PowerPC For Dummies: A simplified guide to understanding the PowerPC Instruction Set

[JimmyKazakhstan]

Yes.

Thanks!

[JimmyKazakhstan]

Hey, This isn't related to the last question but I'm wondering, is my understanding of these instructions correct?

lwzu

Load Word Zero Update

The value in Register A will be loaded into whatever Address is in Register B.
After the instruction is executed, The address in Register B is updated by the value you specify
 

EXAMPLE :
lwzu r1, 4(r2)

* The address in Register B starts as 0x80000004. After execution of this instruction, it becomes 0x80000008. It changed because it was updated by the pointer, which was 0x4. 

---

lwzx

Load Word Zero Indexed

The value in Register A will be loaded into whatever Address is in Register C.
After the instruction is executed, The address in Register C is updated by the value in Register A.
The new address is stored in Register B

 

EXAMPLE :

lwzx r1, r2, r3

*When I figure out how this works, I'll add an example

[Vega]

lwzu looks good but lwzx is wrong, you could write them like this....

lwzu rA, VALUE (rB)

Register A is loaded with the word located at rB+VALUE. Afterwards, rB is incremented by the amount in VALUE.

--

lwzx rA, rB, rC

Register A is loaded with the word located at rB+rC.

--

Simple and straight to the point.

[JimmyKazakhstan]

lwzu looks good but lwzx is wrong, you could write them like this....

lwzu rA, VALUE (rB)

Register A is loaded with the word located at rB+VALUE. Afterwards, rB is incremented by the amount in VALUE.

--

lwzx rA, rB, rC

Register A is loaded with the word located at rB+rC.

--

Simple and straight to the point.

So whatever is the sum of register b and c added together becomes the pointer added to the address in register A? (for lwzx)

[Vega]

It's called the Effective Address and that's how store and load instructions work.

Examples~

stw r0, VALUE (r3) #Effective Address aka EA (where store occurs) is VALUE+r3
lbz r5, VALUE (r10) #EA = VALUE+r10

These calculations are signed because all store and load instructions are signed. Meaning VALUE can be negative. If it is, do the sum accordingly.

Example~

lhzu r11, 0xFFFFFFFC (r9) #EA = -4 + r9, or easier to write it as r9 - 4 = EA.

Final Example (using lwxz)~

r5 = 0xFFFFFFF0 (aka -16 or -0x10)
r6 = 0x80001000

lwxz r4, r5, r6

EA = r5+r6 (0x80001000 minus 0x10)
Thus EA = 0x80000FF0

So the lwzx instruction will load the word located at 0x80000FF0 into r4.