Abstract:
A method for controlling data transfer in a serial-ATA system includes using serial-ATA Native Command Queuing (NCQ) to issue a queue of NCQ commands to at least two serial-ATA devices. The commands include a first plurality of commands for a first one of the devices and a second plurality of commands for a second one of the devices. Each of the commands includes a respective port address of one of the at least two devices and a first command identifier identifying a command for the one of the at least two devices. The method further includes receiving a first acknowledgement, which has a port address of a first target device and a second command identifier identifying a first outstanding command for the first target device. Each of the queues of commands is sent to the at least two serial-ATA devices prior to receiving the first acknowledgement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application incorporates by reference the following patent applications filed on the same day as the present application: U.S. application Ser. No. 10/667,197 entitled Communication Channel between Serial-ATA Host Controllers, U.S. application Ser. No. 10/667,192 entitled Queued Command-based Switching Serial-ATA Port Multiplier U.S. application Ser. No. 10/666,959 entitled Method and Apparatus for Controlling Data Transfer in a Serial-ATA System U.S. application Ser. No. 10/667,196 entitled Serial-ATA Host Controller Supporting Both QDMA Commands and NCQ Commands. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to serial-ATA, and more specifically to a queued command-based switching serial-ATA port multiplier and host controller. 
     2. Description of the Related Art 
     In the Serial-ATA II: Port Multiplier specification, Revision 1.0 provided by the Serial ATA working group (“Serial-ATA II PM 1.0”), a port multiplier is defined as a mechanism for one active host connection to communicate with multiple devices, as shown in  FIG. 1 . A port multiplier  101  has one host port, one control port and a number of device ports. Device port addresses start at zero and are numbered sequentially higher until the last device port address has been defined. The host port is coupled to a host  102 , and the device ports are coupled to devices  103   a  to  103   n.    
     The port multiplier uses four bits, known as the PM Port Field, in all FIS (Frame Information Structure) types to route FISes between the host and the appropriate device. For a host-to-device FIS, the PM Port Field is filled in by the host with the port address of the device to route the FIS to. For a device-to-host FIS, the PM Port Field is filled in by the port multiplier with the port address of the device that is transmitting the FIS. 
     To transmit a FIS to a device connected to a port multiplier, the host sets the PM Port Field in the FIS to the device&#39;s port address. Then the host starts transmitting the FIS to the Port Multiplier according to the Serial ATA: High Speed Serialized AT Attachment, Revision 1.0 (“Serial-ATA 1.0 Specification”). When a port multiplier receives a FIS over the host port, the port multiplier checks the PM Port Field in the FIS to determine the port address that the FIS should be transmitted over. 
     To transmit a FIS to the host, the device proceeds with the transmission of the FISes to the port multiplier according to the Serial-ATA 1.0 Specification. The port multiplier fills in the PM Port Field with the port address of the transmitting device, and then transmits the modified FIS to the host. 
     Serial-ATA II PM 1.0 provides information for two switching types: FIS-based switching and command-based switching. If the host controller does not support hardware context switching based on the value of the PM Port Field in a received FIS, then the host utilizes command-based switching and may only have a command or commands outstanding to one device at any point in time. 
     The host has bandwidth to work with multiple devices simultaneously. However, because command-based switching only allows the host to have a command outstanding to one device at a time, the host cannot use that bandwidth. Therefore, it would be advantageous to provide a method and port multiplier for more efficient switching and data transfer between the host and the devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a queued command-based switching between a host and serial-ATA devices, such as disk drives. According to the present invention, a host controller, that does not support hardware context switching based on the value of the PM Port Field in a received FIS, can issue multiple commands to multiple devices at the same time. As a result, the present invention improves data transfer efficiency between the host and the devices. 
     The serial-ATA port multiplier of the present invention provides a locking mechanism. When a serial-ATA command is started, the port multiplier locks a specific port between a device and the host, and prevents intervention of other devices. Accordingly, other devices trying to access the host through the port multiplier are not acknowledged until the first started serial-ATA command is completed. Once the first started serial-ATA command is completed, the port multiplier releases the locked port and waits for additional commands. 
