reference is made to fig2 , which is a block diagram of a preferred embodiment in accordance with the present invention . a chip set 10 provided in the present invention includes a pci - express ( pci - e ) interface and a south - bridge or north - bridge chip set . the chip set 10 is connected electrically to a transmission control unit 20 . the transmission interface between the transmission control unit 20 and the chip set 10 is the pci - e interface . furthermore , the transmission control unit 20 integrates multiple pci - e lanes to increase transmission bandwidth and data processing speed . the transmission control unit 20 is a chip in this embodiment . the transmission control unit 20 is connected respectively to a first storage device 21 and a second storage device 22 and used to control the data transfer among the chip set 10 , the first storage device 21 and the second storage device 22 . for example , via the transmission control unit 20 , the data may be transferred from the chip set 10 to the first storage device 21 or from the first storage device 21 to the chip set 10 . the second storage device 22 provides storage space to store temporarily the data when the data transfer is performed . thus , via the second storage device 22 , the transmission control unit 20 can adjust the transmission bandwidth flexibly during the data transfer . in this embodiment , the first storage device 21 can be a hard disk with an ide , sata , 1394 or scsi interface . the second storage device 22 is a memory unit having an access speed larger than the hard disk has . during data transfer , the transmission control unit 20 checks the condition of the data reception . if the destination device is busy or cannot provide a sufficient receiving rate , the transmission control unit 20 temporarily stores the data in the second storage device 22 , i . e . the memory unit . the destination device mentioned in this embodiment is the chip set 10 or the first storage device 21 . therefore , via the transmission control unit 20 , the present invention can connect with a hard disk that has a large storage capacity and a memory unit that has a higher data access rate . the hard disk and the memory unit are the first storage device 21 and the second storage device 22 mentioned in this embodiment , respectively . the second storage device 22 of this embodiment is a memory unit plugged in a memory socket of a motherboard extended for and connected to the transmission control unit 20 . in accordance with the quantity of data requiring transmission , this embodiment can have multiple first storage devices 21 and multiple second storage devices 22 to increase the data transmission rate . in this embodiment , the interface between the chip set 10 and the transmission control unit 20 is a pci - e interface ; pci - ex1 means that the interface has a transmission lane having a transmission rate of 250 mb / s . the pci - e interface has various standards , such as pci - ex1 , pci - ex2 , pci - ex4 , pci - ex8 , pci - ex16 and pci - ex32 . in a duplex transmission mode , for example , the pci - ex16 interface has a transmission rate of 8 gb / s , which is much higher than the access speed of a common memory unit or hard disk . based on this fact , in this embodiment , the basic input / output system ( bios ) of a computer system can be set to use the second storage unit 22 for booting . the necessary data for booting can be stored in the second storage unit 22 in advance . in this way , the computer system can have a faster booting operation . in order to prevent the data stored in the second storage unit 22 from being removed when the computer system is turned off , this embodiment has a backup power supply to provide electricity for the second storage unit 22 when the computer system is turned off . the necessary data for booting can be multiple booting files , multiple registration files , multiple executive files or multiple associated files of an operating system ( os ), such as the windows system , the os2 system or the linux system . the necessary data for booting can also be a booting image file . reference is made to fig3 , which is a flow chart showing a data transfer procedure in accordance with the present invention . fig3 shows the procedure of the data transfer from the chip set 10 to the first storage device 21 controlled by the transmission control unit 20 . the data transfer procedure has the following steps . at the beginning , the chip set 10 starts to transmit data for the first storage device 21 ( s 301 ). then , the transmission control unit 20 receives the data transmitted from the chip set 20 ( s 303 ). the transmission control unit 20 determines whether the first storage device 21 can receive and process all of the data immediately ( s 305 ); in other words , the transmission control unit 20 checks whether the data quantity that can be received and processed by the first storage device 21 exceeds that of the data transmitted from the chip set 10 . if the result of the determination is yes , the first storage device 21 immediately receives the data transmitted from the chip set 10 ( s 307 ). otherwise , the transmission control unit 20 temporarily stores a portion of the data transmitted from the chip set 10 into the second storage unit 22 ( s 309 ) and directly sends the remaining portion to the first storage device 21 ( s 311 ). after the first storage device 21 finishes receiving the portion of data sent from the transmission control unit 20 ( s 313 ), the portion of data temporarily stored in the second storage unit 22 is then sent to the first storage device 21 ( s 315 ). reference is made to fig4 , which is a flow chart showing another data transfer procedure in accordance with the present invention . fig4 shows the procedure of the data transfer from the first storage device 21 to the chip set 10 controlled by the transmission control unit 20 . the data transfer procedure has the following steps . at the beginning , the first storage device 21 starts to transmit data for the chip set 10 ( s 401 ). then , the transmission control unit 20 receives the data transmitted from the first storage device 21 ( s 403 ). the transmission control unit 20 determines whether the chip set 10 can immediately receive and process all of the data ( s 405 ); in other words , the transmission control unit 20 checks whether the data quantity that can be received and processed by the chip set 10 exceeds that of the data transmitted from the first storage device 21 , and whether the chip set 10 is not busy . if the result of the determination is yes , the chip set 10 immediately receives the data transmitted from the first storage device 21 ( s 407 ). otherwise , the transmission control unit 20 temporarily stores a portion of the data transmitted from the first storage device 21 into the second storage unit 22 ( s 409 ) and directly sends the remaining portion to the chip set 10 ( s 411 ). after the chip set 10 finishes receiving the portion of data sent from the transmission control unit 20 ( s 413 ), the portion of data temporarily stored in the second storage unit 22 is then sent to the chip set 10 ( s 415 ). accordingly , the transmission control unit 20 provided in this embodiment can control the data transfer between the chip set 10 and the first storage device 21 according to the quantity of data transmitted . when the quantity of data is smaller than that able be received and processed by the destination device , the destination device directly receives the transmitted data . otherwise , when the quantity of data is larger than that able be received and processed by the destination device , only a portion of the data that can be received and processed by the destination device is transmitted . the remaining portion of the data is temporarily stored in the second storage device 22 . after the destination device finishes receiving and processing the data sent from the transmission control unit 20 , the remaining portion of the data is sent from the second storage device 22 to the destination device . since the second storage device is provided to store the data temporarily , thus providing a function similar to that of a cache memory , the transmission rate of the data transferred between the chip set 10 and the first storage device 21 is improved . reference is made to fig5 , which is a flow chart showing a procedure for switching on / off a computer system in accordance with the present invention . the second storage device 22 stores necessary booting data in advance . hence , when the computer system is switched on , the chip set 10 accesses the necessary booting data stored in the second storage device . 22 via the transmission control unit 20 ( s 501 ). thus , the necessary booting data can be accessed speedily . according to the data , the computer system is activated ( s 503 ). the method for booting the computer system provided in this embodiment is faster than the conventional method where the booting data must be acquired from a hard disk . when the computer needs to be switched off or enter a sleeping mode , the booting data are stored in the second storage device in advance ( s 505 ). hence , the next time the computer system is switched on , the booting data can be accessed from the second storage device 22 directly . although the present invention has been described with reference to the preferred embodiment thereof , it will be understood that the invention is not limited to the details thereof . various substitutions and modifications have been suggested in the foregoing description , and other will occur to those of ordinary skill in the art . therefore , all such substitutions and modifications are embraced within the scope of the invention as defined in the appended claims .