Back-side bus

<h2 id="history">History</h2>
<p>BSB is an improvement over the older practice of using a single <a href="/facts/System_bus/p1mxHpbb">system bus</a>, because a single bus typically became a severe <a href="/facts/Von_Neumann_bottleneck/te0yulFa">bottleneck</a> as CPUs and memory speeds increased. Due to its dedicated nature, the back-side bus can be optimized for communication with cache, thus eliminating protocol overheads and additional signals that are required on a general-purpose bus.  Furthermore, since a BSB operates over a shorter distance, it can typically operate at higher clock speeds, increasing the computer's overall performance.
</p><p>Cache connected with a BSB was initially external to the microprocessor <a href="/facts/Die_(integrated_circuit)/XWvdlltE">die</a>, but now is usually on-die.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> In the latter case, the BSB <a href="/facts/Clock_signal/9UfTzGHx">clock</a> frequency is typically equal to the processor's,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> and the back-side bus can also be made much wider (256-bit, 512-bit) than either off-chip or on-chip FSB.
</p>

<p>The dual-bus architecture was used in a number of designs, including the <a href="/facts/IBM/QszA7nwd">IBM</a> and <a href="/facts/Freescale_Semiconductor/mRjBBbeX">Freescale</a> (formerly the semiconductor division of <a href="/facts/Motorola/ztqczx68">Motorola</a>) <a href="/facts/PowerPC/DqmM9y50">PowerPC</a> processors (certain PowerPC <a href="/facts/604e/NM9PomrW">604</a> models, the <a href="/facts/PowerPC_7xx/SfPRudfh">PowerPC 7xx</a> family,<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> and the Freescale <a href="/facts/PowerPC_G4/llrBTyVn">7xxx</a> line), as well as the <a href="/facts/Intel_Corporation/SMF0gJJX">Intel</a> <a href="/facts/Pentium_Pro/mTAN64fo">Pentium Pro</a>, <a href="/facts/Pentium_II/VoHQLV1s">Pentium II</a> and early <a href="/facts/Pentium_III/sV1v2Frq">Pentium III</a> processors,<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> 
which used it to access their L2 cache (earlier Intel processors accessed the L2 cache over the FSB, while later processors moved it on-chip).
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/CPU_cache/x8BYFcdv">CPU cache</a></li>
<li><a href="/facts/Bus_(computing)/R45TKwS7">Bus (computing)</a></li>
<li><a href="/facts/Front-side_bus/LgYSB7Wl">Front-side bus</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Dedicated Backside Cache Bus". PCguide.com. 2001-04-30. <a href="http://www.pcguide.com/ref/cpu/arch/extBackside-c.html" target="_blank">http://www.pcguide.com/ref/cpu/arch/extBackside-c.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Pentium® III Processors for Applied Computing product brief <a href="http://download.intel.com/design/intarch/prodbref/27331106.pdf" target="_blank">http://download.intel.com/design/intarch/prodbref/27331106.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Buses: frontside and backside". ITworld. 2001-04-30. Archived from the original on 2001-05-02. <a href="https://web.archive.org/web/20010502134148/http://www.itworld.com/Comp/1091/CWD010430STO60015/" target="_blank">https://web.archive.org/web/20010502134148/http://www.itworld.com/Comp/1091/CWD010430STO60015/</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Buses: frontside and backside". ITworld. 2001-04-30. Archived from the original on 2001-05-02. <a href="https://web.archive.org/web/20010502134148/http://www.itworld.com/Comp/1091/CWD010430STO60015/" target="_blank">https://web.archive.org/web/20010502134148/http://www.itworld.com/Comp/1091/CWD010430STO60015/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Monday a big day for Apple". CNet. 1997-11-07. <a href="http://news.cnet.com/Monday+a+big+day+for+Apple/2100-1001_3-205119.html" target="_blank">http://news.cnet.com/Monday+a+big+day+for+Apple/2100-1001_3-205119.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>"Backside Bus". Whatis.com. 2001-04-30. <a href="http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci213804,00.html" target="_blank">http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci213804,00.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

Back-side bus open-in-new

Back-side bus