1 will be 140. Example2: if a = 5, b = 6, c = 7, d = 50, and r0 = 3, then the returned value in r0 should be 50 and the value in r1 will be 100. Example3: if a = -5, b = 6, c = 7, d = 10, and r0 = 3, then the returned value in r0 should be -20 and the value in r1 will be -40.

Copyright 2016 Mahmoud El-Sakka.