Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)

On 03/10/2024 02:31, Lawrence D'Oliveiro wrote:

On Tue, 1 Oct 2024 23:38:10 -0000 (UTC), Brett wrote:
 

My wish list for the 68k was a barrel roller for fast shifts, would have
made a HUGE difference for the first Macintosh.

 The 68020 certainly had that. It also added bit-field instructions, on top
of the single-bit instructions of the original 68000.
 And in typical big-endian fashion, they added yet another inconsistency to
the way the bits were numbered ...

Since you mentioned POWER and PowerPC elsewhere, the bit numbering challenges of the m68k world are nothing compared to the PowerPC world. Originally, the PowerPC was 32-bit and numbered bits from 0 as the MSB down to 31 as the LSB.  So your 32-bit address bus had lines from A0 down to A31.  Then it got extended to 64-bit (some devices had only partial 64-bit extensions), and the chips got a wider address bus (you never need all 64-bit lines physically) - the pins for the higher address lines were numbered A-1, A-2, and so on.  For the internal registers, some are 64-bit numbered bit 0 (MSB) down to bit 63.  Some were original 32-bit and got extended to 64-bit, and so are numbered bit -32 down to bit 31 for consistency.  Others are 32-bit but numbered from bit 32 down to bit 63.
I am not really complaining - one of our customers hired us precisely because they wanted to use a particular PowerPC microcontroller but after one look at the ten thousand page reference manual full of this kind of thing, they paid us to write a library and drivers for the peripherals they wanted so that they never had to think about that mess.