[hatari-devel] Re: [Emutos-devel] Aranym blitter

[Eero Tamminen <oak@xxxxxxxxxxxxxx>]

Hi,

On 06/07/2017 10:33 PM, Thorsten Otto wrote:
....
> Another thing that might explain why you had some success with Hatari:
>
> static Uint16 Blitter_ReadWord(Uint32 addr)
> {
> 	Uint16 value;
>
> 	/* When reading from a bus error region, just return a constant */
> 	if ( STMemory_CheckRegionBusError ( addr ) )
> 		value = BLITTER_READ_WORD_BUS_ERR;
> 	else
> 		value = (Uint16)get_word ( addr );
> ...
>
> static void Blitter_WriteWord(Uint32 addr, Uint16 value)
> {
> 	/* Call put_word only if the address doesn't point to a bus error region
> */
> 	/* (also see SysMem_wput for addr < 0x8) */
> 	if ( STMemory_CheckRegionBusError ( addr ) == false )
> 		put_word ( addr , (Uint32)(value) );
> ...
>
> So Hatari simply ignore access to invalid memory addresse in all cases. I
> don't think that this is the right to do.

Looking at Hatari change history:
$ hg blame src/blitter.c | egrep -A8 'static.*Blitter_(Read|Write)Word'
$ hg log | grep -A4 5806:
$ hg diff 5806

You can see that it's a deliberate fix from 2015 by Nicolas.
It's even documented in start of blitter.c:
---------------------------
#define        BLITTER_READ_WORD_BUS_ERR       0x0000  /* This value is 
returned when the blitter try to read a word */
                                               /* in a region that 
would cause a bus error */
                                               /* [NP] FIXME : for now 
we return a constant, but it should depend on the bus activity */
---------------------------

And later in mid 2016, in commit 6278:
---------------------------
+    /* Only CPU will trigger bus error if bit S=0, not the blitter */
     if(addr < 0x8 || (addr < 0x800 && !regs.s))
     {
-      M68000_BusError(addr, 0, BUS_ERROR_SIZE_WORD, BUS_ERROR_ACCESS_DATA);
-      return;
+      if ( BusMode == BUS_MODE_CPU )
+      {
+       M68000_BusError(addr, 0, BUS_ERROR_SIZE_WORD, 
BUS_ERROR_ACCESS_DATA);
+       return;
+      }
+      /* If blitter writes < 0x8 then it should be ignored, else the 
write should be made */
+      else if ( ( BusMode == BUS_MODE_BLITTER ) && ( addr < 0x8 ) )
+       return;
---------------------------


> What's a bit unclear to me is:
> - what happens on real hardware when accessing the word registers with byte
> moves? That is not handled yet by aranym.
> - what happens to the source address, when no data is read? is it stll
> incremented/decremented?


	- Eero