vhdl语法问题

Question

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;

PACKAGE mypack IS
FUNCTION ShiftRight(a:std_logic_vector;n: integer) return std_logic_vector;
END;
PACKAGE BODY mypack IS
function ShiftRight(a:IN std_logic_vector;n: in integer)
return std_logic_vector is
variable b:std_logic_vector(a'length-1 downto 0);
subtype rtype is std_logic_vector(n-1 downto 0);
begin
b(a'length-1-n downto 0):=a(a'length-1 downto n);
b(a'length-1 downto a'length-n):=rtype'(others=>a(a'length-1));该句赋值号后面的语法看不懂
return b;
end function ShiftRight;
end;

Answer 1

第一，s:=conv_interger(s_vec);是将矢量转为整数的函数，这个是怎么个转发啊？比如s_vec="011",转的整数是其十进制数吗？ -- 对, s_vec="011"转的整数就是3. 第二，din'left表示取左边界，比如din="1100",这个左边界取的值是1么？ -- 不对. 左边界是指矢量的边界, 例如 constant k1 : r_vector(10 downto 3) k1'left = 10, k1'right = 3 constant k2 : r_vector(5 to 7) k2'left = 5, k2'right = 7