     It is another object of the present invention to provide a method for queued command-based switching, wherein the port multiplier releases the bus when a command has been queued, and locks the bus again when the device is ready to complete the data transaction of the queued command. 
     To further improve data transfer efficiency, the present invention uses a cache between the device port of the port multiplier and the device. During a read operation, all requested data are taken from the disk and staged on the cache before being transferred to the device port. During a write operation, all data to be written are staged on the cache before being received by the device. Thus, the port multiplier works against the cache, not the device. 
     It is a further object of the present invention to provide a method for controlling a serial-ATA port multiplier by a host controller in queued command-based switching. The host controller sends commands to multiple devices. Each of the commands contains the tag of the command and a device port number. When a device is ready to proceed with the data transaction, it sends back the tag of the command, and the port multiplier provides the information of the device port. Consequently, the host controller knows which type of data transaction to carry out and which device to transfer data with. 
     It is a further object of the present invention to provide a method for NCQ (Native Command Queuing) queued command-based switching, wherein the device sends a DMA (Direct Memory Access) Setup FIS when it is ready to complete the data transaction of the queued command. 
     It is a further object of the present invention to provide a method for QDMA (Queued Direct Memory Access) queued command-based switching, wherein the device sends a Set Device Bits FIS when it is ready to complete the data transaction of the queued command. 
     It is a further object of the present invention to provide a host controller, which provides a command to a device with a tag uniquely identifying the command and a device port address, and uses a request queue and a response queue mechanism to control NCQ queued command-based switching. 
     It is a further object of the present invention to provide a host controller, which provides a command to a device with a tag uniquely identifying the command and a device port address, so as to control QDMA queued command-based switching. 
     It is a further object of the present invention to provide a host controller which supports NCQ and QDMA commands. The host controller has a state machine for each operation mode, and a selection mechanism to choose an operation mode which best fits a particular device. 
     As described herein, the inventive host controller may be implemented in a variety of systems, including computers, servers, network attached storage (NAS) systems, storage area networks (SAN), and systems employing technology known as a redundant array of inexpensive disks (RAID) or JBOD (Just a Bunch of Disks). The device could be any serial-ATA device, which has a direct connection with the host by a non-shared media and has point to point communications with the host, including disk drives and removable storage devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described herein with reference to the accompanying drawings, similar reference numbers being used to indicate functionally similar elements. 
         FIG. 1  shows a port multiplier as defined in Serial-ATA II PM 1.0. 
         FIG. 2  shows a flow chart for a read operation of a queued command-based switching serial-ATA port multiplier in the NCQ mode according to one embodiment of the present invention. 
         FIGS. 3A and 3B  shows a flow chart for a write operation of a queued command-based switching serial-ATA port multiplier in the NCQ mode according to one embodiment of the present invention. 
         FIGS. 4A and 4B  show a flow chart for a read operation of a queued command-based switching serial-ATA port multiplier supporting both NCQ operation mode and QDMA operation mode according to one embodiment of the present invention. 
         FIGS. 4C and 4D  show a flow chart for a write operation of a queued command-based switching serial-ATA port multiplier supporting both NCQ operation mode and QDMA operation mode according to one embodiment of the present invention. 
         FIGS. 5A-5B  show a command request status according to one embodiment of the present invention. 
         FIGS. 5C-5D  show a command response status according to one embodiment of the present invention. 
         FIG. 6  shows a Register—Host to Device FIS issued by a NCQ host controller used for queued command-based switching according to one embodiment of the present invention. 
         FIG. 7  shows a flow chart for the operation of a NCQ host controller used for queued command-based switching according to one embodiment of the present invention. 
         FIG. 8  shows a flow chart for the operation of a QDMA host controller in queued command-based switching according to one embodiment of the present invention. 
         FIG. 9  shows a block diagram of the operation mode selection module of a host. 
         FIG. 10  shows a flow chart of a method used by a host controller to select an operation mode according to one embodiment of the present invention. 
