[RC5] Use of FPU on Intel 486 and P5 processors

gindrup at okway.okstate.edu gindrup at okway.okstate.edu
Wed Jan 7 13:12:28 EST 1998


     Having looked the '486 information, this is what I find:
     
     from
     Intel 486 Microprocessors and Related Products [ISBN: 1-55512-235-3 
     (1995)]
     p. 2-44
        4.1.1
           ... [486 and 486DXx chips] also have an on-chip floating point 
     unit (FPU) that operates in parallel with the arithmetic and logic 
     units.
     pp. 2-278 - 2-285
        Table
     One of the column headings is "Concurrent Execution" and lists the 
     number of core clocks during which concurrent execution may occur.
     
     
     Also, further research on the Pentium and PPro (as well as PII) case 
     indicates that FPU instructions must be executed through the U 
     pipeline, but can be paried with pairable instructions in the V 
     pipeline.
            -- Eric Gindrup ! gindrup at okway.okstate.edu


______________________________ Reply Separator _________________________________
Subject: [RC5] Use of FPU on Intel 486 and P5 processors 
Author:  <rc5 at llamas.net > at SMTP
Date:    1/6/98 10:48 AM


There has been some comment on the possibility of using the floating 
point unit (FPU) on Intel processors in parallel with the integer 
unit to help process more keys.
     
I have tested this idea on a 486 and a P5 and found that instructions 
to the FPU stall the integer pipeline for the duration of the FPU 
instruction.  
     
It may be different on other chips, or even later Intel chips, but I 
don't have access to those.  If after testing you discover that the 
FPU does execute in parallel without stalling the integer pipeline, I 
have available a 34 step FPU sequence that will do one cycle of 
round 1 of the key expansion.  Foolishly, though partly as an 
intellectual exercise, I developed this before testing the 
pipeline stalling.
     
As MMX instructions use the FPU registers I suspect that they too 
will stall the integer pipeline but this should really be tested.
     
Just to add some facts to this discussion.
     
Bruce Ford                                      b.ford at qut.edu.au 
Systems Programmer
Teaching and Learning Support Services          Ph: +61 7 3864 3383 
Queensland University of Technology
--
To unsubcribe, send 'unsubscribe rc5' to majordomo at llamas.net 
rc5-digest subscribers replace rc5 with rc5-digest
     
     


--
To unsubcribe, send 'unsubscribe rc5' to majordomo at llamas.net
rc5-digest subscribers replace rc5 with rc5-digest



More information about the rc5 mailing list