Abstract:
BIOS instructions are transferred from a BIOS ROM to a processor for either execution or storage in a system memory. The BIOS ROM has an address bus coupled to an address bus of the processor and a data bus coupled to the an intelligent drive electronics (“IDE”) controller through the data bus portion of an IDE bus. In operation, the processor applies addresses directly to the address bus of the BIOS ROM, and the corresponding instructions are coupled through the IDE data bus and the system controller to the data bus of the processor.

Description:
TECHNICAL FIELD 
     The present invention relates to computer systems, and, more particularly, to a method of transferring data through a bus bridge in a manner that reduces the number of external terminals of the bus bridge. 
     BACKGROUND OF THE INVENTION 
     When a computer system is powered on or reset, computer instructions are executed that are part of a basic input/output system (“BIOS”) program. The BIOS program is normally in the form of firmware routines stored in a read only memory (“ROM”), which may or may not be a programmable read only memory (“PROM”). The processor may execute the BIOS program directly from the BIOS ROM. However, the BIOS program is usually transferred from the BIOS ROM to system memory, such as dynamic random access memory (“DRAM”), in a process known as “BIOS shadowing.” Following transfer of the BIOS program to system memory, the processor is initialized and then executes initialization routines, or bootstrap routines, that are part of the BIOS program from the system memory. This entire process, including any shadowing of the firmware routines from the ROM to the system memory, is known as “booting” the computer system 
     If the processor executes the BIOS program directly from the BIOS ROM, it must repeatedly apply an address to the ROM and then couple an instruction to the processor that is stored at the address in the ROM. If the BIOS program is shadowed, the processor repeatedly fetches and executes instructions for transferring the BIOS program from the BIOS ROM, as well as the BIOS program itself, in a multi-step process. In either case, the BIOS program instructions are transferred over a relatively low-speed bus through a bus bridge to a processor bus that is connected to the processor. 
     A variety of configurations may be used in a computer system to couple a BIOS ROM to a processor. Examples of such systems are illustrated in FIGS. 1 and 2. With reference to FIG. 1, a computer system  10  includes a processor  14 , such as an Intel® Pentium® processor or Pentium II® processor, although other processor may, of course, be used. For example, the processor  14  may be any microprocessor, digital signal processor, micro controller, etc. The processor  14  is coupled to a processor bus  16  which includes data, control, and address buses (not shown) that provide a communication path between the processor  14  and other devices, as explained below. One device with which the processor  14  communicates is a cache memory device  18 , typically cache static random access memory (“SRAM”), which is also coupled to the processor bus  16 . As is well known in the art, the cache memory device  18  is generally used for the high speed storage of instructions that are frequently executed by the processor  14 , as well as for data that are frequently used by the processor  14 . 
     Also coupled to the processor bus  16  is a system controller  20 . The system controller  20  performs two basic functions. First, the system controller  20  interfaces the processor  14  with a system memory  22 , which is generally a dynamic random access memory (“DRAM”). More specifically, the system memory  22  may be an asynchronous DRAM, a synchronous DRAM (“SDRAM”), a video or graphics DRAM, a packetized DRAM, such as a synchronous link DRAM (“SLDRAM”), or any other memory device. The system controller  20  includes a DRAM controller  24 , which interfaces the processor  14  to the system memory  24  to allow the processor  14  to write data to and read data from the system memory  22 . Basically, the system controller  20  performs this function by receiving and sending data to the processor  14  (although the data may bypass the system controller  20  by being coupled directly to the processor bus  16 ), receives addresses from the processor  14 , and receives high level command and control signals from the processor  14 . In response, the system controller  20  couples the data to and from the system memory  22  via a data bus  32 , generates separate row and column addresses and sequentially applies them to the memory device via an internal address bus  34 , and generates and applies to the system memory  22  lower level command signals via a control bus  36 . 
     The second function performed by the system controller  20  is to interface the processor bus  16  to a peripheral I/O bus, such as a Peripheral Component Interconnect (“PCI”) bus  40 . The PCI bus  40 , in turn, is coupled to a conventional PCI-ISA bus bridge  42  and a conventional VGA controller  44  driving a conventional display  46 . The PCI bus  40  may also be connected to other peripheral devices (not shown) in a manner well known to one skilled in the art. The PCI-ISA bus bridge  42  may also include a disk drive controller, such as an Intelligent Drive Electronics (“IDE”) controller  48 , which controls the operation of an IDE disk drive  50  in a conventional manner. 
     The PCI bus  40  is a relatively high speed peripheral I/O bus. Many peripheral devices are adapted to interface with a relatively slow speed peripheral I/O bus, known as an industry standard architecture (“ISA”) bus. The computer system  10  illustrated in FIG. 1 includes an ISA bus  60  that may be coupled to such I/O devices as a Keyboard Controller, Real Time Clock, and Serial and Parallel Ports, all of which are collectively designated by reference number  62 . The ISA bus  60  may also be coupled to a BIOS ROM  64  as well as other I/O devices (not shown) as is well known in the art. The BIOS ROM  64  stores the BIOS program, which, as explained above, is executed by the processor  14  at boot-up, either directly or after being transferred to the system memory  22  if the BIOS is shadowed. 
