Semiconductor memory unit

1. In a semiconductor memory unit to which a plurality of different functions can be imparted by merely changing a portion of its production process, the improvement comprising:

a plurality of data buses which include first data buses for use only in one of the functions and the remaining data buses for use in the one and the remainder of the functions;

wherein when the semiconductor memory unit performs the remainder of the functions, the first data buses are utilized for the semiconductor memory unit.

2. A semiconductor memory unit according to claim 1 , wherein the one and the remainder of the functions are a double data rate synchronous DRAM (DDR SDRAM) and a single data rate synchronous DRAM (SDR SDRAM), respectively;

wherein when the semiconductor memory unit functions as the SDR SDRAM, the first data buses are utilized as shielding wires by fixing a DC level of the first data buses.

3. A semiconductor memory unit according to claim 1 , wherein the one and the remainder of the functions are a first word organization and a second word organization smaller than the first word organization, respectively;

wherein when the semiconductor memory unit functions as the second organization, the first data buses are utilized as shielding wires by fixing a DC level of the first data buses.

4. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, a DC level of the first data buses is set in a floating state.

5. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, the first data buses are removed.

6. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, the first data buses are short-circuited to the remaining data buses, respectively.

7. A semiconductor memory unit according to claim 1 , wherein the first data buses are utilized at the time of a test mode of the remainder of the functions.

8. A semiconductor memory unit according to claim 1 , wherein the one and the remainder of the functions are a double data rate synchronous DRAM (DDR SDRAM) and a single data rate synchronous DRAM (SDR SDRAM), respectively;

wherein the first data buses are utilized at the time of a test mode of the SDR SDRAM.

9. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, the first data buses are utilized as power lines.

10. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, the first data buses are subjected to an arbitrary voltage.

11. A semiconductor memory unit according to claim 10 , wherein the arbitrary voltage is selected from a plurality of voltage application circuits.

12. A semiconductor memory unit according to claim 10 , wherein the arbitrary voltage is applied from a bonding pad to the first data buses.

13. A semiconductor memory unit according to claim 12 , wherein the arbitrary voltage is selected from a plurality of bonding pads.

14. A semiconductor memory unit according to claim 1 , wherein when the semiconductor memory unit performs the remainder of the functions, a line width of the first data buses is made smaller than that of the remaining data buses.