I am having some trouble with the following question over ARMv8 assembly. The ma

Question

I am having some trouble with the following question over ARMv8 assembly. The main area I am not understanding are the last two lines. Thank you.

What value is left in X0 after the following code is run (Remember ARMv8 is big endian)?

ORR X1, XZR, 0x1234

ORR X2, XZR, 0x4

ADD X10, XZR, XZR

ADD X0, XZR, XZR

STUR X1, [X2, 0]

LDURB X0, [X2, 6]

Note, LDURB operates on bytes instead of words.

What value is left in X0 after the following code is run (Remember ARMv8 is big endian)?

ORR X1, XZR, 0x1234

ORR X2, XZR, 0x4

ADD X10, XZR, XZR

ADD X0, XZR, XZR

STUR X1, [X2, 0]

LDURB X0, [X2, 6]

Note, LDURB operates on bytes instead of words.

Explanation / Answer

XZR register is a zero register

ORR X1, XZR, 0x1234        // X1 gets XZR OR   0x1234

ORR X2, XZR, 0x4             //X2 gets XZR   OR   0x4

ADD X10, XZR, XZR           // X10 general purpose register get XZR + XZR

ADD X0, XZR, XZR

STUR X1, [X2, 0]              //   stores 8bit Bt value [BASE + SIMM9] i.e.,     [X2+0] gets the value of X1 which has OR value from 1st instruction

LDURB X0, [X2, 6]        // loads [base+ simm9] value to X0 i.e.,   X0 gets value of [X2+6] value

i.e., taking XZR value as 0 we get X0 value as 0XA

Syntax

LDURB Wt, [Xn|SP{, #simm}]

Load Register Byte (unscaled) calculates an address from a base register and an immediate offset, loads a byte from memory, zero-extends it, and writes it to a register.

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