Re: [rfa/arm] Handle bx and blx

[Daniel Jacobowitz <drow@false.org>]

On Mon, Mar 08, 2004 at 10:17:53AM +0000, Richard Earnshaw wrote:
> > On Wed, Mar 03, 2004 at 04:01:55PM +0000, Richard Earnshaw wrote:
> > > > The software single-step implementation in GDB doesn't know either BX or
> > > > BLX.  This results in losing control of the inferior when we single-step
> > > > over them.  I based this on the ARM ARM, so I'm pretty sure I've got the
> > > > numbers correct.
> > > > 
> > > > OK to check in?
> > > > 
> > > > -- 
> > > > Daniel Jacobowitz
> > > > MontaVista Software                         Debian GNU/Linux Developer
> > > > 
> > > > 2004-02-28  Daniel Jacobowitz  <drow@mvista.com>
> > > > 
> > > > 	* arm-tdep.c (thumb_get_next_pc): Handle BX.
> > > > 	(arm_get_next_pc): Handle BX and BLX.
> > > 
> > > Yikes!  Yes, this is OK.  However, Thumb has BLX (2 variants) as well.
> > 
> > Right you are.  I've checked in the above; if I'm reading
> > thumb_get_next_pc and the ARM correctly, then the below is all I need
> > for BLX.  The first form is already handled since we don't check H.
> > The second form can be handled identically to BX by relaxing a test.
> > 
> > OK?
> > 
> > -- 
> > Daniel Jacobowitz
> > MontaVista Software                         Debian GNU/Linux Developer
> > 
> > 2004-03-07  Daniel Jacobowitz  <drow@mvista.com>
> > 
> > 	* arm-tdep.c (thumb_get_next_pc): Handle Thumb BLX.
> 
> Very close, and possibly good enough for most purposes.  But the ARM ARM 
> says that in the blx(1) case, the resulting address should be masked with 
> 0xfffffffc.  That means that there are two theoretical encodings for each 
> target ARM-state instruction.  I think you need to add a test for H=01 and 
> if so, to apply the mask to nextpc.

Except it also says:
           Bit[0] for BLX     If H == 01, then bit[0] of the instruction must
			      be zero, or the instruction is UNDEFINED.
                              The offset calculation method described
                              in Usage above ensures that the offset
                              calculated for a BLX instruction is a
                              multiple of four, and that this
                              restriction is obeyed.

So I think the mask really isn't needed, or am I reading that wrong?

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer