Abstract:
A method and apparatus for improving sustainable data throughput between a host system minicomputer and a plurality of peripheral memories utilizes a dual-ported RAM memory, a microelectronic processor with an instruction rate of 10 MIPS or greater and task switching firmware executed by the microelectronic processor for rapid switching between communication on a peripheral data bus to the peripheral memories and communication on a system data bus having a relatively higher data rate. Also disclosed is a method of data caching on the host system using the peripheral controller.

Description:
This application is a continuation of application Ser. No. 07/906,911, filed Jun. 30, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to peripheral controllers and methods for interfacing computers of the minicomputer class to one or more peripheral memories. 
     2. Description of the Background Art 
     Peripheral controllers-are provided to interface minicomputer central processing units (CPU&#39;s) to various numbers of disk and/or tape memories in an overall computer system. The primary objective of such peripheral controllers is to interface a plurality of peripheral memories to the host computer. 
     Typical host systems are provided by various well known suppliers of computer equipment including Digital Equipment Corporation. Currently, such systems are provided with a system bus structure. Peripheral memories are serviced through a peripheral I/O bus. 
     Interfacing peripheral memories to a system bus involves transferring data on the peripheral I/O bus and then transferring the data onto the system bus. The peripheral I/O bus provides a data path which is limited to the speed of the peripheral device. The peripheral memories are serviced in real time by the peripheral controller, and if data is supplied &#34;late&#34;, there is a loss of performance or a loss of data. Some system buses have a high data rate, but none can guarantee transfer of time-dependent data. One approach to interfacing the two buses involves the use of static RAM buffer memories or FIFO&#39;s (first in, first out RAM memories). Use of FIFO&#39;s and data buffers has not removed the problems of system latencies which tend to limit data throughput. 
     SUMMARY OF THE INVENTION 
     The invention relates to a peripheral controller for interfacing a plurality of peripheral memories to a host system processor and host system memory through a host system data bus. 
     The peripheral controller utilizes a dual-ported RAM which stores data that is transferred between the host system memory and the peripheral memories. The dual-ported RAM is interfaced to the system data bus through a first port and is interfaced to the peripheral memories through a second port and a peripheral data bus rated at a lower speed than the system data path. Transfer through both data buses to and from the dual-ported RAM is under DMA control. 
     The peripheral controller includes a very fast processor interfaced to direct memory access (DMA) circuitry to control data transfers to and from the dual-ported RAM. The peripheral controller processor executes instructions organized for rapid switching between two tasks, including a task for initiating DMA transfer of data through the system data path to the host system memory and a task for initiating DMA transfer of data through the peripheral DMA data path to the peripheral memories. The peripheral controller is operable for rapid switching from one task to another which is essential for fully servicing both DMA paths. Preferably, the peripheral controller processor has a very fast instruction execution rate. 
     It is one object of the present invention to improve the overall sustainable rate of data transfer between peripheral memories and host systems with the same or disparate data rates and data path widths. 
     Another object of the invention is to further enhance throughput by providing very large and very flexible data caching using the host system memory. 
     Another object of the invention is to maximize the throughput between these peripheral memories and host systems regardless of the type of I/O requests being made in the target computing environment (e.g. random vs. sequential, contiguous vs. non-contiguous). 
     The peripheral controller includes two sets of high level DMA control which provides for two completely separate DMA paths to be active independently and simultaneously. Dual DMA paths and controls allow DMA transfers between the system bus and the peripheral controller dual-ported RAM circular buffer (system DMA) to occur simultaneous to DMA transfers between the peripheral controller dual-ported RAM circular buffer and the peripheral memory (Interface DMA). The dual-ported RAM structure, very fast peripheral controller processor, separate high level control of the two DMA paths, and high level management of the dual-ported RAM circular buffers keeps both paths fully serviced at all times. Prior products utilized a single DMA path which forced all DMA to be subject to limitations of both peripheral speed and host system latencies. 
     Rapid task switching is assisted by splitting the stack of the peripheral controller processor into two sections, and using one section to serve two tasks for transferring data on the system bus and peripheral bus, respectively. A limited number of data items are saved in the stack for each task, such saved data items including the run state, stack pointer, restart address and user base register address of the task. 
     In addition, a further aspect of the invention, is that with an external peripheral controller based high speed processor and DMA control of the host system bus, a data cache may be established in the host system memory and data can be transferred to and from the data cache under the control of the peripheral controller, with this control being transparent to the host system processor. 
     The host based data cache can subsequently be very large and relatively inexpensive. Further, its size can be configured by the user to optimize use in target computing environments. 
