Apparatus and method for optimizing bus usage by varying the amount of data transferred on a DMA operation
This application is a continuation of application 07/514,475 filed Apr. 25, 1990, and now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to direct memory access (DMA) processes. More particularly, it relates to a technique for achieving efficient bus usage for DMA operations by dynamically varying the amount of data transferred on a DMA operation based on the amount of buffered data awaiting transfer and the recent data rate of arrival. 
     BACKGROUND OF THE INVENTION 
     Many direct memory access (DMA) controllers used for processing asynchronous data, such as bytes or other size blocks from terminals, transfer the data to memory via DMA operations as the blocks are received. This is done irrespective of the width of the memory bus, because it is not known if, and when, another block of data will be received. Moreover, for some environments, it may be necessary to effect DMA transfers relatively quickly. Such might be the case for echoing data back to terminals, for example. For slow rates of data arrival, e.g., 300 bits per second, the DMA transfer of bytes as they arrive carries little penalty. It is also appropriate when data arrives intermittently at slow rates. However, as data arrival rates increase, it becomes increasingly inefficient to perform a DMA operation for each block or byte of received data, if additional bus width is available for multiple data block transfers. Each DMA operation requires an arbitration for the memory bus which, in turn, consumes system resources. 
     The prior art contains many attempts to maximize bus usage and improve performance during DMA transfers. IBM Technical Disclosure Bulletin, Vol. 30, No. 12, May, 1988, page 211, describes a technique in which a DMA bus arbitration is requested only when the receive buffer contains a prescribed number of data blocks. This results in a larger data transfer to reduce the total amount of bus arbitrations. IBM Technical Disclosure Bulletin, Vol. 24, No. 10, Mar., 1982, page 5295, discloses an arrangement which provides for an automatic memory bus request for a subsequent DMA operation before a block presently being transferred is completed. This reduces buffer size requirements and the amount of microcode required for performing DMA transfers. U.S. Pat. No. 4,912,632, which issued on Mar. 27, 1990, relates to a memory subsystem controller including means for causing a DMA memory transfer to be interrupted at a cache memory line boundary to service a processor request and thereafter resume data transfer. U.S. Pat. application Ser. No. 297,778 relates to a DMA controller requiring no intervention from a main processor once the first of a series of block transfers has begun. 
     In many DMA architectures, the size and frequency of DMA transfers is fixed, as discussed above. In some applications, DMA performance can be improved by better understanding the data being processed. For example, in applications where data is being received rapidly, moving more data per DMA operation improves system performance because of the reduced overhead of negotiating for the bus. In applications where small or infrequent amounts of data are processed, moving the data as it is received may be optimal because of reduced latency of the processor acting on the information. The size of a typical DMA transfer is fixed at design time based on the constraints for that system. This poses a problem in that most applications can tolerate a relatively small variation in data rate before system performance deteriorates. 
     SUMMARY OF THE INVENTION 
     A system includes a processor, a main memory, one or more peripheral devices and a DMA controller. A system bus interconnects the processor, memory and the DMA controller. The controller improves bus usage during direct memory access transfers of data blocks by dynamically varying the size of DMA transfers. The controller includes means for buffering data blocks from the device as they arrive and means for dynamically activating a DMA operation and for varying the amount of data transferred on a DMA operation depending on the recent arrival rate of data and the amount of data already buffered for DMA transfer. 
     In a preferred embodiment, a timer monitors the time between the arrival of data blocks. The timer is reset each time a new data block is received. The output of the timer is continually decoded. At the same time, a count is kept of the number of data blocks already received, but not yet transferred via DMA operation. The timer output and received block count are combined to control the activation and size of DMA operations. As the interval between the receipt of data blocks increases (low data rate), the invention requires fewer blocks before performing a DMA operation. In the limit, a DMA operation is performed for each data block. At high data block arrival rates, a DMA operation is performed less often and, in the limit, only when the number of received data blocks is sufficient to fill at least the width of the memory bus. 
