Systems Architecture

Question 1

Does ARM assembly support a RISC or CISC instruction set?

Answer 1

RISC

Question 2

What is the purpose of the CPSR?

Answer 2

To record anomalous arithmetic outputs

Question 3

What are the flags for the CPSR?

Answer 3

N bit for negatives
Z bit for zero results
C bit for a carry
V bit for negatives

Question 4

Which registers can the programmers us?

Answer 4

R0-R7

Question 5

Which register should contain the output of a function?

Answer 5

R0

Question 6

What is an unaligned transfer?

Answer 6

Reading 4 bytes across more than one 4 byte region

Question 7

What does the stack store?

Answer 7

Variables and function arguments

Question 8

What does the heap store?

Answer 8

Data allocation during runtime

Question 9

What does the global data area store?

Answer 9

Global variables

Question 10

What does BSS store?

Answer 10

Uninitliased global variables

Question 11

What does the text segment store?

Answer 11

Machine code instructions

Question 12

Define pipelining

Answer 12

A method to improve efficiency where multiple fetch-decode-execute cycles are run at the same time with each using a different component

Question 13

Define program relative addressing

Answer 13

An addressing mode where an isntruction specifies an address relative to the current program counter

Question 14

What is the effective address?

Answer 14

The PC + an offset

Question 15

What does a compare instruction do?

Answer 15

Updates and evaluates the value of the bits in the CPSR and then discards them

Question 16

What should we always ensure when returning from a function?

Answer 16

LEAVE NO TRACE

Question 17

What is the first convention for writing good functions?

Answer 17

We should only use r0-r3 to pass parameters to functions

Question 18

What do you do if you have more than 4 parameters to pass to a function?

Answer 18

Allocate memory from the heap to a pointer and then store those pointers in r0-r3

Question 19

What does the link register store?

Answer 19

The return address

Question 20

What should we do to the stack pointer when we call a function to ensure we have enough memory?

Answer 20

Decrement the value of the stack pointer so that the stack grows into the unused memory space

Question 21

What is the second convention for writing good functions?

Answer 21

We must preserve the contents of registers

Question 22

What does it mean to preserve registers?

Answer 22

Registers must be restored after a procedure call and if we avoid using these regsiters than we avoid register spilling

Question 23

What is register spilling?

Answer 23

The contents of registers spilling into main memory

Question 24

What is the third convention for writing good functions?

Answer 24

Push contents of registers to the stack before your function and pop them afterwards to restore them

Question 25

How do we ensure that assembly instructions stay in place during compilation?

Answer 25

Compiling with no optimisation

Question 26

Which command allows inline assembly and for assembly code to be embedded into c?

Answer 26

asm

Question 27

What is a clobbered register?

Answer 27

A CPU register that is overwritten by the assembly code but not used as an explicit output operand in the GCC inline assembly syntax

Question 28

Why are clobbered register important?

Answer 28

We need to explicitly tell the compiler that the contents of the register has changed otherwise it may assume the value remains the same and use it as such

Question 29

What does the preprocessor do?

Answer 29

Deal with macros such as directives and replaces macros throughout the program Outputs what is known as pure C code

Question 30

What does the compiler do?

Answer 30

Transforms C into assembly code Dependent on machine architecture

Question 31

What does the assembler do?

Answer 31

Outputs machine code

Question 32

What does the linker do?

Answer 32

Combines several object files together

Question 33

What is the difference between dynamic and static linking?

Answer 33

Static linking embeds the additional functions into your program where as dynamic linking links your program with the file containing the other functions whilst keeping the two files separate

Question 34

What is an exception?

Answer 34

Any unexpected change in the flow control

Question 35

What is a synchronous exception?

Answer 35

Caused by an instruction in the running program and includes things like arithmetic exceptions, invalid memory addresses

Question 36

What is an asynchronous exception?

Answer 36

Caused by an IO device requesting the CPU and is known as a hardware interrupt

Question 37

What does the CPU do when it receives an interrupt?

Answer 37

It will create an exception interrupt which is a special function called by the CPU. It will save the current state of the CPU and then change the code execution to the exception handler to deal with the interrupt. It does this by using a vector table that contains a branch instruction to the exception handler