On Wed, Mar 14, 2001 at 10:47:36PM +0100, Marcus Comstedt wrote:
> PSX and PSX2 use MIPS CPU's, and it's no problem to get docs for those
> either.

ah, that makes sense.  but, let me share my reasoning quickly 
before I shut up:

1. QLogic masks their own MIPS CPU's, in that MIPS cores are embedded 
   into the ISP SCSI chips.  They are custom, in that they also have 
   some SCSI l;ogic in them.  so they can't be shared with things 
   besides SCSI controllers and devices too much.

2. NEC masks their own MIPS CPU's, the ``VR'' series which is in my NEC 
   laptop.  The chips have extra NEC laptop stuff in them, so they can't 
   be shared too much with things other than PDAs.  or, so the naive 
   and shortsighted would think, anyway.

3. Probably more PS2's will sell than NEC laptops and QLogic ISP's 
   combined.

4. Sony has advertised ad nauseum about their ``emotion engine'', an 
   apparent add-on to the MIPS chip that sounds sort of like a graphics 
   accelerator or SIMD module or something.  this is also not standard 
   MIPS, like the SCSI glue and the LCD controller.

so, (1) MIPS cores often get licensed to fairly random and 
tight-budgeted companies, and (2) MIPS cores are often embedded 
in medium-volume chips that do other stuff as well.  Why shouldn't 
the MIPS CPU in the PS2 be a custom chip?

Certainly, it ought to take ordinary MIPS instructions, but that's not 
what I'm afraid of.  The Dreamcast port gained a lot of ground because 
all those extra jacks like serial and whatnot on the SH4 are strictly 
pin-by-pin documented.  The Dreamcast is almost SH4 + powersupply.  If 
this Sony chip is merely ``MIPSish'', then that is a different story 
entirely.