PowerPC lwzx

[ryanebola16]

I am actually trying to understand PowerPC assembly.

The green bar indicates the next instruction that will be performed. If r0 = 0000006c and r3 = 817d34e0, what will the result of lwzx r3, r3, r0 be?

Dolphin says the result is r3 = 920d5fd0 when I step into that instruction. I though the result would be the sum of r3 and r0 stored in r3 so r3 should equal 817d354c.

Where have I gone wrong?

Thanks!

[JonnyH]

lwzx[0] appears to be a "load word from address" instruction - from my understanding it doesn't store the result of (r3 + r0) in r3, but it stores the memory value at address (r3 + r0) into r3.

So presumably there's the value 0x920d5fd0 in the memory at address 0x817d354c?

[0]
https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.aix.alangref/idalangref_lwzx_lx_lwzi_instrus.htm

[ryanebola16]

That is correct. Thank you!

Memory is not the same thing as a register, duh.

[medevo]

(10-20-2017, 07:57 AM)ryanebola16 Wrote: I am actually trying to understand PowerPC assembly.

The green bar indicates the next instruction that will be performed. If r0 = 0000006c and r3 = 817d34e0, what will the result of lwzx r3, r3, r0 be?

Dolphin says the result is r3 = 920d5fd0 when I step into that instruction. I though the result would be the sum of r3 and r0 stored in r3 so r3 should equal 817d354c.

Where have I gone wrong?

Thanks!

I think you might have gotten confused between an immediate load and an addressed load. From the documentation for the instruction at https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.aix.alangref/idalangref_lwzx_lx_lwzi_instrus.htm

LWZX Effectively just adds the values of RA and RB, but uses this value EA as the effective address for a memory load and stores it in Register RT

In your case, after executing line 8084fe98, R3= [R3+R0] or R3 = [817d354c]. If you pull up that location in memory I bet you will find 920d5fd0

[CosmoCortney]

lwzx stands for load word zero indexed.
in your case the effective address is computed by the sum of r3 and r0. the value stored at the effective address is then loaded into r3
For reverence I recommend: http://wiibrew.org/wiki/Assembler_Tutorial#Load_and_Store_Instructions