     Various objects and advantages, besides those discussed above, shall be apparent with those familiar with the art from the description of the preferred embodiment of the invention which follows. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a computer system incorporating the peripheral controller of the present invention; 
     FIG. 2 is a block diagram of the task scheduling firmware used in the peripheral controller of FIG. 1; and 
     FIG. 3 is a block diagram showing firmware and data elements for fast task switching utilized in the peripheral controller of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the invention is incorporated in a disk controller 10 which may be supported on an individual circuit board 11. The overall function of the disk controller 10 is to interface one or more peripheral disk memories 12 and 13 to a host computer system 14 of the minicomputer class, such as those commercially offered by Digital Equipment Corporation. 
     The following is a brief description of the host system 14. The host computer 14 has a host system processor 15 and a host system memory 16. The host system memory includes some target locations 17 and locations for an I/O data cache 18 to be explained in more detail below. The host computer system 14 described includes a system bus 19, referred by the trade designation TURBOchannel. The TURBOchannel provides a 32-bit data path and multiplexed address lines. For further specification of the TURBOchannel bus, reference is made to the literature of Digital Equipment Corporation and others which defines the signals in this bus. The host computer system connects to the disk controller 10 through a connector 20 which includes enough lines to accommodate the TURBOchannel bus. 
     On the other side, the disk controller 10 connects to the disk memories 12, 13 via a daisy chain cabling arrangement which carries signals for an Intelligent Peripheral Interface (IPI) bus 21 which is an industry standard peripheral bus. The disk controller 10 includes a connector 22 for connecting to cables carrying IPI bus signals between the controller 10 and the disk memories 12 and 13. 
     The disk controller 10 includes a dual-ported static RAM 23 of 16 kilobytes capacity has Port A connected to the TURBOchannel connector 20 through a 32-bit data bus (T DATA). The number of lines in the buses to be discussed are indicated by the &#34;/&#34; symbol and the number in parentheses. Also, it should be understood that various buffers and drivers may be used in the address and data paths indicated, but as these are well understood in the art and form no part of the invention, they have not been illustrated in the drawings. 
     The disk controller 10 also includes a high speed processor 24, which in this example, is preferably a Model RTX 2001A microelectronic CPU commercially offered by Harris Semiconductor of Melbourne, Fla. This is a reduced instruction set (RISC) machine with a capacity for executing 10 MIPS (million instructions per second). This processor 24 has a certain amount of internal RAM 25, including registers or locations forming what is known as the &#34;stack&#34;. 
     The disk controller processor 24 is connected through its ASIC port and a 16-bit data bus (GDATA) to a group of DMA (direct memory access) registers 26. The DMA registers 26 include two 16-bit registers which store 16 bits of address each, to be transmitted over the TURBOchannel 19, and a memory address register for coupling a 12-bit address to Port A of the dual-ported RAM 23. The disk controller processor 24 is also connected through a control bus (CONTROL) to DMA control circuitry 27 for initiating and controlling a 32-bit DMA data transfer operation between the host memory 16 and Port A of the dual-ported RAM 23 (System DMA). 
     The disk controller processor 24 has address (ADD) and data (DATA) ports connected to a 20-bit address bus (M ADDRESS) and a 16-bit data bus (M DATA). The disk controller processor 24 is further connected through these buses to a static RAM 28 of 64 kilobytes capacity and 16 bits per addressable location, an EPROM 29 of 64 kilobytes capacity and 16 bits per addressable location, and an EEPROM 30 (electrically erasable PROM) of 8 kilobytes capacity and 8 bits per addressable location. The RAM 28 is used as a scratchpad for storing data and temporary results during operation of the disk controller processor 24, as well as for the storing of certain control tables used in a data caching operation to be discussed below. The EPROM 29 stores a program of firmware operating instructions 31 which are read and executed by the disk controller processor 21. The EEPROM 30 stores system operating parameters 32, which may be altered to effect different modes of operation for the disk controller 10. 
     The dual-ported RAM 23 has a Port B for address (ADD) and data (DATA) connected through thirteen address lines in the M ADDRESS bus and sixteen data lines in the M DATA bus to a disk memory controller chip and an associate error checking chip represented by block 33 in FIG. 1. The disk memory controller chip 33 has DMA control circuitry within, which is responsive to control signals on the CONTROL bus from the disk controller processor 24 to initiate a DMA data transfer between the dual-ported RAM 23 and the disk memories 12, 13 through the 16-bit data path (Interface DMA). 
     The dual-ported RAM 23 has an area organized as a circular buffer 34, in which the addressing from the I/O side (disk memory side) accesses a different buffer than the addressing from the host system side. This is represented by the pointers 35, 36 in FIG. 1. Circular buffers are known in the computer arts, as exemplified by Floro et al., U.S. Pat. No. 4,809,217, issued Feb. 28, 1989. 
