Abstract:
A display memory system consisting of a controller and a display memory that has a read display memory cycle time that is a non-integer multiple of the write display memory update rate. The display memory being partitioned into two frames of display memory with each frame having a plurality of subframes. The controller consisting of a read display memory decoder, a read display memory next subframe generator a read display memory latch and consisting of a write display memory decoder, a write display memory next subframe generator and write display memory latch.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a display memory system and, more particularly, to a display memory system wherein the write display memory rate and read display memory update rate are non-integer multiples of each other, alternately known as a Gerbil Wheel Memory (GWM). 
     Prior art display memories cannot have a read rate and a write rate which are non-integer multiples of each other. For example, it may be advantageous to write a memory buffer with video data at an update rate of 20 Hz, while simultaneously reading video data from the same buffer at an update rate of 66 Hz. A conventional PING/PONG video memory in which the input and output update rates are integer multiples of one another (for example having a 30 Hz input rate and a 60 Hz output rate) is not a feasible solution. 
     A conventional PING/PONG memory configuration utilizes two full frames of dual port random access memory (RAM). The frame being updated is called write display memory. The frame being displayed is called read display memory. The write display memory device writes to one frame while a read display memory device simultaneously and synchronously reads from the other. When both devices have finished their respective memory accesses, which will occur at the same time, the read display memory device then reads from the frame previously written to by the write display memory device, and the write display memory device begins writing to the frame previously read by the read display memory device. This configuration is adequate only when the input writing rate and output reading rate are integer multiples of one another. If the frame rates are not integer multiples of one another, the update rate of the read display memory device will approach the output rate of the write display memory device, resulting in loss of data or overwriting of data. 
     Prior art solutions have not addressed the particular problem of input and output rates that are not equal or not equal multiples of each other. Current display memory systems demand that the write update rate be slaved to the input video source or to the rate the display requires. As a result, most, if not all current systems, use a PING/PONG configuration. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide the capability, in a display memory system, to read from display memory at a non-integer multiple of the rate at which the display memory is updated. 
     It is a further object of the invention to allow simultaneous access by write display memory and read display memory devices to the same frame of memory in a display memory system. 
     It is a further object of the invention to allow synchronous input/output operations to be performed, with no restrictions on the rates at which these operations are performed. 
     It is yet another object of the invention to provide a system having a display system read frame rate which is independent of the write frame rate of an input source. 
     The gerbil wheel memory (GWM) provided by the present invention solves the problems present in the prior art by partitioning each frame of memory into subframe blocks which can be controlled independently. Like the PING/PONG configuration, the GWM uses two full frames of dual port RAM. The GWM, however, allows the write display memory device and read display memory device to access a part of each frame simultaneously. This means that both devices can act on different parts of the same frame at the same time. 
     The GWM as embodied in one example of the invention utilizes a video memory. The memory is organized as two full frames of dual port RAM. Each frame is divided into subframes. Each subframe is of equal size and can be any fraction of the entire frame, such as 1/2 frame, 1/4 frame, . . . down to a single pixel. Input/output operations occur on subframes. This allows the write display memory and read display memory devices to access the same frame at the same time, allowing the update rate of the read display memory device to be independent of the update rate of the write display memory device. The input and output update rates become independent because neither read or write devices have to wait to gain access to a frame. 
     Steps are taken to ensure that the read display memory device and the write display memory device do not interfere with one another. Before reading from a particular frame, a controller checks to see if the read display memory device will catch up with the write display memory device and thus read inconsistent display data (e.g., a subframe that is not completely rewritten). If the read device will catch the write device, the read device is instructed to go back and read from the previous frame for a second time. 