     Although the BIOS ROM  64  is shown in the computer system  10  of FIG. 1 coupled to the ISA bus  60 , it will be understood that it has conventionally been coupled to other components or buses, including the PCI bus  40 , the IDE controller  48  within the PCI-ISA bridge  42 , and a controller within the system controller  20 . For example, an alternative example of a conventional computer system  70  shown in FIG. 2 includes many of the same components used in the computer system  10  of FIG.  1 . Therefore, in the interest of brevity, an explanation of their structure and operation will not the repeated. The system  70  uses a system controller  80  that includes not only a DRAM controller  82  and a PCI bus controller  84 , but also an accelerated graphics processor (“AGP”) controller  86  and an IDE controller  88 . The computer system  70  shown in FIG. 2 thus reflects the trend in computer architecture to couple as many components as possible to the system controller  80 . The AGP controller  86  is coupled to an accelerated graphics processor  90  which is, in turn, coupled to a display  94 . The IDE controller  88  is coupled through an IDE data bus  96  and an IDE control bus  98  (sometimes known as PC AT Attached (“ATA”) buses) to a BIOS ROM  100  as well as to a pair of IDE devices  102 ,  104 , such as disk drives. Not shown in FIG. 2, as will be apparent to one skilled in the art, is circuitry for multiplexing the data bus  96  between an address bus port of the BIOS ROM  100  and a data bus port of the BIOS ROM  100  since the IDE, or ATA, bus does not include an extensive address bus. Instead, the IDE bus includes only 4 address bits. 
     In operation, the system controller  80  is used to interface the processor with all of the other components of the computer system  70  except the cache memory device  18 , i.e., the system memory  22 , the PCI bus  40 , the accelerated graphics processor  90 , and the BIOS ROM  100  and IDE devices  102 ,  104 . When a BIOS instruction is to be transferred, the IDE controller  88  outputs the address of the instruction&#39;s storage location on the IDE data bus  96 , and the BIOS ROM then outputs the instruction which is coupled to the IDE controller  88  through the IDE data bus  96 . 
     One problem with the computer system  10  illustrated in FIG. 1, and particularly the computer system  70  illustrated in FIG. 2, is a proliferation of external terminals that the system controllers  20 ,  80  and the PCI-ISA bridge  42  must have to interface with all of the components to which they are connected. Increasing the number of terminals on an integrated circuit, such as a bus bridge, increases the cost of packaging the integrated circuit, increases the size of the integrated circuit package, increases the cost and complexity of mounting the integrated circuit on a circuit board, and increases the likelihood all of a faulty interconnection. It is therefore desirable to minimize the number of external terminals on an integrated circuit, such as a bus bridge. Although this problem exists to some degree with many integrated circuits in a computer system, it is particularly serious for system controllers and bus bridges since they generally have more external terminals than other integrated circuits in computer systems. 
     The problems resulting from the proliferation of external terminals are exacerbated by two trends in computer system architecture. First, the sizes of data buses continue to increase to support the faster transfer of data, and the sizes of address buses continue to increase to allow addressing larger capacity system memories. As the size of these buses have increased, the number of terminals that the system controller or bus bridge must have to interface with these buses had correspondingly increased. For example, data buses have grown from 16 data bits, to 32 data bits to currently 64 data bits. Even larger data buses can be expected in the future. Second, as mentioned above, there has been a tendency to relocate the interface with peripheral devices closer to the processor to decrease the time required to access the peripheral devices. This trend is illustrated by comparing the computer system  10  of FIG. 1 with the computer system  70  of FIG.  2 . However, as this trend continues, the system controller must interface with additional buses, as also exemplified by the computer system  70  of FIG.  2 . Both of these trends have increased the number of external terminals that the system controller must include and, and as a result, have increased the resulting problems. 
     There is therefore a need to reduce the number of external terminals on the system controllers of computer systems despite industry trends tending to increase the number of such external terminals. 
     SUMMARY OF THE INVENTION 
     An inventive method couples data from an addressable device to a processor in a computer system. For example, initialization instructions may be coupled from a memory device to a processor. In accordance with the method, an address is output from the processor and coupled directly to the addressable device. Data, such as an initialization instruction, is then output from the addressable device, and the data is coupled to the processor through a system controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional computer system in which a BIOS ROM is coupled to a processor through two I/O buses, a bus bridge, and a system controller. 
     FIG. 2 is a block diagram of a conventional computer system having a more modem architecture in which a BIOS ROM is coupled to a processor through a system controller. 
     FIG. 3 is a block diagram of a computer system in accordance with one embodiment of the invention. 
