e_stwu/stwu problem

[Routh]

Hi,

I have problem about stack on my 5604b system.

I want to create a section of stack frame, the size of which is 0xA8 byte.

the code like this:

e_stwu r1, -0xA8 (r1)

but the result is r1=r1-0x58 , I don't know what happened.How can I reliaze this purpose?

Very grateful for your assistance

[TomE · Accepted solution]

You need to read up on the instruction you're trying to use.

Documented in "Variable-Length Encoding (VLE) Programming Environments Manual," as:

    e_stwu rS,D8(rA)

There are only 8 SIGNED bits available in the VLE version for the offset, so you're limited to -0x80 to 0x7f (0x70 or 0x7c for alignment).

Initially I thoughy you could code it as:

e_stwu r1, -0x70 (r1)

e_stwu r1, -0x38 (r1)

BUT that would end up with the wrong value stored on the stack, breaking the stack linkage, so it needs something a little smarter than that involving another temporary register.

I'd suggest you write a small C function with more than 0x80 bytes of function-local variables/storage, compile it and then disassemble it to see how the compiler solved this problem. Then copy what it did into your assembly code.

Tom

[Bloodhound]

Would have been great to see the code produced because I have run in to the same problem with the command

e_stwu

r1,-0x84(r1)

[pradeepmc]

Posting the reply even though it is late. Since it may help other people. 

We can use the below when the offset is more the 0x7f. 

e_stw r1, -0x84(r1)

e_add16i r1,r1, -0x84