Strings

Question 1

true/false

strings should always have a value equal to the total number of cells declared

Answer 1

false, strings may have an actual value less then the total number of cells declared

Question 2

string instructions and meaning (5)

Answer 2

1. LODS - load string
2. STOS - store string
3. MOVS - move string
4. CMPS - compare string
5. SCAS - scan string

Question 3

required operand/s for LODS

Answer 3

source

Question 4

required operand/s for STOS

Answer 4

destination

Question 5

required operand/s for MOVS

Answer 5

src and dest

Question 6

required operand/s for CMPS

Answer 6

src and dest

Question 7

required operand/s for SCAS

Answer 7

dest

Question 8

what register/s do string instructions use as operand/s?

Answer 8

rsi (src) and rdi (dest)

Question 9

true/false

string instructions automatically increment/decrement the index registers used by them

Answer 9

true

Question 10

controls the direction of string processing

Answer 10

direction flag

Question 11

in what direction will string operations proceed if the direction flag is clear (DF = 0)?

Answer 11

forward (head to tail)

Question 12

instructions to explixitly manipulate the direction flag (2)

Answer 12

std: set DF (DF = 1)
cld: clear DF (DF = 0)

Question 13

MOVS format

Answer 13

movs dest_str, src_str

Question 14

a prefix accepted by string instructions to repeatedly execute the operation

Answer 14

repetition prefix

Question 15

prefix categories (2)

Answer 15

1. unconditional repetition
2. conditional repetition

Question 16

unconditional repeat prefix

Answer 16

rep

Question 17

rep repeats the instruction until ____

Answer 17

the value of the rcx register reaches 0

Question 18

repeats the instruction until either the rcx register reaches 0 or the zero flag (ZF) is equal to 0

Answer 18

repe/repz

Question 19

repeats the instruction until either the rcx register reaches 0 or the zero flag (ZF) is not equal to 0

Answer 19

repne/repnz

Question 20

copies the value from the source string (rsi) to the a register

Answer 20

LODS

Question 21

copies the value from the A register to the destination string (rdi)

Answer 21

STOS

Question 22

what will this block of code do?

mov rcx, qword[strlen]
mov rsi, string1
mov rdi, string2

loop1:
    lodsb
    stosb
    loop loop1

Answer 22

it copies the content of string1 into string2

Question 23

compares the two strings at RSI and RDI and sets the flags

Answer 23

CMPS

Question 24

true/false

CMPS performs RDI - RSI and sets the flags according to the comparison made

Answer 24

false, it should be RSI - RDI

Question 25

useful in searching for a particular value or character in a string

Answer 25

SCAS