Separate Compilation & ASCII

Question 1

What is relocatable object code?

Answer 1

Means you can put machine instructions anywhere

Question 2

How is source code divided?

Answer 2

Into several modules (compilation units) like separate .c or .s files which can be compiled into relocatable object code.

Question 3

Describe the process of separate compilation and linking

Answer 3

1. Modules (compilation units) are compiled into ROC (put in corresponding .o files)
2. Object code is linked together to create an executable
   - this is done by a loader ld (invoked by gcc)
   - ld resolves any references to external data or functions in other modules
   - code & data are relocated in mem as necessary to form contiguous text, data & bss sections

Question 4

What does ASCII stand for and what does it do?

Answer 4

American Standard Code Information Interchange has 128 characters

encodes characters using 7 bits, stored in a byte. The 8th bit may be used to extend ASCII to include characters beyond english

Question 5

In assembly, how are character constants denoted?

Answer 5

With hex code: mov w19, 0x5A

OR

The character in single quotes (better but may interfere w/m4): mov w19, ‘Z’

Question 6

True or false: these both initialize a string:

.byte ‘c’, ‘h’, ‘e’, ‘e’, ‘r’, ‘s’

.ascii “cheers”

Answer 6

True. A string is an array of characters.

Question 7

In C, strings are null-terminated. What does this mean?

Answer 7

Strings end with \0

Question 8

How to indicate null-termination in assembly?

Answer 8

Use two pseudo-ops:
.ascii “cheers”
.byte 0

OR

Use .asciz or .string:
.string “cheers”

Question 9

What is a string literal?

Answer 9

• A read-only array of characters allocated in the text section. A string literal is immutable.
• In c code, delimited with “…”

Question 10

What section is a string stored in?

Answer 10

.data

Question 11

What does the label for a string literal represent?

Answer 11

The address of the first character in the array
- this address can be passed as a pointer argument into a function using an x register:
Adrp x0, fmt

Question 12

How are external arrays of pointers created?

Answer 12

With lists of labels

Question 13

Command-line arguments allow you to pass values from the ____ into your program

Answer 13

Shell

Question 14

In this C code, what are argc and argv?
main(int argc, char *argv[])

Answer 14

argc: the number of arguments

argv: an array of pointers to the arguments
- short for argument vector
- represented as strings