     Other objects, features and advantages of the present invention will become apparent to those skilled in the art through the Description of the Preferred Embodiment, Claims, and Drawings herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To illustrate the invention, a preferred embodiment of this invention will be described hereinafter with reference to the accompanying drawings, in which: 
     FIG. 1 hows a high level diagram of the GWM display memory circuitry including the partitioning of the GWM memory system by dividing display memory into frames and subframes; 
     FIG. 2 shows an example of read and write cycles in relation to time for the GWM; and 
     FIGS. 3A, 3B, 3C and 3D show a state machine diagrams of the operation of the preferred embodiment of the invention; and 
     FIG. 4 illustrates, in more detail, the GWM read and write system components. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a block diagram of one embodiment of the GWM invention is shown comprising a GWM controller 20 and a display memory or a dual port display memory 10. The GWM controller receives input signals on two communication buses, a write communication bus 6 and a read communication bus 8. Each bus carries three signals. The first GWM controller input signal is received on the write pixel clock line 22 and is generated by a system utilizing the GWM, such as a computer based display system (not shown). Pixel clock line 22 is connected to controller 20 at a write pixel clock port 23. The second GWM controller signal is received on the write GWM address bus 24 and comprises the address of the pixel being written. This signal is received at a write GWM address port 25. The third GWM controller signal is received on the write GWM data bus 26 and comprises the content or value of the pixel being written. The data on write GWM data bus 26 is received on a write GWM data port 27. The fourth GWM controller signal is received on the read pixel clock line 28 and again is generated by the system utilizing the GWM. Read pixel clock line 28 is connected to GWM controller 20 at a read pixel clock port 29. The fifth GWM controller signal is received on the read GWM address bus 30 and comprises the address of the pixel being read. Read GWM address bus is connected to GWM controller at a read GWM address port 31. The sixth GWM controller signal is received on the read GWM data bus 32 and comprises the content or value of the pixel being read. Read GWM data bus 32 is connected to GWM controller at a read GWM address port 33. 
     The GWM controller is connected at its output portion to a dual port display memory 10 by two communication buses, a first communication bus 18 and a second communication bus 19. The dual port display memory 10 is divided into two frames; FRAME --  0 12, and FRAME --  1 14. Each frame 12 &amp; 14, then is divided into subframes 16. In the present embodiment the frames are each divided into four subframes 16. Each frame of the dual port contains one full screen of display information. All information sent to dual frame display memory 10 is generated by the GWM controller 20. 
     Connected between GWM controller 20 and dual port display memory 10 are first communication bus 18 and second communication bus 19 which each are comprised of a number of control lines for reading from and writing to dual port display memory 10. The control lines within first communication bus 18 include a write enable line 34, a write subframe select line 36, a write address bus 38 and a write data bus 40. Similarly, the control lines within second communication bus 19 include a read enable line 42, a read subframe select line 44, a read address bus 46, and a read data bus 48. GWM controller 20 utilizes these connections to communicate with dual port display memory 10. 
     First communication bus 18 is connected to controller 20 at the appropriate output port. Specifically, write enable line 34 is connected at a write enable port 134, write subframe select line 36 is connected at a write subframe select port 136, write address bus 38 is connected at a write address port 138, and write data bus 40 is connected at a write data port 140. Similarly, second communication bus 19 is connected to controller 20 at the appropriate output parts. Specifically read enable line 42 is connected at a read enable port 142, read subframe select line 44 is connected at a read subframe select port 144, read address bus 46 is connected at a read address port 146, and read data bus 48 is connected at a read data port 148. Dual port memory 10 has similar connections to facilitate communication by first communication bus 18 and second communication bus 19. 
     Referring now to FIG. 2, an example of GWM operation is depicted by a chart depicting operation mode v. time In the present example the write display memory has a frame rate of 50 Hz. as shown in write display memory bar graph 100 and the read display memory has a frame rate of 90 Hz as shown in read display bar graph 102. Each block in write bar graph 100 represents a write cycle and each block in read bar graph 102 represents a read cycle. As shown in FIG. 2, in the time it takes the write display memory device to write to FRAME --  0, the read display memory device can read from FRAME  --  1 1.75 times. As can also be seen in the figure, the write display memory device writes to each frame consecutively. The read display memory device operates differently. When the read display memory device finishes reading a frame, it must determine if it should move to the next frame and read it or read from the same frame again. This decision is based on the current status of the write display memory device, the writing rate of the write display memory device and the reading rate of the read display memory device. In summary, the read display memory device ensures that it will not &#34;catch&#34; the write display memory device, causing the display of incomplete and inconsistent data. This process is described in further detail later in the present description. 