     FIG. 4 is a flow chart showing the initialization operation of the computer system of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A computer system  120  in accordance with one embodiment of the invention is illustrated in FIG.  3 . The computer system  120  includes a processor  122  of conventional design, such as a Pentium® or Pentium II® microprocessor. The processor  122  is coupled to a processor bus  124 , which includes a processor address bus  126  and processor data and control buses  128 . The processor address bus  126  and the processor data and control buses  128  are coupled to a processor interface  129  in a system controller  130 . The system controller includes a system memory controller  132  that is coupled to a system memory  134  through a system memory bus  138 . The system controller  130  also includes a PCI bus controller  140  that is coupled to various PCI devices  144  through a PCI bus  146 . Finally, the system controller  130  includes and IDE controller  150  that is coupled to an IDE data bus  152  and an IDE control bus  156 . Coupled to the buses  152 ,  156  are a BIOS ROM  160  and first and second IDE devices  162 ,  164 . In contrast to conventional practice exemplified by the computer system  70  of FIG. 2, an address of the BIOS ROM  160  is not coupled to the IDE controller  150  and applied to the BIOS ROM  160  through the IDE data bus  152 . Instead, the processor address bus  126  is coupled to the address bus port of the BIOS ROM  160  through a separate ROM address bus  170 . The BIOS ROM  160  is selectively enabled by a chip select (“CS”) signal applied to the BIOS ROM  160  from the IDE controller  150  through line  178 . 
     In operation, the processor  122  writes data to and reads data from the system memory  134  in a conventional manner through the system memory controller  132  in the system controller  130  and through the memory bus  138 . Similarly, the processor  122  interfaces with I/O devices, such as the PCI device  144 , in a conventional manner through the PCI controller  140  in the system controller  130  and through the PCI bus  146 . Finally, the processor  122  interfaces with the IDE devices  162 ,  164  in a conventional manner through the IDE controller  150  in the system controller  130  and the IDE data bus  152  and the IDE control bus  156 . What is not conventional is the manner in which the processor  122  interfaces with the BIOS ROM  160 . The processor  120  reads instructions from the BIOS ROM  160  by first applying the address where the instruction is stored to the ROM  160  through the processor address bus  126  and ROM address bus  170 . When enabled by a chip select signal coupled through the line  178 , the instruction is coupled from the BIOS ROM  160  to the processor  122  through the IDE data bus  152 , the IDE controller  150 , the processor interface  129  and the processor data bus  128 . 
     One advantage of the computer system  120  of FIG. 3 is that the system controller  130  need not include the large number of external terminals that would be required to couple the address bus of the BIOS ROM  160  to the system controller  130 . Furthermore, circuitry for multiplexing the IDE data bus  152  to the data bus port and the address bus port of the BIOS ROM  160  is not required. 
     The operation of the computer system  120  of FIG. 3 during initialization is illustrated in FIG.  4 . The IDE controller  150  (FIG. 3) waits at for an address from the processor  122  at  200 . When an address is received from the processor  122 , a determination is made at  202  whether the received address is in the address space of either the IDE device  162  or the IDE device  164 . If so, a chip select (“CS”) signal for the appropriate IDE device  162 ,  164  is asserted at  204 , and the IDE bus cycle is run at  206 . 
     If a determination is made at  202  that the received address is not in the address space of either the IDE device  162  or the IDE device  164 , then a check is made at  210  to determine if the address is in the address space of the BIOS ROM  160 . If not, the method returns to  200  to wait for another address from the processor. If a determination is made at  210  that the address is in the address space of the BIOS ROM  160 , the IDE controller  150  applies a chip select signal to the BIOS ROM  160  at  214 . The IDE bus cycle is then run at  206  to transfer the instruction from the BIOS ROM  160  to the processor  122 . The above sequence is repeated each time that instruction is transfer from the BIOS ROM  160  to the processor  122 . 
     It will be appreciated that, although a specific embodiment of the invention has been described for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Those skilled in the art will appreciate that many of the advantages associated with these circuits and processes described above may be provided by other circuit configurations and processes. For example, although the BIOS ROM  160  has been described in FIG. 3 as being coupled to an IDE bus, it will be understood that the principles exemplified by this architecture exist for other bus systems, such as a PCI bus, in addition to IDE and Enhanced IDE (“EIDE”) bus systems. Further, although the system  120  shown in FIG. 3 includes a BIOS ROM coupled to the processor in accordance with one embodiment of the invention, it will be understood that ROMs containing other information or other components addressable by the processor may be coupled to the processor in the same or similar manners. Also, although the BIOS ROM is shown with its address bus coupled directly to the address bus of the processor and its data bus coupled to the data bus of the processor through the system controller, it will by understood that the data bus of the BIOS ROM or other device may be coupled directly to the data bus of the processor and the address bus or other buses of the BIOS ROM or other device may be coupled to the processor through a system controller or other device. Finally, although the BIOS ROM is shown as being coupled to a system controller that is coupled to the processor bus, it will be understood that it may be coupled to other bus bridge devices in the same or a similar manner, or even to bus bridge devices, such as a PCI/ISA bus bridge, that are coupled to the processor through a system controller or other bus bridge. Accordingly, the invention is not limited by the particular disclosure above, but instead the scope of the invention is determined by the following claims.