         FIG. 11  shows a disk drive as an example of a serial-ATA device to which the present invention is applicable. 
         FIG. 12  shows an embodiment of a serial-ATA system employing one or more serial-ATA host controllers according to the present invention. 
         FIG. 13  shows a block diagram of a port multiplier according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     I. Introduction 
     According to the Serial-ATA 1.0 Specification, communication layers between a host and a device include a physical layer, a link layer, a transport layer and an application layer. The link layer transmits and receives frames. A frame is a group of Dword (32 bits of data) that conveys information between host and device. Each frame includes a user payload called a FIS, as mentioned earlier. The transport layer constructs FISes for transmission and decomposes received FISes. 
     The Serial-ATA 1.0 Specification defines eight types of FIS. Several of them are relevant here. The first is a Register—Host to Device FIS used to transfer the contents of Control Block Register from the host to the device. Control Block Register are interface registers used for delivering commands to the device or posting status from the device. 
     The second relevant FIS type is DMA Activate—Device to Host, which is used by the device to signal its readiness to receive DMA data from the host, so that the host can proceed with a DMA data transfer from the host to the device. When the host needs to send multiple Data frames in order to complete the overall data transfer request, the host waits for a successful reception of a DMA Activate FIS before sending each of the Data FISes that are needed. 
     The third relevant FIS type is Set Device Bits—Device to Host FIS, which is used by the device to set the SERV bit in the Status Register to request service for a bus release command. The Set Device Bits—Device to Host includes a bit to signal the host controller to generate an interrupt if the BSY bit and the DRQ bit in the Shadow Status Register are both cleared to zero when this FIS is received. 
     The fourth relevant FIS type is DMA Setup—Device to Host or Host to Device FIS, which is used to request the host or device to program its DMA controller before transferring data, and to establish DMA hardware context for one or more data transfers. 
     A further relevant FIS type is Data—Host to Device or Device to Host. 
     Also according to the Serial-ATA 1.0 Specification, transmission of a Register—Host to Device FIS can be initiated by a write operation to the Command Register in the host controller&#39;s Control Block Register. Upon initiating transmission, the current contents of the Control Block Register are transmitted, and the host controller sets the BSY bit in the Shadow Status Register. Upon reception of a valid Register—Host to Device FIS from the Command Register, the device updates its local copy of the Command Register contents, and initiates execution of the command indicated in the Command Register. 
     II. Queued Command-Based Switching Serial-ATA Port Multiplier 
       FIG. 2  shows a flow chart for a read operation of a queued command-based switching serial-ATA port multiplier in the NCQ mode according to one embodiment of the present invention. 
     Serial-ATA II: Extensions to Serial ATA 1.0 (“Serial-ATA II Extensions 1.0”) defines Native Command Queuing, which utilizes the reserved 32-bit field in the Set Device Bits FIS to convey the pending status for each of up to 32 outstanding commands. The state of the BSY bit in the Status Shadow Register is checked prior to attempting to issue a new queued command. If the BSY bit is set to one, issue of the next command is deferred until the BSY bit is cleared to zero. The native queuing commands include a tag value that uniquely identifies the command. The tag value is in the range 0 through 31 inclusive. 
     As defined in Serial-ATA II Extensions 1.0, Serial-ATA Native Command Queuing allows the host controller to issue concurrent commands to a device. Along with the command, the host controller provides a tag whose value uniquely identifies the command. When the device is ready to perform one of the NCQ commands, regardless of the command&#39;s order in the queue, it sends a DMA Setup FIS, which notifies the host controller with the TAG of the command. The host controller then identifies the command according to the TAG and restores the command parameters to execute the data transaction. A serial-ATA device supports up to 32 concurrent queued commands and may perform them out of order. As discussed below, the NCQ command sent by the host controller of the present invention also contains a device&#39;s port address in the PM Port field, and consequently can issue multiple NCQ commands to multiple devices at a time. 
