The Stack

Question 1

ARMv8 uses ______ addresses

Answer 1

64-bit

Question 2

The ARMv8 can address ____ bytes of memory

Answer 2

2^64

Question 3

The virtual memory space is mapped to physical memory by __ and ________

Answer 3

OS and Hardware

Question 4

A register can be loaded with _,_,_ or _ byte data from RAM Where is data placed? If necessary, higher order bits are?

Answer 4

1,2,4, and 8 Data is placed into the lower-order bits Sign-extended if loading signed data (1 or 0) Zero-extended if loading unsigned data (0 only)

Question 5

Which kind of data can be stored into RAM?

Answer 5

byte, halfword, word, or doubleword Low-order bit of a register when the data is just a part of a register

Question 6

Which part of a register can be stored in RAM when the data is just a part of the register?

Answer 6

Low-order bit (Right most bit) of a register

Question 7

What is stack memory?

Answer 7

It is the space in RAM provided by the OS to store data for functions

Question 8

What happens when a function is called?

Answer 8

A stack frame is pushed onto the stack which holds the function’s parameters, local variables, local variables, and return values The frame is popped when the function returns

Question 9

The stack uses ____ memory

Answer 9

high

Question 10

The stack grows _________

Answer 10

backwards (toward 0) low OS Program

Question 11

Where are programs loaded?

Answer 11

Into low memory, right above the space reserved for the OS

Question 12

The ____ is used for dynamically allocated memory in a program It is Done in C using ______( ) and ______( )

Answer 12

heap malloc freeze

Question 13

Where does the SP register point and what happens to it when the stack grows?

Answer 13

Points to the top of the stack Is decremented when the stack grows

Question 14

How is stack memory allocated by a program?

Answer 14

sub sp,sp,16 16 is the bytes needed

Question 15

Why should the stack be quadword aligned?

Answer 15

The address in SP must be evenly divisible by 16 May need to allocate more space than actually needed

Question 16

To guarantee alignment:

Answer 16

Add a negative number instead of subtracting Clear the low 4 bits of this number: AND it with -16 (1111 … 11110000)

Question 17

To allocate 20 bytes:

Answer 17

add sp,sp,-20&-16 Actually allocates 32 bytes

Question 18

What is the FP used for

Answer 18

To point to local variables in a stack frame

Question 19

Stability of FP and SP

Answer 19

FP is stable, once set at the beginning of a function SP is unstable as it is allowed to change as the function executes

Question 20

A ___________ is pushed onto the stack when entering a function and it should be ________ long

Answer 20

stack frame 16 bytes It is created by the pre-increment part of the stp instructions main: stp x29,x30,[sp,-16]! mov x29, sp …

Question 21

What does the stp do? What’s this part of the stack frame called?

Answer 21

Stores the contents of x29 (FP) and x30 (LR) to these 16 bytes This part of the stack frame is called the frame record

Question 22

What does mov do?

Answer 22

Sets x29 (FP) to point to the frame record

Question 23

What does the ldp do?

Answer 23

negate the number used in stp to pop the stack frame when function returns to calling code It also restores x29 and x30 from main ( )’s frame record

Question 24

How are stack variables created?

Answer 24

By allocating extra space in the stack frame when entering a function (in addition to frame record’s 16 bytes)

Question 25

How are stack variables addressed?

Where are addresses specified?

Answer 25

Question 26

Many architectures require aligned memory access so an n-byte unit must be at an address evenly divisible by n and pad bytes are inserted to ensure this

On ARMv8, the exceptions for unaligned memory access are:

Answer 26

Addresses in SP must be quadword aligned
Frame record must be at an address divisble by 16 (otherwise, bus error)

Machine instructions must be word aligned (.balign 4)

Exclusive load/store accesses must be aligned

Question 27

Load Instructions

Answer 27

Question 28

Store Instructions

Answer 28

Question 29

The following are possible for basic load/store instructions:

Answer 29

Question 30

M4 macros are used for offsets to _______________

Can also be done with _____________

____________ are useful for renaming x29 and x30 to FP and LR

Answer 30

improve readability (e.g. define(a_s, 16))

assembler equates(e.g. a_s = 16)

register equates (e.g. fp .reqx29)

Question 31

How are local variables declared in C?

Answer 31

in a block of code { … }

Any inside {} are local to that

Question 32

How are local variables implemented in assembly?

Answer 32

As stack variables