     The invention is primarily useful for asynchronous operation. For synchronous data, USARTS may provide END-OF- FRAME indications, at which time DMA operations may be performed. However, the invention may also be used for synchronous operation without penalty. 
    
    
     DESCRIPTION OF THE DRAWING 
     In the drawing, 
     FIG. 1 shows one illustrative block diagram of a system suitable for practicing the invention, including a DMA controller containing the novel bus request circuit; 
     FIG. 2 shows the details of the bus request circuit; and 
     FIGS. 3 and 4 show flow charts of the operations carried out by the illustrative bus circuit. 
    
    
     DETAILED DESCRIPTION 
     An illustrative system, as shown in FIG. 1, includes a processor 100, a main memory 102, a peripheral device 104 and a DMA controller 106, including a data block buffer 116, for arbitrating DMA data block transfers between the main memory and the peripheral device. Device 104 may be, for example, a terminal at which a user inputs data from a keyboard and receives data via an electronic display screen. Controller 106 includes a novel bus request circuit 114 which incorporates the invention. Other circuits of controller 106 are entirely conventional and are not discussed in detail. 
     The details of bus request circuit 114 are shown in FIG. 2. A flowchart of the internal timing operations is shown in FIG. 3. A counter 200 counts clock cycles to determine the time span between the receipt of data blocks from peripheral device 104. Of course, the invention is not limited to this illustrative arrangement and may pertain to the receipt of blocks from other devices as well, including the main memory 102. The state of counter 200 is decoded by combinatorial logic 202 to generate time count thresholds based on the rate of arrival of data blocks. A four bit up-down counter 204 is incremented each time a data block is buffered and decremented for each block that is transferred. The states of the buffer counter 204 stages are outputted directly via latch 230 to logic 208. The count threshold signals from logic 202 are combined with the output from up-down counter 204 by logic 208 to generate DMA bus request signals to the conventional circuits of controller 106. These request signals activate a DMA operation and identify the number of data blocks to be transferred when a bus grant signal is received. Once transferred, the logic 208 causes up-down counter 204 to be decremented by the number of transferred data blocks. 
     For illustrative purposes of this disclosure, it is assumed that a data block is one byte in length (8 bits) and that eight bytes can be buffered by the controller 106. It is further assumed for illustration that the memory bus 103 is 32 bits wide. A halfword transfer illustratively means a transfer of two bytes, while a fullword incorporates a full 32-bit bus transfer. 
     When a data block becomes valid, controller 106 notifies bus request circuit 114 by a signal on data valid lead 210. This signal is stored by latch 212 and used to reset the time counter 200 (FIG. 4-402). Counter 200 is continually incremented by system clock pulses on lead 214 (FIG.3-300). The system clock operates illustratively at a 125 microsecond rate. As will be shown, in the illustrative embodiment, this clock frequency supports a wide range of rates of data block arrival, including rates of anywhere between 300 bits per second (bps) to 64 kilobits per second (kbps). Of course, other clock rates can be used for different data arrival rates, as required by the environment in which the invention is used. At 64 kbps, a byte is valid every 125 microseconds. Therefore, counter 200 will reach at least the value of one (1) between the arrival of each byte. For continuous data arrival at 300 bps, approximately 37 bytes arrive every second. At a clock rate of 125 microseconds, this translates to a counter 200 count of at least 213 for each block arrival. Counter 200 is illustratively 8 bits wide to support this count of 213. 
     Latch 216 is ordinarily enabled. Therefore, the count from counter 200 is ordinarily passed to logic 202, which operates on it asynchronously. When a bus request signal is generated by logic 208, the output of latch 216 is locked by a low-going signal on lead 218 to stabilize the request circuit 114 signals to controller 106 while awaiting a bus grant signal. The DMA request signal from OR gate 226 is sent to the controller 106 and also to AND gate 228. The bus grant signal from controller 106 is inverted by inverter 229 to fully enable AND gate 228 before the bus grant is received. The output of AND gate 228 is inverted by inverter 230 to disable latch 216. The same signal also disables latch 233 between up-down counter 204 and logic 208, as discussed below. 