     Referring to FIG. 2, the control program 31 for the disk controller processor 24 includes an initialization portion 40 that is executed on power up. The processor 24 then enters an executive portion of its control program 31 which includes a task scheduler, which is a high level control task that includes a task switching routine represented by shaded time slices 41, 42, 43, 44 in FIG. 2. The task switching routine, written in FORTH programming language, is listed in Appendix A to this specification. As a result of executing the task switching routine 41, the disk controller processor is switched first to execute a first cycle 45 of a task for DMA transfer of data between one or more of the disk memories and Port B of the dual-ported RAM. This task cannot be interrupted except at certain points (non-preemptive). When it runs to a certain point and is either completed or interrupted, control returns to the task scheduler, as represented by the arrow. A second cycle of the task switching routine 42 is executed to switch from the task for disk I/O transfer 45 to a task 46 for DMA transfer between Port A of the dual-ported RAM and the host system memory. When it runs to a certain point and is either completed or interrupted, control returns to the task scheduler. In this example, the task switching routine is executed for a third cycle 43 to switch from the task for host DMA transfer 46 to a second cycle 47 of the task for disk DMA I/O transfer. The task switching routine is effective to switch tasks in 4 microseconds or less, which is significant in aiding data throughput through the disk controller 10. This high speed switching is made possible by limiting the number of data items that must be saved on the stack of the disk controller processor 24, in order to maintain context for continuing operations. 
     The stack is maintained within the internal RAM 25 of processor 24 for temporary storage of data items and subroutine return addresses during program execution. Referring to FIG. 3, the stack for the disk controller processor 24 is divided into two portions 52, 53. One portion serves as the stack for the high level control task for scheduling and task switching and the other serves the two data transfer tasks, the host DMA transfer task and the disk DMA I/O task. These two portions are provided with stack limits or boundary addresses which are observed by the disk controller in its operations. Each task has a task header block associated with it. 
     As seen in FIG. 3, the header 54 for the disk DMA I/O task includes data items for the run state 48, a stack pointer 49, a restart address 50, and a user base register address 51. The run state 48 determines whether the task is ready for execution. The stack pointer 49 indicates the next free location on the stack. The restart address 50 is the position in the task program code where execution is to commence when the task is next run. The user base register 51 is used for fast access to a 64 byte block of memory. The task switcher maintains a value for each task, so that cache may access independent memory areas in this manner. 
     The header 55 for the host DMA task includes all of these except the stack pointer 49. Since this task makes only temporary use of the stack 52, it does not have a separate reserved stack area. Whenever the host DMA task relinquishes control of the stack portion 52, it must leave the stack portion in the same condition as it took control of the stack, so that the stack portion 52 may be utilized by the disk DMA I/O task. Both the host DMA task and disk DMA I/O tasks set up and control DMA which runs simultaneously and independently on the separate DMA paths. 
     The header 56 for the high level control task operates in the second portion 53 of the stack area of the processor 24 and includes the same header items as described above for disk DMA I/O task. 
     Host system access to data in the peripheral disk memories is limited by the speed of the attached disk memories, which may be rated in a range from 10 to 25 Mb/second, and is further limited by seek times and rotational delays for accessing data on the disks. Since a cache in host memory can deliver data at much faster rates with no access delays, it is therefore deemed an enhancement to provide an I/O data cache in the host system, so that I/O data is more readily available. The disk controller of the present invention is utilized to provide such a cache. 
     The disk controller processor 24 initializes tables in the RAM 28 for storing pointers to target locations 12 in regular host system memory and locations in a set-aside data cache 18 area of the host system memory. The host system requests I/O data from the cache 18 via control data transferred to the dual-ported RAM 23, where it is read by the disk controller processor 24. When I/O data is requested by the host system and the disk controller processor 24 determines that such data is already in the cache 18, the required data blocks are copied from the cache buffer 18 in the host memory into the controller and back out through the TURBOchannel to the target memory locations 17. These data transfers are accomplished at rates of up to 100 Mb/sec. The operation of the disk controller processor and its associated DMA control circuitry is transparent to the host system. Using this technique, up to 32 megabytes of system memory can be used for I/O data cache, as compared to prior products providing 1 to 4 megabytes of on-board data cache. A block of 8 kilobytes of data can be moved between the target locations and the data cache in less than 1 millisecond. 
     With disk memories rated at 9.33 Mb/sec peak rates, disk transfers are accomplished at data rates of 6 Mb/sec between the host system and each disk memory. This is in comparison to competitive rates of 2 Mb/sec using other disk controllers with the same host system and the same disk memory. 
     The portion of the firmware program which enables the disk controller processor to carry out the data caching operations in the host system is listed in Appendix B to this specification. This portion of the program is written in the FORTH programming language. 
     This has been a description of a preferred embodiment and methods for carrying out the invention. Those of ordinary skill in the art will recognize that variations may be made in arriving at other detailed embodiments, and these embodiments will come within the scope of the invention. Such variations may include, but are not limited to, a wide range of system buses and a wide range of tape, disk, or system peripheral memories. 
     Therefore, to apprise the public of the scope of the invention and the embodiments covered by the invention, the following claims are made.