     To initiate a read operation, the host controller updates all Command Registers and issues a read command at step  201 . At step  202 , the host controller sets the BSY bit in the Shadow Status Register. At step  203 , the host controller transmits to a device a Register—Host to Device FIS. The Register—Host to Device FIS contains the device&#39;s port address in the PM Port Field indicating the device to be read, and a TAG in another field uniquely identifying that command. The port multiplier determines the device to be read according to the value of the PM Port Field in the FIS. The port multiplier then locks the device port between this device and the host. If there is an existing locked port, the port multiplier releases that device port. 
     At step  204 , when the device has queued the command and wishes to release the bus, it sends a Register—Device to Host FIS with bits BSY=0 and DRQ=0. The port multiplier then releases the device port. 
     At step  205 , when the device is ready to complete the data transaction of the queued command, it sends DMA Setup FIS to the host controller, sending back the TAG of the command to be performed. The port multiplier then determines the device that sends the DMA Setup FIS, locks the device port between the device and the host. 
     At step  206 , the host DMA is programmed with the transfer parameters associated to the command identified by the received TAG. At step  207 , the device sends Data—Device to Host FISes. At step  208 , the host controller conceptually attaches data from the device to the host DMA. The data are added to the tail of a FIFO. Then the host DMA pulls the data from the head of the FIFO and forwards the data to its destination in the memory. Meanwhile, the FIFO flow control prevents the FIFO from being overloaded. 
     If it is determined at step  209  that the data transfer is successfully completed, at step  210 , the device sends a Set Device bits—Device to Host FIS with command complete status, and the port multiplier releases the device port. Alternatively, if the data transfer contains an error, at step  211 , the device sends a Set Device bits—Device to Host FIS with the ERR bit in status register of the FIS set, the BSY and DRQ bits in the Shadow Status Register are cleared to zero, and the port multiplier releases the device port. 
       FIGS. 3A and 3B  show a flow chart for a write operation of a queued command-based switching serial-ATA port multiplier in the NCQ mode according to one embodiment of the present invention. 
     To initiate a write operation, the host controller updates all Command Registers and issues a write command at step  301 . At step  302 , the host controller sets the BSY bit in the Shadow Status Register. At step  303 , the host controller transmits a Register—Host to Device FIS. The Register—Host to Device FIS contains in its PM Port Field the port address of the target device, and in another field a TAG uniquely identifying the write command. The port multiplier determines the device to be written to according to the value of the PM Port Field in the FIS. The port multiplier then locks the device port between the device and the host. If there is an existing locked device port, the port multiplier releases that device port. 
     At step  304 , when the device has queued the command and wishes to release the bus, it transmits a Register—Device to Host FIS with bits BSY and DRQ cleared to zero. The port multiplier then releases the device port. 
     At step  305 , when the device is ready to complete the data transaction of the queued write command, it transmits DMA Setup FIS to the host, sending back the TAG of the command to be performed. The port multiplier determines the device that sends the DMA Setup FIS, and locks the device port between the device and the host. 
     At step  306 , the host DMA is programmed with the transfer parameters associated to the command identified by the received TAG. If it is determined at step  307  that the Auto-Activate in the DMA Setup FIS is set, the operation proceeds to step  309 . 
     When the device is ready to receive data, the device sends to the host a DMA Activate FIS at step  308 . At step  309 , the host sends a single Data—Host to Device FIS. 
     At step  310 , if additional Data—Host to Device FISes are required to complete, then steps  308  to  309  are repeated. Otherwise, if it is determined at step  311  that the data transfer is successfully completed, at step  312 , the device sends a Set Device bits—Device to Host FIS with command complete status, and port multiplier releases the device port. Alternatively, if the data transfer contains an error, at step  313 , the device sends a Set Device bits—Device to Host FIS with the ERR bit of the status register of the FIS set, the BSY and DRQ bits in the Shadow Status Register are cleared, and the port multiplier releases the device port. 
     To speed up data transfer between the host and the device, a memory cache can be used to bridge the device and the port multiplier. As a result, when the host switches from one device port to another device port, it works against the high speed cache, instead of the low speed device. 
