Question

Explain steps also please. Thanks.

- We want to implement "if (m != n) m--;" in LEGv8.

Suppose m and n are in X20 and X21, respectively. What should we use in the blank in the following code segment?

SUB X9,X20,X21

_____ EXIT

SUBI X20,#1

EXIT:

- Suppose X20 contain the decimal value 5 and X21 contains the decimal value -5. Which condition codes are set to 1 after the following instruction is executed?

ADDS X9,X20,X21

- We want to implement "if (m != n) m--;" in LEGv8.

Suppose m and n are in X20 and X21, respectively. What should we use in the blank in the following code segment?

SUB X9,X20,X21

_____ EXIT

SUBI X20,#1

EXIT:

Answer 1

Part 1:

SUB X9,X20,X21

CBZ X9 EXIT

SUBI X20,#1

EXIT:

Knowledge behind above:

CBZ register, L1    if (register == 0) branch to instruction labeled L1;

CBNZ register, L1 if (register != 0) branch to instruction labeled L1;

Here, we will exit only when m==n otherwise we will do m--. So, we used here CBZ command.

Part 2:

The Z flag is set if the result of the flag-setting instruction is zero.

Here,

ADDS X9,X20,X21 result in X9 as 0

So, Z flag set to 1.

Knowledge behind above:

Condition codes, set from arithmetic instruction

negative (N): result had 1 in MSB

zero (Z): result was 0

overlow (V): result overflowed

carry (C): result had carryout from MSB

Part 3:

It is same as part 1. So, same answer as in part 1 can be used.

Hope it helps.

Thank you!