     Logic 202 generates signals on its outputs based on the time count from counter 200. Logic 202 decodes this count state and produces a signal on lead 220 if the count equals or exceeds 96. Similarly, logic 202 generates signals on leads 222 and 224 for time counts equal to or exceeding 24 and 3, respectively. These time threshold signals are combined with the buffered data count from up-down counter 204 to generate an appropriate DMA request. To accomplish this, latch 233 is also disabled by the output of inverter 230 to lock in and stabilize the count from counter 204 to logic 208. Logic 208 functions as follows. In the case in which only one byte is buffered (lead 232 high) (FIG. 3-304), if logic 202 generates a signal on lead 220, meaning that the count from counter equals or exceeds 96 (FIG. 3-302), AND gate 238 of logic 208 generates a request on the BYTE lead of 232 to request a transfer of one byte (FIG. 3-306). If another byte is buffered before counter 200 reaches a state of 96, the time counter 200 is reset (FIG.4-402) and the up-down counter 204 is incremented by one by the signal from latch 212. Now two bytes of data are buffered. The time counter 200 increments from zero until the halfword count threshold (24 in this illustrative circuit) is met (FIG. 3-308), at which time a halfword DMA operation is requested by logic 208 (FIG. 3-312). If a third byte is buffered before the time count threshold of 28 is exceeded, a halfword transfer will still be requested at that time. Continuing, if two or three bytes are buffered (lead 234 high) and the time count from counter 200 equals or exceeds 28, AND gate 240 requests a transfer of a halfword (two bytes). Similarly, if four through seven bytes are buffered (FIG. 3-314) and the time count threshold equals or exceeds 3 (FIG. 3-308), AND gate 242 requests a fullword (four bytes) transfer (FIG. 3-316). 
     Now consider the case when data is arriving so quickly that counter 200 is incremented only a few times before being reset. In this case, the lowest time count threshold from counter 200 that is recognized by logic 202 (three, which results in a fullword transfer) may never be reached. However, this is not a problem, because when the data buffer count from counter 204 reaches 2 fullwords (FIG. 4-404), logic 208 generates a request on lead 238 to transfer two fullwords, (FIG. 4-406) regardless of the time count. A request to transfer two fullwords requires the conventional circuits of controller 106 to perform two full bus transfer operations, assuming a 32-bit bus. Alternatively, some controllers also have the ability to operate in what is called &#34;burst&#34; mode. Burst mode allows the transfer of successive blocks slated for storage in successive addresses. However, blocks following the initial block of a burst mode do not need to be accompanied by a storage address. Whenever data is transferred, the number of transferred data blocks is subtracted from the block count in up-down counter 204 to properly control block transfer so that odd amounts of data pose no residual data problems. The decrementing of counter is performed by logic 208 in response to signals to logic 208 on bus 240. 
     Table 1 below shows illustrative block transfers requested by bus request circuit 114, assuming that data is continuously arriving at the stated rates and that counter 202 is incremented at a 125 microsecond rate. 
     
                       TABLE 1                                                     
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DATA    COUNTER    COUNTER 200  TRANSFER                                  
RATE    200        THRESHOLD    SIZE                                      
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64   KBS    1          NEVER      2 FULLWORDS                             
                       REACHED                                            
38.4 KBS    2          NEVER      2 FULLWORDS                             
                       REACHED                                            
32   KBS    2          NEVER      2 FULLWORDS                             
                       REACHED                                            
19.2 KBS    3           3         FULLWORD                                
9600 BPS    6           3         FULLWORD                                
4800 BPS    13          3         FULLWORD                                
2400 BPS    26         24         HALFWORD                                
1200 BPS    54         24         HALFWORD                                
600  BPS    111        96         BYTE                                    
300  BPS    213        96         BYTE                                    
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     It is to be understood that the above described arrangements are merely illustrative of the application of principles of the invention and that other arrangements may be devised by workers skilled in the art without departing from the spirit and scope of the invention.