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 Post subject: MULU & MULS accurate timingsPosted: Tue Sep 09, 2008 10:16 am

Joined: Wed Jun 27, 2007 2:08 pm
Posts: 6
Location: France
It appears to me that the instructions MULU & MULS are always weighted with the number of cycles corresponding to the worst possible case (70 cycles + n cycles for calculating the effective address) in v3.7.11 Beta.

According to M68000 User's Manual :

Quote:
MULS, MULU â€” The multiply algorithm requires 38+2n clocks where n is defined as:
MULU: n = the number of ones in the <ea>
MULS: n=concatenate the <ea> with a zero as the LSB; n is the resultant number of 10
or 01 patterns in the 17-bit source; i.e., worst case happens when the source
is \$5555.

For exemple in v3.7.11 Beta :
MULU #160,d1
"costs" 74 cycles whereas in real life it should "cost only" 38+2*2+4=46 cycles if I understand correctly the User's Manual.

Will we see these accurate timings in a next version ?

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 Post subject: Posted: Wed Sep 10, 2008 1:03 pm

Joined: Thu Dec 16, 2004 6:42 pm
Posts: 1115
From EASy68K Help.

"The cycle counter does not take into account pipeline efficiencies. The cycle number displayed tends to be a worst case value and is only intended for relative comparison purposes."

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Prof. Kelly

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 Post subject: Posted: Thu Sep 11, 2008 3:20 am

Joined: Thu Dec 16, 2004 6:42 pm
Posts: 1115
Sim68Kv3.9.1 will use the aforementioned 38+2n formula to calculate execution times.

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Prof. Kelly

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