     The methods shown in  FIGS. 2 and 3  not only allow the host controller to have multiple commands outstanding to multiple devices, but also save the device&#39;s data transaction preparation time, thus improving data transfer efficiency between the host and the device. The use of cache further increases data transfer speed. 
     In another embodiment, a queued command-based switching serial-ATA port multiplier of the present invention supports both the QDMA operation mode and the NCQ operation mode.  FIGS. 4A and 4B  show a flow chart for a read operation of such a serial-ATA port multiplier, and  FIGS. 4C and 4D  show a flow chart for a write operation of such a serial-ATA port multiplier. Steps  401 ,  402  and  403  are similar to steps  201 - 203  in  FIG. 2 . Steps  410 ,  411  and  412  are similar to steps  206 - 208  in  FIG. 2 . Steps  451 ,  452  and  453  are similar to steps  301 - 303  in  FIG. 3 , and steps  460 - 464  are similar to steps  306 - 310  in  FIG. 3 . For the sake of brevity, only steps which differ from those shown in  FIGS. 2 ,  3 A and  3 B will be described here. 
     During a read operation, when the device has queued the command, it sends a Register—Device to Host FIS with bits REL=1, BSY=0 and DRQ=0 at step  404 . At step  405 , when the device is ready to complete the data transaction, it sends Set Device bits FIS with bits SERV=1, and the port multiplier locks the device port. Then the host issues a Service Command at step  406 , sets BSY bit in the Shadow Status Register at step  407 , and transmits a Register—Host to Device Service Command FIS to the device at step  408 . At step  409 , the device sends a Register—Device to Host FIS conveying a tag of the command to be performed. At step  413 , when the data transfer is completed, the device sends a Register—Device to Host FIS with command complete status, and the port multiplier releases the device port. 
     Similarly, during a write operation, when the device has queued the command, it sends a Register—Device to Host FIS with bits REL=1, BSY=0 and DRQ=0 at step  454 . At step  455 , when the device is ready to complete the data transaction, it sends Set Device bits FIS with bits SERV=1, and the port multiplier locks the device port. Then the host issues a Service Command at step  456 , sets BSY bit in the Shadow Status Register at step  457 , and transmits a Register—Host to Device Service Command FIS to the device at step  458 . At step  459 , the device sends a Register—Device to Host FIS conveying the TAG of the command to be performed. At step  465 , when the data transfer is completed, the device sends a Register—Device to Host FIS with command complete status, and the port multiplier releases the device port. 
     Thus, the port multiplier of the present invention allows a host, which does not support FIS-based switching, to issue multiple Native Command Queuing commands and/or Queued DMA commands to multiple serial-ATA devices. The port multiplier can use a number of registers to carry out the locking mechanism of the present invention, e.g., a port multiplier Lock Control Register and a port multiplier Lock Status Register. 
     One embodiment of the port multiplier of the present invention comprises two serial-ATA host controllers, which communicate with each other via a serial-ATA interface.  FIG. 13  shows a block diagram of a port multiplier according to such an embodiment. The two communication channels between serial-ATA host controller cards  1301  and  1302  use the serial-ATA interface described above. Other ports of the two cards are used to connect devices. Such a configuration is described in detail in U.S. application Ser. No. 10/667,197 filed on the same day as the present application, which is incorporated herein by reference. 
     III. Host Controller for Queued Command-Based Switching—NCQ 
       FIGS. 5A-5B  show a command request status according to one embodiment of the present invention. The Command Request Queue is the interface that the CPU uses to request a data transaction between the host and the devices. The Command Request Queue is a 32-entry long, circular queue (FIFO) whose location is configured by a Request Queue In-pointer Register and a Request Queue Out-pointer Register. A queue is empty when the Request Queue Out-pointer reaches the Request Queue In-pointer as shown in  FIG. 5A , and a queue is full when the Request Queue In-pointer is written with the same value as the Request Queue Out-pointer. For example,  FIG. 5B  shows a queue which has one empty block; addition of another 32-byte Command Request Block (CRQB) would result in the Request Queue In pointer being written with the same value (Entry 3) as the Request Queue Out-pointer. A queue contains N entries when the request Queue Out-pointer is N less than the Request Queue In-pointer, taking into account a wraparound condition. Each entry, a 32-byte Command Request Block (CRQB), contains DMA parameters and commands for the device. The CRQB data structure is written by the CPU, and involves a number of registers, e.g., ID registers, Flag registers, and Time Stamp registers. The Command Request Queue provides the tag of a command. In one embodiment, the tag of a command corresponds to the location of an entry in the Command Request Queue. 