     For the example in FIG. 2, at the end of 0.1 second the write display memory has written to FRAME --  0 three times and FRAME --  1 twice. In the same period of time, the read display memory has read FRAME --  0 five times and FRAME --  1 four times. This is due to different write and read update rates. 
     A PASCAL program that determines which subframe to read based on the system time is shown in Table 1 below. 
     
                       TABLE 1______________________________________ 1     if (time = nextr) then 2      begin 3       if (pr &lt;&gt; 3) or (pr &lt;&gt; 7) then pr := Pr+1 4        else if (pr=3) then 5         begin 6          if    (ps(time) = 4) or 7                (ps(time + Tr) = 5) or 8                (ps(time + 2*Tr) = 6) or 9                (ps(time) + 3*Tr) =7) then pr:=010             else pr :=4;11           end12        else if (pr = 7) then13         begin14          if    (ps(time) = 0) or15                (ps(time + tr) = 1) or16                (ps(time + 2*tr) = 2) or17                (ps(time + 3*tr) = 3) then pr:=4 18            else pr:= 0;19           end;20      end;______________________________________ 
    
     The program variables are defined as follows: 
     ps: Write Pointer--Pointer to subframe being accessed by the write display memory device. 
     pr: Read Pointer--Pointer to subframe being accessed by the read display memory device 
     ts: Time required for write display memory device to write to one subframe. 
     tr: Time required for read display memory device to read from one subframe. 
     time: Current time. 
     nextr: Time at which the read display memory device will access the next subframe. 
     At any given time, the subframe which the write display memory device is accessing can be determined from the following simple equation. The relationship between ps (the write pointer) and time is described by: 
     ps(time)=(Trunc(time/ts)) Mod 8 
     The term &#34;(Trunc(time/ts))&#34; determines how many subframes have been accessed up to this time. The term &#34;Mod 8&#34; limits PS to range from 0 to 7 with the module function. For example, if the input frame rate is 50 Hz, then ts would equal 20 ms. The relationship for PS to time shown above indicates that every 20 ms (ts) the subframe being accessed by the write display memory device increments by 1. When the write display memory device completes writing to SF7 it returns to SF0 and continues in a daisy chain fashion. 
     The calculation which determines which subframe the read display memory device accesses is somewhat more complicated since it is dependent not only on the current time, but also on which frames the write display memory device is accessing, and which frames the write display memory device will be accessing during the upcoming read cycle. The read display memory device is allowed to access the same display memory frame as often as is necessary which helps to insure that the read display memory device will not attempt to access a subframe that is currently being accessed by the write display memory device. The read display memory device will finish reading any frame it has started. 
     Referring now to the program in Table 1, Line 1 shows that the read pointer will change only if the read display memory device is finished reading the current subframe. In this program the dual port display memory contains two frames, each of which are broken into four subframes. The subframes within FRAME --  0 are labeled SF0, SF1, SF2, and SF3. Similarly, the subframes in FRAME --  1 are labeled SF4, SF5, SF6 and SF7. In the program in Table 1, the subframes are simply identified by numbers 0-7. 
     In line 3 of the program, the read pointer is incremented by 1 if the current subframe just read is not the last subframe of the current frame. This ensures that an entire frame is read before the read display memory device begins reading the other frame. 
     Lines 5-8 and 13-19 demonstrate the predictive nature of the algorithm. If the current subframe being accessed by the read device is the last subframe of the frame, then the write pointer is checked to see which frame the write display memory device will be writing to during the upcoming subframe read cycles. This satisfies the objective that both devices are not accessing the same subframe simultaneously. For example, line 4 tests whether the read pointer is on the last subframe of FRAME --  0. If this condition is met, the write pointer is checked. Lines 6-9 test whether the writer pointer will be pointing at a subframe which could be read during the next read cycle. Line 6 tests whether the write pointer is current writing to subframe 4, and if so, it is not desired to have the read device read FRAME --  1 so the read pointer is brought back to the beginning of FRAME --  0. Line 7 tests whether the write pointer will be pointed at SF5 during the time in which the read device could access SF5. If this is satisfied, the read pointer is again directed back to the beginning of FRAME --  0. Similar test are done in lines 8 &amp; 9. Finally, if none of these tests are met, them the read device is free to access FRAME --  1. 
