5.96 · exercise

Predict: memory and the stack

Trace the memory addresses and register values through these snippets. Write hex answers as 0x followed by the digits, and decimal answers as plain integers.

0 of 10 correct

Predict the result

mov x20, 0x1000
ldr x21, [x20, 16]!

What hex address is left in x20 after this code executes? Write it with no leading zeros, for example 0x1234.

Predict the result

mov x28, 0x2000
str w20, [x28], 8

What hex memory address does the str instruction write to? Write it with no leading zeros, for example 0x1234.

Predict the result

mov x28, 0x2000
str w20, [x28], 8

What hex address is left in x28 after this code executes? Write it with no leading zeros, for example 0x1234.

Predict the result

mov x21, 5
ldr x22, [x29, x21, lsl 3]

What decimal offset is added to x29 to form the memory address?

Predict the result

add sp, sp, -28 & -16

The assembler evaluates -28 & -16 before it emits the instruction. What decimal value does that expression produce?

Predict the result

fp .req x29
c_s = -4
mov fp, 0x8000
mov w21, 10
str w21, [fp, c_s]

The frame pointer holds 0x8000. What hex address does the str write to? Write it with no leading zeros, for example 0x1234.

Predict the result

main: stp x29, x30, [sp, -(16 + 12) & -16]!

How many bytes does this instruction allocate on the stack? Answer in decimal.

Predict the result

main: stp x29, x30, [sp, -32]!

If this frame holds only three 4-byte local integers, how many pad bytes are left unused? Answer in decimal.

Predict the result

ldrsb w20, [x29]
// address holds the byte 0xff

After ldrsb loads the byte 0xff, what value is in w20? Write the full 32-bit value as 0x followed by eight hex digits.

Predict the result

ldrb w20, [x29]
// address holds the byte 0xff

After ldrb loads the byte 0xff, what value is in w20? Write the full 32-bit value as 0x followed by eight hex digits.