[Info-vax] Microsoft: Alpha architecture responsible for poor Windows file compression

[Bob Gezelter]

On Wednesday, November 2, 2016 at 8:33:09 PM UTC-4, Chris wrote:
> On 11/02/16 22:10, John Reagan wrote:
> > No, bit twiddling is VAX INSV, EXTV, BBC, etc.
> > Potato potAtdo
> 
> I guess it must mean something different, but while many embedded
> processors have dedicated instructions to set or clear a bit in a
> register or memory, not that common in mainstream processors afaik.
> 
> Any instruction that has a capabilty to set or clear a bit, has
> bit twiddling capabilty, but I guess we have to agree to differ :-)
> 
> Chris

Chris,

Actually, setting/clearing a bit is straightforwardly accomplished using a shift (or table lookup) and the standard logical instructions (e.g., AND, OR, NOT, XOR).

Spent a fair amount of time deriving what would now probably be called "templates" for so-called "bit twiddling" on IBM System/360 and DEC PDP-11, neither of which had purposely designed instructions. 

For example, on a PDP-11, one could do a bit count for a 16-bit word with two registers in seven instructions without branches with a 16-word auxiliary table. On the System/360, one could do a 32-bit word in a few more instructions.

The non-aligned operations added to Alpha make some things slightly more efficient, but I would not want to place any bets on how much faster in the referenced example.

- Bob Gezelter, http://www.rlgsc.com