     In addition, the physical memory region to be transferred is described by the Physical Region Descriptor (PRD). The data transfer proceeds until all regions described by the PRDs in the table have been transferred. 
       FIGS. 5C-5D  show a command response status according to one embodiment of the present invention. The response queue is the interface that the DMA uses to notify the CPU that a data transaction between the host and the device has been completed. The response queue is a 32-entry long, circular queue (FIFO) whose location is configured by the Response Queue In-Pointer Register and the Response Queue Out-pointer Register. A queue is empty when the Response Queue Out-pointer reaches the Response Queue In-pointer as shown in  FIG. 5C , and a queue is full when Response Queue In-pointer is written with same value as a Response Queue Out-pointer. For example,  FIG. 5D  shows a queue which has one empty block; addition of another 8-byte Command Response Block (CRPB) would result in the Request Queue In-pointer being written with the same value (Entry 3) as the Request Queue Out-pointer. A queue contains N entries when the Response Queue Out-pointer is N less than the Response Queue In-pointer, taking into account wraparound condition. Each entry is a 8-byte Command Response Block (CRPB). 
       FIG. 6  shows a Register—Host to Device FIS issued by a NCQ host controller used for queued command-based switching according to one embodiment of the present invention. As shown, the FIS contains the port address of the device in the PM Port Field, and the TAG of the command in the Sector Count Field. 
       FIG. 7  shows a flow chart for the operation of a NCQ host controller used for queued command-based switching according to one embodiment of the present invention. 
     At step  701 , the NCQ host controller sends a queue of up to 32 commands via Register—Host to Device FISes to multiple serial-ATA devices via a serial-ATA port multiplier. The NCQ host controller attaches to each of the FISes a TAG provided in the CRQB as the tag of the command, and the port address of a device. The NCQ host controller stores the PRD pointer of each queued FIS. 
     Whenever a device is ready to perform one of the NCQ commands, regardless of its order in the queue, it sends to the host controller a DMA Setup FIS at step  702 . The A Setup FIS notifies the host controller with the TAG of the command. Also, the port multiplier provides the port address of device sending the DMA Setup FIS. 
     At step  703 , the host controller reads the TAG and the port address of the device, restores the command PRD pointer, and performs the data transaction indicated by the TAG with the device indicated by the port address. 
     At step  704 , the host controller checks status registers in the device to verify normal completion of the command. 
     If the command is successfully completed, the host controller updates the response queue at step  705 , and then returns to step  701 . Otherwise, the host controller generates an interrupt, releases control of the device to the CPU at step  706 , and returns to step  701 . 
     In this embodiment, the device sends the TAG back to the host. It should be understood that the device can send to the host other type of information identifying the command that the device is ready to perform. 
     When a NCQ host controller is used to send a queue of commands to only one device, the serial-ATA port multiplier is not involved, and operations related to PM Port Field and device port address can be omitted. 
     IV. Queued Command-Based Switching—QDMA 
     The Queued Command-based switching of the present invention supports Queued DMA commands (QDMA), which is also called Tag Command Queuing (TCQ). 
     QDMA commands allow the CPU to issue concurrent commands to a device. Along with the command, the host controller provides a tag to uniquely identify the command. When the device is ready to perform one of the QDMA commands, regardless of the command&#39;s order in the queue, it sends a Set Device bits FIS. In response to a Service Command from the host, the device then notifies the host controller with the TAG of the command. After that, the host controller identifies the command according to the TAG and restores the command parameters to execute the data transaction. A serial-ATA device supports up to 32 concurrent QDMA commands and may perform them out of order. When a port multiplier is used, the QDMA command of the present invention also contains a device port number in the PM Port Field, and consequently can issue multiple QDMA commands to multiple devices. 