     The GWM cycle time is derived from the access times of the memory devices themselves. Those skilled in the art will appreciate that it is desirable to have the fastest cycle times available within the limits of the chosen memory technology. 
     Referring now to FIG. 3A, a write flow diagram 180 of the operation of the invention&#39;s display memory write process is shown. Generally, the write display memory device begins writing to subframe 0 (SF0) and sequentially writes to all of the other subframes (SF1 thru SF7). In the case of write flow diagram 180, the display memory write process begins at block 182 where subframe zero (SF0) is written to until the end of subframe signal (EOS) goes active at block 184. When the end of the subframe signal (EOS) is active, the next frame (SF1) is accessed at block 186 for writing. This process repeats until the end of subframe signal (EOS) is active for the final subframe (in this case, SF7), then SF0 is accessed again. The whole process repeats indefinitely. 
     Referring now to FIG. 3B, a read flow diagram 190 of the operation of the invention&#39;s display memory read cycle is shown. The process starts at block 200 by reading from FRAME --  1. Reading of FRAME --  1 is accomplished by reading each subframe 201 until the End of Frame signal EOF is active at block 202. When the EOF is active, GWM must determine whether to move on to the next frame or read again from the same frame. This decision is made by the process in the next subframe (next SF) block 206. More detail of next subframe block 206 is discussed in conjunction with FIG. 3D. 
     Referring now to FIG. 3C, a state machine diagram of the operation of the invention&#39;s display memory read subframes cycle is shown. Read subframes process 201 reads the appropriate subframes for the frame being accessed. If FRAME --  0 is being read the process starts at subframe zero (SFO) then increments through subframe three (SF3). Similarly, if FRAME --  1 is being read the process starts at subframe four (SF4) then increments through subframe 7 (SF7). 
     Referring now to FIG. 3D, a state machine diagram of the operation of the read next frame decision block 206 is shown. The process starts following an end of frame (EOF) indication by block 202. The state machine diagram 206 is shown generically to apply following the reading of FRAME --  0 or FRAME --  1. Therefore, both possible subframes are indicated where applicable (e.g. in the first block 222 of the process, &#34;SFO/SF4&#34; indicates that the process is polling SF0 if FRAME --  1 was the last frame read and SF4 if FRAME --  0 was the last frame read.) The process will be described as if FRAME --  1 has just been read and next subframe block 206 is determining if FRAME --  0 can now be read. It is understood that the same process is applicable following the vending of FRAME --  0. The process must determine if the subframes of the next frame will be written to during the upcoming read cycles (i.e. will SF0-SF3 or SF4-SF7 be written to in the time in which the read device desires to read those subframes.) First it must be determined if SF0 is presently being written to (See block 222). If SF0 is currently being written to, then it is not desirable to read from SF0 also, therefore the read device should go back and read from same frame. If SF0 is not being written to, the process then asks if SF1 will be written to within one read cycle. If SF1 will be written to in the next read cycle, this indicates that the read device will catch up to the write device in one read cycle. Again, it would be undesirable to write to and read from the same subframe, therefore the read device is instructed to read the same frame again. If SF1 will not be written to within one read cycle, the process must move on and determine if SF2 will be written to within two read cycles. Again, if SF2 is to be written to within two read cycles, the read device would catch the write device and this is not desirable. If this condition exists, the read device should go back to read the same frame that was previously read. Next, it must be determined if SF3 will be written within three read cycles. This condition would also indicate that the read device would catch the write device, thus causing the system to read to and write from the same subframe. This should be avoided so the read device is instructed to read from the same frame that was previously read if this condition exists. Finally, if the system has gone through all of the previously mentioned tests and determined that the read device can move on to the next frame without catching the write device, the system is instructed to read from the next frame. Appropriate signals are emitted from next frame block 206 to cause the display memory read process 190 to loop to the correct frame. 
