STM - Get assembler error when updating base register using ^

I assume this question is for the ARM instruction set -- but you don't say which ARM version. The ARM7TDMI Data Sheet (August 1995) says, in section 4.11.4 (Use of the S bit, with LDM/STM) that "base write-back should not be used" when R15 is not in the register list and the S bit is set (i.e. the ^ is used), just as in your case.

The Data Sheet does not give a reason for this prohibition, but I guess it has to do with the fact that this instruction (thanks to the ^ modifier) deals with two register sets, the User bank registers (transferred) and the current-mode registers (the Data Sheet does not explicitly say so, but I assume that the base register, here R0, is always the current-mode register, not the User bank register).

HTH

Thanks, but it generates the same error msg even if you specify r15 in the list. And sorry, yes, I'm referring to the ARM processor. Any other ideas?

Op Fri, 29 Aug 2008 06:12:21 +0200 schreef Michael :

My ARM ARM (from 2005) only specifies two forms of STM in ARM mode:

STM{} {!}, STM{} , ^

Your form conforms to none of these.

There are different versions of the ARM processor, and you still did not tell which.

Are you running in a mode that is not using the user register set (not user or system)? The ^ modifier temporarily accesses the user register set from a privileged mode.

You should also leave either the ^ modifier or the ! modifier out - it is not allowed to have both.

I believe I'm using arm7tdmi. If both ^ and ! cannot be used at the same time, then you've answered my question - thanks. Too bad, because it would be nice to push registers onto the user mode stack, and update the stack pointer.

In fact the statement "base write-back should not be used" appears in the same ARM7TDMI Data Sheet also when R15 is in the register list for STM and the ^ modifier is used, which agrees with what you experience. My apologies if my first reply implied that the presence of R15 is significant for STM (I looked only at the specific point in the Data Sheet that matched your instruction).

According to this Data Sheet, it seems that combining write-back and ^ is allowed for LDM with R15 in the register list. I don't know why LDM differs from STM on this point.

Thanks for your helpful comments.

Assuming the stack pointer is in r0 and the stack is "full descending", how about this:

SUB r0, r0, #(14 * 4) STMIA r0, {r1-r14}^

Not exactly what you were looking for, but should work fine.

Regards

-- Marcus Harnisch Consultant

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