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Back-side bus

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Title: Back-side bus  
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Subject: Front-side bus, BSB, Computer bus, Ultra Port Architecture, Bus contention
Collection: Computer Buses
Publisher: World Heritage Encyclopedia

Back-side bus

In personal computer microprocessor architecture, a back-side bus (BSB), or backside bus, is a computer bus used to connect the CPU to CPU cache memory, usually L2. If a design utilizes it along with a front-side bus (FSB), it is said to use a dual-bus architecture, or in Intel's terminology Dual Independent Bus (DIB)[1] architecture.


BSB is an improvement over the older practice of using a single system bus, because a single bus typically became a severe bottleneck 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.

Cache connected with a BSB was initially external to the microprocessor die, but now is usually on-die.[2] In the latter case, the BSB clock frequency is typically equal to the processor's,[3] and the back-side bus can also be made much wider (256-bit, 512-bit) than either off-chip or on-chip FSB.

A Pentium II processor module with its cover removed showing the processor on the left and the L2 cache memory on the right

The dual-bus architecture was used in a number of designs, including the IBM and Freescale (formerly the semiconductor division of Motorola) PowerPC processors (certain PowerPC 604 models, the PowerPC 7xx family,[4] and the Freescale 7xxx line), as well as the Intel Pentium II processor,[5] 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).

See also


  1. ^ "Dedicated Backside Cache Bus". 2001-04-30. 
  2. ^ "Buses: frontside and backside".  
  3. ^ "Buses: frontside and backside".  
  4. ^ "Monday a big day for Apple". CNet. 1997-11-07. 
  5. ^ "Backside Bus". 2001-04-30. 

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