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Altera_Forum
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13 years ago

Subtracting from a std_logic_vector

library IEEE; use IEEE.std_logic_1164.all; use IEEE.std_logic_arith.all; use IEEE.std_logic_unsigned.all; . . . signal count_val : std_logic_vector(5 downto 0) := "000000"; . . ...
Altera_Forum's avatar
Altera_Forum
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13 years ago

std_logic_vector is undefined in terms of mathematical value until you either ask compiler to interpret as either signed or unsigned.

if signed: 000000 + 1 => 000001

000000 - 1 => 111111 (-1)

if unsigned: 000000 + 1 => 000001 same as above

0000000 -1 => illegal (-1), what you get is up to tool if allowed

if the tool does let you add/subtract on std_logic_vector then it is using default and is not safe.