Abstract:
Circuitry is described for interfacing a software-based modem in a computer system. Memory/modem interface circuitry is integrated within a system controller coupling a main memory with a microprocessor. A dedicated region of the main memory is configured as separate transmit data and received data buffer regions. Buffer address registers included within the system controller store values pointing to address locations within the buffer regions of the memory for next data in and next data out. The values programmed in these registers are incremented responsive to associated data transfers in/out of the buffer regions, and the transmit data and received data buffer regions function as FIFOs. The frequency and duration of processor utilization imposed by software-based modems is significantly reduced, due to FIFO operations functioning at main memory access speed.

Description:
TECHNICAL FIELD 
     The present invention relates generally to circuitry associated with the interface of a modem in a computer system, and more particularly, to an apparatus associated with the interface of a software-based modem in a computer system. 
     BACKGROUND OF THE INVENTION 
     The use of computers, especially personal computers (PCs), is widespread. Increasingly, PCs are equipped with a modem, which allows a user to link one computer with another for purposes of sharing data. Modems modulate digital data from one computer to an analog audio signal for transmission to another computer via the publicly switched telephone network (PSTN). Modems receiving the analog audio signal then demodulate the signal and reconstruct the digital data. 
     A modem may be a device external to a PC, which is then connected to one of the serial communications ports on the PC. Alternatively, modem circuitry may be included within the PC, typically on a printed circuit board or card connected to one of the PC bus slots, such as an industry standard architecture (ISA) bus slot. Both external and internal modems traditionally include a number of well-known circuits, such as a universal asynchronous receiver/transmitter (UART), a digital signal processor (DSP), a microcontroller, a coder/decoder (CODEC), and a phone line interface such as data access arrangement (DAA) circuitry. 
     With the availability of high-speed processors such as the Pentium® processor, software-based modems have been made possible by the DSP-like instruction sets executable by these processors. Thus, much of the DSP functionality can be replaced by software execution, thereby reducing the hardware requirements for the modem itself. Software-based modems require a circular buffer, or memory arranged as a first-in-first-out (FIFO) memory, for both the transmitted and received data. Such FIFOs are required because the speed at which data is transmitted or received by a modem is typically greater than the data processing speed of a PC running a commercially available operating system, such as Windows NT or Windows 95. 
     Circuitry associated with currently available software-based modems is typically coupled with the ISA bus, although newly available modems are available for coupling with the peripheral component interconnect (PCI) bus. These modems require application specific integrated circuitry (ASIC) coupling the modem CODEC with the associated bus, providing the necessary control signals and data routing to match the CODEC and bus protocols. The ASIC also couples the CODEC with a FIFO, which may or may not be integrated within the ASIC. 
     Currently available software-based modems, while improving modem performance relative to traditional external and internal hardware-based modems, still do not fully exploit the capability of today&#39;s high-speed processors. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a computer system includes a processor operable to transmit and receive data, a memory operable to receive data for storage, and a modem input/output circuit operable to transmit and receive data to and from a telephone network. The modem input/output circuit is connected to a system controller, which couples the processor and memory and controls data transfer therebetween. The system controller is operable to transfer data directly to and from the modem input/output circuit and correspondingly transfer data from and to the memory. The system controller may transfer a serial data stream to and from the modem input/output circuitry and correspondingly transfer a parallel data stream from and to the memory. 
     In one embodiment, the memory includes first and second addressable regions. The first region stores data transferred from the modem input/output circuitry, and the second region stores data for transfer to the modem input/output circuitry. The first and second addressable regions may be operable as FIFOs. The system controller may include a plurality of registers, each storing a value corresponding to a memory address in a respective one of the addressable regions of the memory. The values stored in the registers are incremented, or otherwise adjusted, responsive to corresponding data transfers in or out of the respective addressable regions of the memory. 
     By integrating modem interface circuitry within the system controller and configuring a portion of the computer system main memory as associated FIFOs, significant improvements in processor utilization result. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a computer system including a memory/modem interface in accordance with an embodiment of the present invention. 
     FIG. 2 is a functional block diagram showing certain details of the memory/modem interface of FIG. 1. 
     FIG. 3 is a graphical representation of a buffer address register set included in the memory/modem interface depicted in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a computer system and apparatus for interfacing computer circuitry and modem circuitry is described with certain specific details set forth in order to provide a sufficient understanding of the present invention. It will be clear to one skilled in the art, however, that the present invention may be practiced without these details. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been described in detail in order not to unnecessarily obscure the invention. 
