Week 12: Block move, Block move encoding/decoding, Subroutine call/return, and summary

Question 1

The ARM block move instructions are

Answer 1

LDM and STM

Question 2

STM does what?

Answer 2

Push a group of registers’ contents to memory

Question 3

LDM does what?

Answer 3

Pop values from memory and load them to a group of registers

Question 4

What is the syntax of using STM or LDM?

Answer 4

ADR r0, DATA

STMIA r0!, {r1 - r3, r5}

Question 5

What indicates auto indexing for LDM and STM?

Answer 5

The ! exclamation mark:

STM r0!
LDM r0!

Question 6

Arm STM and LDM instructions always store the ___ first and in _____

Answer 6

The lower register number in the LOWEST memory address first and the higher register number in the HIGHEST memory address first.

Question 7

What do the following mean?

FD
IA
IB
DA
DB
ED
EA
FA

Answer 7

FD = Full Descend
IA = Increment After
IB = Increment Before
DA = Decrement After
DB = Decrement Before
ED = Empty Descend
EA = Empty Ascend
FA = Full Ascend

Question 8

What are the load/store suffixes of the following stack types?

FD
FA

ED
EA

Answer 8

FD (Full Descend)
Load: IA
Store: DB

FA (Full Ascend)
Load: DA
Store: IB

ED (Empty Descend)
Load: IB
Store: DA

EA (Empty Ascend)
Load: DB
Store: IA

Question 9

FD, FA, ED, and EA are examples of

Answer 9

Pseudo notations

Question 10

The difference between ARM’s block move instructions are:

Answer 10

The direction (up/down) and whether the pointer points to free memory or not

Question 11

How does the “register list” portion of a block move instruction work?

Answer 11

From right to left, every bit that is a 1 is included in the register list from r0 - r15

Question 12

What is a subroutine?

Answer 12

A set of instructions that may be repeatedly called by a program to do a given function

Question 13

What are the two main characteristics in any subroutine?

Answer 13

1. A subroutine can be called from anywhere in the program

2. Once the subroutine is completed, it should return to the instruction directly AFTER the subroutine calling location

Question 14

What is ARM’s support for Subroutines?

Answer 14

BL, branch with link automatically saves the return address in register 14

Question 15

BL does what on top of branching?

Answer 15

The processor also copies the return address into R14 (the link register)

Question 16

To return from a subroutine call, you simply

Answer 16

Copy the return address into the PC

Question 17

A subroutine call can also be implemented by means of

Answer 17

Pushing the return address onto the stack and branching to the target address, then returning by popping the return address from the stack and copying the address into the PC

Question 18

If the routine is not a leaf routine, you cannot

Answer 18

Call another routine without first saving the link register

Question 19

To save register data before a subroutine call, what must be done?

Answer 19

You must save the information in the registers by pushing the information onto the stack by means of a STM and then afterwards pop them off by means of a LDM

Question 20

If you are using a block move to restore registers from the stack, you can also include the

Answer 20

Program Counter

Question 21

If you plan on including the program counter with the STM and LDM, what must be done?

Answer 21

You must ensure the return address is saved in r4, the LR