Chapter 5

Question 1

What is the stack pointer?

Answer 1

It points to the top of the stack, that is, last occupied stack memory element

Question 2

What is the frame pointer?

Answer 2

It holds a stored copy of the stack pointer before it is changed to provide more storage

Question 3

When does the frame pointer actually get moved?

Answer 3

It stays in the same place until we move the stack pointer

Question 4

How do you access an item in an array?

Answer 4

1. Get the index that you’re looking to access
2. Multiply the index by the element size (the size of the variable type)
3. Add the frame pointer to the address of the array
4. Add the multiplied index

Question 5

What register is used for %sp?

Answer 5

%o6

Question 6

Which register is used for %fp?

Answer 6

%i6

Question 7

What is the difference between “the stack” and a stack machine?

Answer 7

Although they are both first-in-last-out data structures, a stack machines uses popping and pushing to perform arithmetic and logical operations on the top two items. The stack does not.

Question 8

How does the stack work with memory allocation, and how does it grow?

Answer 8

It grows downwards from the top of memory. When we allocate space for our use, we subtract from the stack pointer, moving the %sp to a new address, and keeping the %fp at the old location of the %sp

Question 9

How do we keep the %sp double word aligned?

Answer 9

By making sure it’s divisible by 8

Question 10

How much memory do we need for our registers?

Answer 10

92 bytes

Question 11

How does the code work for chopping?

Answer 11

We start with our -92 bytes for registers, subtract the amount of memory we need for any automatic variables, and then we chop it with “& -8”

Question 12

How does chopping work?

Answer 12

Chopping is a bitwise-and operation; for example, if you wanted chop something that is halfword aligned, you perform a bitwise-and operation with the number you wish to chop (in binary) and two (in binary). The result is your halfword-aligned address

Question 13

What does the save instruction do?

Answer 13

The save instruction both performs addition and saves the content of the stack pointer is %fp.

Question 14

What are the six load instructions?

Answer 14

ldsb: load signed byte
ldub: load unsigned byte
ldsh: load signed halfword
lduh: load unsigned halfword
ld: load
ldd: load double

Question 15

If you wanted to load a first variable, called a0, into %l1, what instruction would you use?

Answer 15

ld [%fp - 4] %l1

Question 16

If you wanted to load a first variable, called a1, into %l1, what instruction would you use?

Answer 16

ld [%fp-4] %l1

Question 17

How would you store a new value back into the first variable from %l3?

Answer 17

st %l3 [%fp - 4]

Question 18

How does the ifelse macro work?

Answer 18

It takes four arguments. It compares the first two: if they are equal, it returns the third argument. Otherwise, it returns the 4th.