Set on less that operation??
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Set on less that operation??

 
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Paminu
Guest
Posted: Mon Aug 01, 2005 4:15 pm    Post subject: Set on less that operation?? Reply with quote
I am trying to make an ALU that does SLT correct. But if I wan't to do -7<6
I get:

1001 - 0110 = 1001 + 1010 = 0011

And the signbit of the result is 0 but it should be 1. How do I make SLT
work correct? My first idea was to shift the result left by two but I am
not sure this will work.
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Joe Pfeiffer
Guest
Posted: Mon Aug 01, 2005 4:15 pm    Post subject: Re: Set on less that operation?? Reply with quote
Paminu <jadajada@asd.com> writes:

Quote:
I am trying to make an ALU that does SLT correct. But if I wan't to do -7<6
I get:

1001 - 0110 = 1001 + 1010 = 0011

And the signbit of the result is 0 but it should be 1. How do I make SLT
work correct? My first idea was to shift the result left by two but I am
not sure this will work.

Think about the overflow bit.
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skype: jjpfeifferjr
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Grumble
Guest
Posted: Mon Aug 01, 2005 4:15 pm    Post subject: Re: Set on less that operation?? Reply with quote
Paminu wrote:
Quote:
I am trying to make an ALU that does SLT correct. But if I wan't to do -7<6
I get:

1001 - 0110 = 1001 + 1010 = 0011

And the signbit of the result is 0 but it should be 1. How do I make SLT
work correct? My first idea was to shift the result left by two but I am
not sure this will work.

You seem to be using 4-bit two's complement.

Values which can be represented = -8 to 7

You tried to compute 6+7=13 => overflow

13 is congruent to -3 (modulo 16)

Why don't you use 5 bits to hold the sum?
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