Subroutines

Question 1

What is the equivalent of a subroutine in java?

Answer 1

A method

Question 2

What types of subroutines are there?

Answer 2

Open(inline) & closed

Question 3

What does an open(inline) subroutine do

Answer 3

• Code is inserted inline when subroutine is invoked
  
  • done using a macro preprocessor instead of cutting & pasting
• arguments are passed in/out using registers
• efficient since overhead of branching & returning is avoided
• There are many many copies of your subroutine

Question 4

What does a closed subroutine do?

Answer 4

• Only one copy in RAM (machine code appears only once in RAM) so more compact
• when invoked, control branches to the first instruction of routine (PC is loaded w/address of first instruction instead of being incremented each line like in regular code
• When finished, control returns to next instruction in calling code (PC is loaded w/return address)
• Arguments are placed in registers or on the stack
• Slower than open routines bc of the call/return overhead

Question 5

What should a subroutine not do?

Answer 5

Change the state of the machine for the calling code
- when invoked, it should save any registers it uses on the stack
- When it returns, it should restore the og value of the registers

Question 6

What type of variable are arguments to the subroutine considered?

Answer 6

Local. The subroutine may change their value

Question 7

What does $ mean

Answer 7

Argument

Question 8

Which type of subroutine is usually implemented using macros?

Answer 8

Open

Question 9

Which type of subroutine uses labels (the kind we used for the assignment)?

Answer 9

Closed

Question 10

Which subroutine type is alloc used for and why?

Answer 10

Closed. It is the # of bytes that the subroutine uses and is negated to allocate for the subroutine’s stack frame

Question 11

What is the minimum number of bytes a closed subroutine can use?

Answer 11

16

Question 12

What pseudo-op must we use for a closed subroutine?

Answer 12

.balign 4

Question 13

When we invoke a subroutine with bl, where is the return address stored?

Answer 13

In the link register. The return address is PC +4 which points to the instruction after bl

Question 14

When we use ret after a subroutine, where is the control transferred to?

Answer 14

Calling code. The address stored in the lr x30

Question 15

What do the stp instructions do for each subroutine?

Answer 15

Create a frame record for each function’s stack frame. This safely stores the lr in case it’s altered by a bl in the body of the function

Question 16

What is a stack frame?

Answer 16

A section of memory allocated for a single function call.

Question 17

What is a frame record?

Answer 17

A data structure associated with a single stack frame about the context of the function’s execution.

Question 18

The __ and the stored __ ____ in the frame records form a ___ ___

Answer 18

FP, FP values, linked list

Question 19

If a called function uses any of the registers x__ to x__, it must save their data to the stack at the beginning of the function and then restore at the end. What are these registers called?

Answer 19

19, 28

callee-saved

Question 20

The callee can also use registers x__ to x__ and these are not saved/restored by the callee. What is the only time it is safe to use these?

What are these registers called?

Answer 20

9, 15

In between function calls

Caller-saved

Question 21

How many arguments can be passed into a function without doing extra work?

Can the subroutine overwrite these registers?

Answer 21

8 using registers x0-x7

These can be overwritten (contents not preserved over a function call)

Question 22

Which types of numbers use w registers?

Answer 22

Chars, Short Ints, Ints

Question 23

What types of numbers use x registers?

Answer 23

Long ints

Question 24

Which register numbers (x and w) is a subroutine free to overwrite? Register contents are not preserved over a function call. Caller-saved

Answer 24

x0-x18, w0-w18

Question 25

Pointer arguments: In calling code, the _____ of a variable is passed to the subroutine.

Answer 25

Address

• This assumes the variable must be in RAM, not in a register
• The called subroutine dereferences the address of the variable to manipulate the variable being pointed to (usually with a ldr or str instruction)

Question 26

How does a function return a structure if it is too large to fit in a register?

Answer 26

The calling code provides memory on the stack to store it. The address of this memory is put in x8 before the function call

Question 27

What is x8?

Answer 27

The indirect result location register. A called subroutine writes to a memory at this address, using x8 as a pointer to it.

Question 28

What are leaf subroutines?

Answer 28

Subroutines that don’t call any other subroutines

Question 29

What are some things that happen/don’t happen in a leaf subroutine?

Answer 29

A frame record is not pushed onto the stack
- The routine doesn’t do a bl so the LR won’t change and we don’t need to save
- The routine doesn’t call a subroutine so FP won’t change
- No need for stp/ldp

Question 30

What happens to the stack frame if a leaf subroutine only uses registers x0-x7 and x9-x15?

Answer 30

A stack frame is not pushed at all. No need to save/restore registers and no stack variables are used

Question 31

What happens to arguments 9+?

Answer 31

Not passed on the stack. The calling code allocates mem at the top of the stack and writes “spilled” arguments there

Question 32

How many bytes are allocated for each spilled argument?

Answer 32

8 bytes

Question 33

How does the callee read spilled arguments?

Answer 33

Using an offset