Kürzer geschrieben ginge es etwa so:
1 | entity MSB is
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2 | Port ( data : in STD_LOGIC_VECTOR (31 downto 0);
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3 | msb1 : out STD_LOGIC_VECTOR (4 downto 0));
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4 | end MSB;
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5 |
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6 | architecture Behavioral of MSB is
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7 | signal msbint1 : integer range 0 to 31;
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8 | begin
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9 | msbint1 <= 31 when data(31)='1' else
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10 | 30 when data(30)='1' else
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11 | 29 when data(29)='1' else
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12 | 28 when data(28)='1' else
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13 | :
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14 | 4 when data( 4)='1' else
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15 | 3 when data( 3)='1' else
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16 | 2 when data( 2)='1' else
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17 | 1 when data( 1)='1' else
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18 | 0;
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19 |
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20 | msb1 <= std_logic_vector(to_unsigned(msbint1,msb1'length));
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21 | end Behavioral;
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Das ergibt dann eine recht aufwendige Kombinatorik:
1 | Selected Device : 3s50pq208-5
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2 | Number of Slices: 23 out of 768 2%
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3 | Number of 4 input LUTs: 40 out of 1536 2%
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4 | :
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5 | Maximum combinational path delay: 19.263ns
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Kompakter liest sich eine For-Schleife:
1 | process (data)
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2 | variable foundbit : std_logic;
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3 | begin
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4 | foundbit := '0';
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5 | msbint2 <= 0;
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6 | for i in 31 downto 0 loop
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7 | if (data(i)='1' and foundbit='0') then
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8 | msbint2 <= i;
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9 | foundbit := '1';
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10 | end if;
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11 | end loop;
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12 | end process;
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13 |
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14 | msb2 <= std_logic_vector(to_unsigned(msbint2,msb2'length));
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Aber das Ergebnis ist suboptimal, weil die for-Schleife weiter
durchlaufen wird, auch wenn eine '1' gefunden wurde:
1 | Selected Device : 3s50pq208-5
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2 | Number of Slices: 51 out of 768 6%
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3 | Number of 4 input LUTs: 87 out of 1536 5%
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4 | :
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5 | Maximum combinational path delay: 24.339ns
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Dritter Versuch: eine While-Schleife.
1 | process (data)
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2 | variable i : integer range 0 to 31;
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3 | begin
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4 | i := 31;
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5 | while (data(i)='0') loop
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6 | i := i-1;
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7 | end loop;
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8 | msbint2 <= i;
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9 | end process;
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10 |
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11 | msb2 <= std_logic_vector(to_unsigned(msbint2,msb2'length));
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Damit lässt sich auch in der Laufzeit (auf Kosten einiger Slices) noch
was holen:
1 | Selected Device : 3s50pq208-5
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2 | Number of Slices: 28 out of 768 3%
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3 | Number of 4 input LUTs: 47 out of 1536 3%
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4 | :
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5 | Maximum combinational path delay: 18.821ns
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Eine For-Schleife mit EXIT sieht so aus:
1 | process (data)
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2 | begin
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3 | msbint2 <= 0;
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4 | for i in 31 downto 0 loop
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5 | if (data(i)='1') then
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6 | msbint2 <= i;
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7 | exit;
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8 | end if;
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9 | end loop;
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10 | end process;
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11 |
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12 | msb2 <= std_logic_vector(to_unsigned(msbint2,msb2'length));
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Hier wird exakt die selbe Logik wie mit dem auscodierten Encoder
erzeugt:
1 | Selected Device : 3s50pq208-5
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2 | Number of Slices: 23 out of 768 2%
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3 | Number of 4 input LUTs: 40 out of 1536 2%
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4 | :
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5 | Maximum combinational path delay: 19.263ns
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