     The read and write operations of queued command-based switching for QDMA operation mode are shown in  FIGS. 4A to 4D . One difference between NCQ and QDMA operation modes is that a device in the QDMA operation mode sends Set Device bits FIS with its SERV bit set, instead of DMA Setup FIS, when it is ready to execute a QDMA command. Another difference is that the host controller in the QDMA operation mode issues Service Commands to the device upon receipt of the Set Device bits FIS by transmitting a Register—Host to Device Service Command FIS to the device. In response, the device sends a Register—Device to Host FIS conveying the TAG of the command to be performed with its DRQ bit set. When the data transfer is successfully completed, the device sends a Register—Device to Host FIS with command complete status, BSY=0 and DRQ=0, and the port multiplier releases the device port. 
     V. Host Controller for Queued Command-Based Switching—QDMA 
       FIG. 8  shows a flow chart for the operation of a QDMA host controller in queued command-based switching according to one embodiment of the present invention. 
     At step  801 , the QDMA host controller sends a queue of up to 32 commands via Register—Host to Device FISes to multiple serial-ATA devices. The QDMA host controller attaches to each of the FISes a tag provided in the CRQB as the tag of the command, and port address of a device. The QDMA host controller stores the PRD pointer of each queued FIS. 
     Whenever a device is ready to perform one of the QDMA commands, regardless of its order in the queue, it sends to the host controller a Set Device bits FIS with its SERV bits set at step  802 . 
     At step  803 , the QDMA host controller issues Service Command to the device by a Register—Host to Device Service Command FIS. At step  804 , the host controller reads the TAG of the command to be performed from a Register—Device to Host FIS from the device and the port address of the device, restores the command PRD pointer, and performs the data transaction indicated by the TAG with the device indicated by the port address. 
     At step  805 , the host controller checks status registers in the device to verify normal completion of the command. 
     If the command is successfully completed, the host controller updates the response queue at step  806 , and then returns to step  801 . Otherwise, the host controller generates an interrupt, releases control of the device to the CPU at step  807 , and returns to step  801 . 
     Again, when a QDMA host controller is used to send a queue of commands to only one device, the serial-ATA port multiplier is not involved, and operations related to PM Port Field and device port address can be omitted. 
     VI. Non-Queued Commands 
     The port multiplier of the present invention is compatible with non-queued commands, wherein the host issues next command to a device only when the previous command has completed. In such a case, the port multiplier locks a device port when it receives a Register—Host to Device FIS. 
     VII. Host Controller for Queued Command-Based Switching—Supporting both NCQ and QDMA 
     In one embodiment of the present invention, the host controller for queued command-based switching supports both NCQ operation mode and QDMA operation mode. 
       FIG. 9  shows a block diagram of the operation mode selection module of a host. A DMA  901  supports all operation modes: NCQ, QDMA, and non-queued DMA. The DMA  901  can work as state machine for each of operation modes, but can support one operation mode at a time. The DMA  901  controls the host DMA, the host transport layer and a device  903 . A host  900  tests the capability of the device  903  by Identity Device commands, and an operation mode selector  902  selects an operation mode that best fits the device according to predetermined priorities. In one embodiment, the priority sequence is as follows: NCQ, QDMA, and non-queued DMA. The host  900  then configures the DMA  901  according to the selected operation mode. When a specific operation mode is selected, the host  900  is allowed to insert into the CRQB only the associated group of commands corresponding to this operation mode. In one embodiment, the host software is in charge of testing device capability and selecting operation mode. It should be understood that hardware could have similar functions. 
       FIG. 10  shows a flow chart of a method used by a host controller to select an operation mode according to one embodiment of the present invention. 
