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Altera_Forum's avatar
Altera_Forum
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13 years ago

using the LPM_ADD_SUB mega function

Hi, I developed a 4-bit SIGNED adder with a carry bit using the lpm_add_sub megafunction in VHDL in Quartus II 11.1. I'm using a DE1 board with a Cyclone II to verify the component. Here's the pro...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
13 years ago

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unsigned and signed 2's complement addition/subtraction are identical and that is why 2's complement has gained popularity over say sign and magnitude.

However, there is one anomaly here as FvM pointed at; you must ignore carry bit for signed to be correct.Moreover you must do sign extension first

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Thanks. What is sign extension? More importantly, why do we need to designate the SIGNED attribute when building an adder in the Mega function library if the UNSIGNED adder is identical?