     FIG. 1 shows an embodiment of a computer system 10 in accordance with the present invention. A microprocessor 12 is connected to a processor bus 14 which carries address, data, and control signals. The processor bus 14 is in turn connected to a system controller 16. The processor bus 14 is also connected to a cache memory 18, which is commonly a static random access memory (SRAM) device. The system controller 16 includes memory controller circuitry accessing a main memory 20 via a memory bus 22 carrying memory address, data, and control signals. The main memory 20 may include any of a wide variety of suitable memory devices, such as a dynamic random access memory (DRAM) devices, including synchronous DRAMs, SyncLink DRAMs, or Rambus DRAMs. 
     The system controller 16 also functions as a bridge circuit (sometimes called the North Bridge) between the processor 14 and a system bus, such as I/O bus 26. The I/O bus 26 may itself be a combination of one or more bus systems with associated interface circuitry (e.g., AGP bus and PCI bus with connected SCSI and ISA bus systems). Multiple I/O devices 28-32 are coupled with the I/O bus 26. One or more data input devices 28, such as a keyboard, mouse, etc., are coupled with the I/O bus 26. Also, one or more data output devices 30, such as visual display devices, printers, etc., are coupled with the I/O bus 26. Additionally, one or more data storage devices 32, such as disk drives, tape drives, CD-ROM drives, etc., are coupled with the I/O bus 26. Also coupled with the I/O bus 26 are expansion slots 34 to provide future accommodation of other I/O devices not selected during the original design of the computer system. 
     In accordance with an embodiment of the present invention, memory/modem interface circuitry 36 is integrated within the system controller 16. The memory/modem interface 36 provides an interface between the main memory 20 and modem input/output circuitry 24 via a communications bus 25. The modem input/output 24 is in turn connectable to the PSTN via a conventional telephone line 37. This configuration differs significantly from conventional computer system architectures, in which modem circuitry is coupled with the I/O bus 26. 
     Referring to FIG. 2, particular embodiments of the modem input/output 24, the memory/modem interface 36, and the main memory 20 are shown. The modem input/output 24 includes modem CODEC circuitry 38 and telephone line interface circuitry 40, which may include DAA circuitry. The communications bus 25 includes a serial CODEC interface 44 for transferring digital data in a serial data stream between the CODEC 38 and the memory/modem interface 36. 
     The memory/modem interface 36 includes a CODEC protocol state machine (CPSM) 42, which receives a clock signal CLK from a clock circuit 43 included in the system controller 16. As will be understood by those skilled in the art, the CPSM 42 produces the requisite control signals associated with CODEC and main memory access protocols. The CPSM 42 moves digital data in a serial data stream to and from the CODEC 38 via the serial CODEC interface 44, and initiates movement of data to and from the main memory 20 in a parallel data stream. As shown in the embodiment of FIG. 2, the CPSM 42 interfaces directly with the CODEC 38 and directly controls the data stream to and from the CODEC. The CPSM 42 may similarly directly interface with the main memory 20, but preferably passes memory access control to memory controller circuitry 46 included within the system controller 16, as will be understood by those skilled in the art. 
     The main memory 20 includes a plurality of addressable memory locations 21 to which data can be transferred for storage and from which stored data can be retrieved. Of these memory locations 21, a particular portion or dedicated region 48 of the main memory 20 acts as circular buffers for CODEC data (i.e., data received from or to be transmitted to the CODEC 38). Data received from the CODEC is stored in a received data buffer region 50, and data to be transmitted to the CODEC is stored in a transmit data buffer region 52. The sizes of the received data buffer 50 and the transmit data buffer 52 may be selected according to values programmed in associated registers, or are preferably fixed in size. A suitable fixed buffer size is, for example, 64 kB for each of the received data and transmit data buffers 50, 52, but may alternatively be fixed at different sizes. The particular location or address range of the dedicated region 48 may be determined by a value programmed into a base-address register, or is preferably hard coded into logic circuitry included within the system controller 16, as will be understood by those skilled in the art. 
     The memory/modem interface 36 includes buffer address registers 54 for storing programmable values associated with memory address locations, such as memory address pointer values, and for providing these values to the memory controller 46. Referring to FIG. 3, four registers are provided, with two registers 56, 58 associated with the received data buffer 50, and two registers 60, 62 associated with the transmit data buffer 52. 
     When a data word is received from the CODEC 38, the CPSM 42 arranges or initiates arrangement of the data in a parallel configuration and initiates the writing of this data word to the received data buffer region 50 of the main memory 20. An Rx NextDataln register 56 stores a memory address pointer value associated with the memory address to which the received data word is to be written. An Rx NextDataOut register 58 stores a memory address pointer value associated with a memory address from which a previously received data word is next in line for retrieval by the microprocessor 12 or other computer system circuitry accessing CODEC data (see FIG. 1). 
