tvcache.htm

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                            <td colspan="3" valign="top"><span class='include'>Last 
                              updated July 9, 1997</span>
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                            <td colspan="3" height="40" valign="middle"><span class='sectionheader'>Introduction</span></td>
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                            <td valign="TOP" colspan="3" align="left"><i>TVCACHE 
                              </i>is a replacement for VCACHE that implements 
                              compression on cached blocks in an attempt to improve 
                              performance (our benchmarks show that it doesn't 
                              fare too great). In any case, we are posting the 
                              complete source for <i>TVCACHE </i>because the VCACHE 
                              interface is undocumented. The source presented 
                              here is the only complete specification for the 
                              VCACHE interface that we are aware of. Note that 
                              the source is presented solely for educational purposes 
                              and is not licensed for commercial use.</td>
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                            <td colspan="3" height="40" valign="middle"><span class='sectionheader'>How 
                              it Works</span></td>
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                            <td valign="TOP" colspan="3"> In yet another brilliant 
                              design move, the current implementation of Windows 
                              95 does not provide any official way to replace 
                              the default VCACHE with an alternate VxD. However, 
                              TVCACHE overcomes this limitation by replacing the 
                              function pointers in the VCACHE service table with 
                              pointers to its own routines before VCACHE is activated 
                              for the first time. Thus, using VCACHE's Device 
                              Declaration Block, TVCACHE subsumes VCACHE's role 
                              as the cache manager for Windows. 
                              <p>Once active, TVCACHE actually does much more 
                                work than VCACHE needs to do. This extra work 
                                involves fragmented memory management and compressed 
                                buffer management. VCACHE only has to deal with 
                                page sized (4K) chunks, whereas TVCACHE must deal 
                                with almost arbitrary sized chunks of data (in 
                                128 byte increments). TVCACHE mimics VCACHE's 
                                exact policy of cache growing and shrinking and 
                                our benchmarks show VCACHE and TVCACHE (with compression 
                                disabled) have identical performance characteristics. 
                              </p>
                              <p>Whether you are writing your own Windows 95 file 
                                system driver, just curious about how VCACHE works, 
                                or want to experiment with VCACHE grow/shrink 
                                policies, TVCACHE serves as the only source of 
                                precise and accurate information on how VCACHE 
                                actually functions internally. 
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                            <td colspan="3" height="40" valign="middle"><span class='sectionheader'>Building 
                              TVCache </span></td>
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                            <td valign="TOP" colspan="3"> TVCACHE is a Windows 
                              95 VxD that is written in C, and it requires VtoolsD 
                              Version 2.0 or greater to build. Defining the TESTING 
                              flag has the build produce a stand-alone Windows 
                              console program that stress tests the TVCACHE code 
                              by simulating multi-threaded cache accesses. Setting 
                              COMPRESSON to 0 disables compression so that TVCACHE 
                              functions in a manner similar to VCACHE.</td>
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                            <td colspan="3" height="40" valign="middle"><span class='sectionheader'>Installing 
                              TVCache </span></td>
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                            <td valign="TOP" colspan="3"> To install TVCACHE, 
                              add the following line to the [386enh] section of 
                              system.ini: 
                              <ul>
                              </ul>
                              <p>device=&lt;path&gt;\tvcache.vxd </p>
                              <p>where &lt;path&gt; is the directory in which 
                                tvache.vxd is located. You must reboot your machine 
                                for TVCACHE to start.
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                            <td colspan="3" height="40" valign="middle"><span class='sectionheader'>More 
                              Information </span></td>
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                            <td valign="TOP" colspan="3"> The VCACHE interface 
                              is only partially documented in the Windows 95 DDK, 
                              and there are no guidelines for using it. While 
                              we're not taking the time to write up a document 
                              on the VCACHE interface specification, the source 
                              to TVCACHE indirectly documents everything you need 
                              to know. In its stress testing of TVCACHE code, 
                              the main() test function at the bottom of vcache.c 
                              serves as an example of how the VCACHE interface 
                              is used. 
                              <p>Let us know if you find TVCACHE useful.
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                            <td colspan="3" height="40" valign="middle" align="center">
                              <p>&nbsp;</p>
                              <p>&nbsp;</p>
                              <p><b><a href="../../files/tvcache.zip">Download TVCACHE 
                                (72KB)</a></b><br>
                                <br>
                                <a href="#top"><b>Back to Top</b></a> </p>
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