[hatari-devel] Conditional breakpoint code, please comment!

[npomarede at corp.free.fr]

On Sat, 4 Jul 2009, Eero Tamminen wrote:

> Hi,
>
>> On Friday 03 July 2009, npomarede at corp.free.fr wrote:
>>> I committed a change to handle addresses between 0xff8000 and 0xffffff.
>>>
>>> In the case of indirect addresses, I also added a mask to only keep the
>>> 24 lower bits of the address (when checking value and reading memory).
>>> This way $ffff820a is also recognized as io mem $ff820a (which is the
>>> same in the 68000).
>
> Btw. Why the masking is done both when checking breakpoints:
> --------------------------------
> @@ -102,6 +102,8 @@
>  */
> static Uint32 BreakCond_ReadSTMemory(Uint32 addr, const bc_value_t
> *bc_value)
> {
> +	addr &= 0x00ffffff;				/* use a 24 bit address */
> +
> 	switch (bc_value->bits) {
> 	case 32:
> 		return STMemory_ReadLong(addr);
> --------------------------------
>
> And when parsing them:
> --------------------------------
> @@ -301,7 +303,9 @@
> 		EXITFUNC(("-> true (DSP)\n"));
> 		return true;
> 	}
> -	if ((addr > STRamEnd && addr < 0xe00000) || addr > 0xff0000) {
> +
> +	addr &= 0x00ffffff;				/* use a 24 bit address */
> +	if ((addr > STRamEnd && addr < 0xe00000) || (addr >= 0xff0000 && addr <
> 0xff8000)) {
> --------------------------------
>
> Why parsing stage shouldn't give a warning if given address is > 0x00ffffff?
> (user probably mistyped it and masking it would be misleading?)
>

When parsing, we should check only for the lowest 24 bit of the address,
as done in the 68000, since bits 24-31 are ignored anyway.

In fact, we could consider that the address needs a warning if bit 23 (0 
or 1) is not correctly extended to bit 24-31 in case those bits are not 
all 0 or all 1.

So, valid addresses without warning would be :
  $00ff820a, $ffff820a, $00123456

And valid addresses with a warning would be :
  $0fff820a, $f0ff820a, $55ff820a, $ff123456, $07123456

sthg like :

   bit23 = ( address >> 23 ) & 1;
   highbyte = ( address >> 24 ) & 255;

   if ( ( bit23==0 && highbyte!=0 ) || ( bit23==1 && highbyte!=0xff ) )
     print_warning ( address );

With this, we're reaching the limit of the refinement we can make :)


Nicolas