     Again, this same process is applicable following the reading of FRAME --  0, however subframes 4 thru 7 (SF4-SF7) are tested. As also previously discussed, those tests are to determine if the display memory read device 51 would catch up with the display memory write device 50, if the read device is instructed to read from the next frame. 
     Now referring to FIG. 4, a more detailed block diagram of the GWM controller is shown. The GWM controller is comprised of two sets of circuitry, one for an input apparatus or write GWM address device 50 which writes to display memory 10, and one for an output apparatus, or read GWM address device 51 which reads display memory 10. 
     The block of circuitry which writes to display memory 10 consists of a write address decoder 52, write state machine 53, and a write enable latch 54. The system write GWM address bus 24 is connected to an input of 58 write address decoder 52. The system write GWM address bus 24 is also connected by line 70 to an output 96 of write enable latch 54. The write address decoder 52 decodes the write memory addresses received on write GWM address bus 24 to determine if these addresses are the last subframe address in the subframe currently being accessed. Write decoder 52 generates a write end of subframe signal on an output 81 which is connected to an input 80 of write state machine 53 via line 62. If the write memory address is the last address within the current subframe, write address decoder 52 enables state machine 53 and write latch 54. Write state machine 53 generates the next subframe signal on an output 82 which is connected to an input 84 of write enable latch 54 via line 64. 
     The block of circuitry which reads from display memory 10 consists of a read address decoder 55, a read state machine 56, and a read enable latch 57 The system read GWM address bus 30 is connected to an input 59 of read address decoder 55. The system read GWM address bus 36 is also connected to an output 98 of read enable latch 57 by line 72. Read address decoder 55 decodes the read memory addresses, received on read GWM 30 to determine if these addresses are the last subframe address in the subframe currently being accessed. Read decoder 55 generates end of subframe signals on an output 86, which is connected to an input 88 of read state machine 56 via line 63. If the read memory address is the last address in the subframe, decoder 55 enables state machines 56 and latch 57. Further, read state machine 56 receives a signal a second input 90 from the write state machine on line 68. Second input 90 can indicate which subframe is being written to by the write GWM address device 50. Read state machine 56 that generates a next subframe signal on an output 92 which is connected to an input 94 of read enable latch 57 via line 65. 
     State machines 53 and 56 output next subframe signals which are consistent with the logic previously discussed. Specifically, the write state machine 53, increments the next subframe signal so as to allow input apparatus 50 to begin writing to subframe zero (SF0) and sequentially step through each subframe of the memory. As previously discussed, when the write device reaches subframe seven (SF7) the GWM loops steps back to subframe zero (SF0). This process was previously described with reference to FIG. 3A. 
     Read state machine 56 similarly controls the reading from display memory 10. State machine 56 outputs the appropriate signals on line 64 to allow the read device to implement the logic previously discussed with reference to FIGS. 3B, 3C &amp; 3D. 
     Write GWM address device 50 and read GWM address device 51 are synchronized via write pixel clock 22 on line 60 and read pixel clock 28 on line 61. Those skilled in the art will note that synchronization of the pixel clocks transmitted on lines 60 and 61 and the size of display memory 10 depend on each specific implementation. 
     Write enable latch 54 produces a signal on an output 96 latch output 96 is connected with line 70 wherein the output bits from write enable latch 54 are concatenated with the address transmitted on line 70 to fan the full input address bus 76. 
     Similarly, read enable latch 57 produces a signal on an output 98. Latch output 98 is connected with line 72 wherein the signals on output 98 and line 72 are concatenated with the address on line 72 to fan the full output address bus 78. 
     Input address bus 76 and output address bus 78 contain all the necessary addressing required to write to and read from dual port display memory 10. With reference to FIG. 1, input address bus 76 includes write enable input 34, write subframe select 36, and write address bus 38. Similarly, output address bus 78 includes read enable input 42, read subframe select 44 and read address bus 46. 
     This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, can be accomplished without departing from the scope of the invention itself.