Introduction

When we write an ARM Assembly Language program, we can represent immediate operands $($constant values$)$ by prefixing a value with $\text{'}$#$\text{'}.$ For example, in the MOV instruction below, an immediate operand is represented in decimal $($base $10)$ using the syntax #$74.$

MOV R$1,$ #$74$

ARM Assembly Language programs are just strings of ASCII characters so the immediate operand #$74$ is just an ASCII string containing the characters $\text{'}$#$\text{'},\text{'}7\text{'}$ and $\text{'}4\text{'}.$

Part $1$: Decimal Immediate Operand to Value

Write an ARM Assembly Language program that, given a NULL$-$terminated ASCII string representing an immediate operand in decimal form, will calculate the value of the operand and store it in $R0.$ The input string starts in memory at the address in $R1.$ The string is terminated by the NULL character $($i$.$e$.$ ASCII character code $0).$

For example, given the string $"$#$74",$ your program should store the value $74$ in $R0.$

Part $2$: Decimal Immediate Operand to Hexadecimal String

Extending your program from Part $1,$ write an ARM Assembly Language program that, given a NULL$-$terminated ASCII string representing an immediate operand in decimal form, will create a new ASCII string in which the same immediate operand is represented in hexadecimal form. For example, given the string $"$ #$74",$ your program should create the new string $"$#$0$x$4$A$".$

Your program should store the new string in memory at the address in R$2.$ All alphabetic hexadecimal digits $(\text{'}A\text{'}..\text{'}F\text{'})$ must be UPPERCASE and the $\text{'}x\text{'}$ in the prefix $"0x"$ must be lowercase. Your program should also continue to store the value of the operand in R$0.$ The new

string should be NULL$-$terminated. The value represented in the new string should not contain leading zeros $($e$.$g$.$ the new string should be $"$#$0$x$4$A$"$ instead of $"$#$0$x$0000004$A$").$

Part $3$: Decimal to Hexadecimal in an ARM Instruction

Given a string containing a line from an ARM Assembly Language program, modify and extend your program from Part $2$ to create a copy of the original string in which any occurrence of an immediate operand in decimal form is replaced with its equivalent in hexadecimal form.

For example, given the string $*.$

"ADD R$8,$ R$8,$ #$90"$

Your program should create the new string $...$

"ADD R$8,$ R$8,$ #$0$x$5$A$"$

Note that the string might not end with the immediate operand and your program should be able to handle strings such as $...$

MOVR$9\frac{,}{\text{\#}}180$@$y=180$

The new copy of the string should again be stored in memory at the address in R$2.$

You can make the simplifying assumption that an occurrence of the character $\text{'}$#$\text{'}$ always represents the start of a valid immediate operand in decimal form. You can identify the end of a immediate operand by the occurrence of any character other than $\text{'}0\text{'}...\text{'}9\text{'}.$