     Looking at a hard disk drive as an example of a serial-ATA device, each hard disk drive has a command set which defines how to work with the particular drive. During the initialization process of a hard disk drive, when the DMA  901  is disabled, the host  900  issues Identity Device commands at step  1001 . The disk drive informs the host  900  of its capability at step  1002 . Based on such capability information, at step  1003 , the host  900  decides whether NCQ is an operation mode supported by the disk drive. If yes, NCQ is selected as the operation mode at step  1004  and the process goes to step  1011 , where the host  900  adjusts the disk drive to NCQ operation mode by SET FEATURES commands, adjusts the DMA  901  to NCQ operation mode, inserts NCQ commands into CRQB, and executes NCQ commands. If NCQ is not supported, it is determined at step  1005  whether QDMA is an operation mode supported by the disk drive. If yes, QDMA is selected as the operation mode at step  1006 , and the process goes to step  1012 , where the host  900  adjusts the disk drive to QDMA operation mode by SET FEATURES commands, adjusts the DMA  901  to QDMA operation mode, inserts QDMA commands into CRQB, and executes QDMA commands. If QDMA is not supported, non-queued DMA is selected as the operation mode at step  1007 , and the process goes to step  1013 , where the DMA  901  executes non-queued DMA commands. Accordingly, the selection of operation mode is a part of the initialization process of the serial-ATA device. 
     QDMA and NCQ are two types of commands currently known for Queued Command-based Switching. It should be understood that the host controller of the present invention can be used for other types of commands for Queued Command-based Switching. In addition, the host controller of the present invention uses a number of registers to generate, transmit and receive the FISes involved. 
     Finally,  FIG. 11  shows a disk drive as an example of a serial-ATA device to which the present invention is applicable. As shown in  FIG. 11 , an exemplary hard disk drive system comprises a disk  1101 . A motor  1102  spins the disk  1101  at a substantially constant speed and under control of a motor controller  1103 . An actuator  1104  positions a recording head  1105  over the proper data track on the disk  1101 . Data is transmitted through the recording head  1105  via a communications channel  1106 . The communications channel  1106  communicates with a host through a serial-ATA interface  1107 . Skilled practitioners in the art will recognize that other disk drive configurations are possible. 
     The host controller described herein has application in many different kinds of systems, from computers and servers, to network attached storage (NAS) systems, storage area networks (SAN), redundant arrays of inexpensive disks (RAID), and JBOD (Just a Bunch of Disks).  FIG. 12  shows an embodiment of a serial-ATA system employing one or more serial-ATA host controllers according to the present invention.  FIG. 12  shows a plurality of host controllers or computers or servers  1200 - 1  to  1200 - n . Each host controller or computer or server  1200 - n  is in communication with a plurality of serial-ATA devices  1200 - n - 1  to  1200 - n - m  (e.g., element  1200 - 1  is in communication with serial-ATA devices  1200 - 1 - 1  to  1200 - 1 - m ; element  1200 - 2  is in communication with serial-ATA devices  1200 - 2 - 1  to  1200 - 2 - m ; element  1200 - n  is in communication with serial-ATA devices  1200 - n - 1  to  1200 - n - m ). Each element  1200 - 1  to  1200 - n  may function as a stand-alone host controller, computer, or server. The serial-ATA devices  1200 - 1 - 1  to  1200 - 1 - m , for example, may be attached in either a RAID or a JBOD configuration, as skilled artisans will appreciate. In addition, the elements  1200 - 1  to  1200 - n  may function by themselves to provide a NAS configuration, or may be connected together in various SAN configuration, again as known to skilled artisans. The invention is not limited to specific stand-alone, NAS, or SAN configuration. Skilled artisans will appreciate readily that such host controllers will work with many types of storage systems and configurations, as well as with systems employing other types of Serial-ATA devices. 
     The Serial-ATA 1.0 Specification and Serial-ATA II PM 1.0 Specification are incorporated herein by reference. It should be understood that the present invention is intended to apply to serial-ATA devices configured according to these Specifications, all preceding Serial-ATA Specifications, and also all succeeding Serial-ATA Specifications. 
     While the invention has been described in detail above with reference to some embodiments, variations within the scope and spirit of the invention will be apparent to those of ordinary skill in the art. Thus, the invention should be considered as limited only by the scope of the appended claims.