     When data is to be transmitted to the CODEC 38, the CPSM 42 retrieves or reads a data word stored in the transmit data buffer 52 and provides it to the CODEC 38 in a serial format. A Tx NextDataOut register 60 stores a memory address pointer value associated with the memory address from which the data word is to be read. A Tx NextDataln register 62 stores a memory address pointer value associated with the memory address to which a data word is next to be written by the microprocessor 12 or other computer system circuitry prior to subsequent transmission to the CODEC 38. 
     As data is read from or written to the buffers 50, 52, the corresponding register is incremented or decremented, as desired. The actual physical memory address is calculated (preferably in hardware logic) by summing a base address value 64 (whether hard coded in logic or stored in a separate register) with the contents of the appropriate one of the registers 56-62. The unit for data movement between the buffers 50, 52 and the CODEC protocol state machine 42 is preferably fixed, with a single data word sufficing. As desired, improved memory bandwidth may be attained by longer bursts of data, in which case the unit for a data movement may be multiple data words. 
     In accordance with the above-described embodiments of the present invention, significant performance improvements are achieved over currently available software-based modem architectures. The transfer of data between the CODEC 38 and the dedicated region 48 of the main memory 20 is performed with minimal processor operations. Once initiated, transfer of CODEC data occurs at full memory sub-system speed. The CODEC data is transferred directly between the main memory 20 and the CODEC 38 without requiring processor read/write operations. This contrasts significantly with current designs, in which a processor must regularly perform I/O read/write and corresponding memory write/read operations to effect data transfer between a modem CODEC and main memory. In accordance with the above-described embodiments, the microprocessor 12 need only perform read or write operations when the received data buffer 50 or transmit data buffer 52 is full or empty, all respectively. 
     The relative values of the registers 56-62 associated with each of the buffers 50, 52 indicate the status of the buffers--namely whether the buffers are full, empty, nearly full, nearly empty, etc. Correspondingly, requests for service from the microprocessor 12 and control of the phone line interface 40 are initiated as appropriate (see FIG. 2). During transmission of CODEC data, for example, the relative values stored in registers 60 and 62 might indicate that the transmit data buffer 52 is nearly empty. In this case, requesting service from the microprocessor 12 to write data to the transmit data buffer 52 avoids undue interruptions to the CODEC data stream. If the relative values stored in registers 60 and 62 indicate the transmit data buffer 52 is nearly full, operations of the phone line interface 40 are initiated to transmit CODEC data via the PSTN 37. 
     Referring to FIG. 2, the memory/modem interface 36 includes phone line interface controller circuitry 66 and interrupt request circuitry 68. The interrupt request circuitry 68 asserts an interrupt request signal IRQ to request execution of the appropriate interrupt service routine by the microprocessor 12. The system controller 16 may include an interrupt controller 70 for receiving the interrupt request signal IRQ and correspondingly producing an interrupt signal INT for provision to the microprocessor 12. The interrupt controller 70 may be a conventional programmable interrupt controller (PIC), an I/O advanced programmable interrupt controller (I/O APIC), or other suitable interrupt controller, and may be integrated within the system controller 16 (as shown in FIG. 2) or included elsewhere within the computer system 10. 
     It is desirable that access to CODEC data by the microprocessor 12 or other computer circuitry not differ significantly from current approaches. In other words, addressing the buffers 50, 52 in accordance with the above-described embodiments should appear (to the microprocessor 12, for example) much the same as addressing conventional modem circuitry coupled to the I/O bus 26. A separately dedicated I/O address or memory address range can be used for such purposes, with the address/range being set in a configuration space, as will be understood by those skilled in the art. The microprocessor 12 or other circuitry accessing CODEC data then provides this address/range, thereby initiating the address calculation described above, in which the actual physical location in the main memory 20 is determined from the contents of the registers 56-62 in the system controller 16. 
     Those skilled in the art will appreciate that the above-described embodiments of the present invention achieve a number of advantages over currently available software-based modem architectures. By integrating CODEC interface circuitry within the system controller 16, the requirement for a separate FIFO memory device and associated ASIC circuitry is avoided. Further, in accordance with the above-described embodiments, the microprocessor 12 has reduced cycle latency relative to conventional designs, since data may be transmitted at full memory sub-system speed. Thus, the frequency and duration of processor utilization imposed by software-based modems is significantly reduced. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. As one example, the CODEC 38 could itself be integrated within the memory/modem interface circuitry 36 of the system controller 16. Accordingly, the invention is not limited by the disclosed embodiments, but instead its scope is determined by the following claims.