multiplication bug
Information:
Type :: BUG
Status :: CLOSED
Assigned to ::
Steve, Rhoads
Description:
Hi,
I found a problem when multiplying 0x80000000 by 0x80000000.
The result should be 0x4000000000000000, but it is 0xC000000000000000.
I found it with the following test code:
li $t2, 0x80000000
mult $t2, $t2
mflo $t0
bne $t0, 0, fail
mfhi $t1
bne $t1, 0x40000000, fail
See you
Comments:
| Rhoads, Steve | Sep 3, 2010 |
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Your solution is much better. I checked it in.
Thanks, Steve |
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| ilardo, santo | Aug 31, 2010 |
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What do you think about the following change? Maybe it requires less logic.
mode_reg <= MODE_MULT; if b(31) = '0' then aa_reg <= a; bb_reg <= b; -- sign_reg <= a(31); else aa_reg <= a_neg; bb_reg <= b_neg; -- sign_reg <= a_neg(31); end if; sign_reg <= a(31) xor b(31); -- added line sign2_reg <= '0'; ... |
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| Rhoads, Steve | Aug 31, 2010 |
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Multiplying 0x80000000 times a negative number causes the bug. This is because -0x80000000 = ~0x80000000+1 = 0x80000000.
Here is a possible fix to mult.vhd: when MULT_SIGNED_MULT => mode_reg <= MODE_MULT; if b(31) = '0' then aa_reg <= a; bb_reg <= b; sign_reg <= a(31); --Special case of 0x80000000 * negative number elsif a(31) = '1' and (a(30 downto 0) = ZERO(30 downto 0) or b(30 downto 0) = ZERO(30 downto 0)) then aa_reg <= a; --convert to unsigned mult bb_reg <= b; sign_reg <= '0'; else aa_reg <= a_neg; bb_reg <= b_neg; sign_reg <= a_neg(31); end if; |
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