Micro Firmware Tech Support

Write-Back/Write-Through Cache on Intel P24T Overdrive CPU's

Filename: P24TWB.TXT
WWW URL:  http://www.firmware.com/support/bios/p24twb.htm
FTP URL:  ftp://ftp.firmware.com/text/p24twb.txt
Revision: 06/17/96  TLS  Micro Firmware Technical Support
Keywords: P24T, POD, Pentium, cache, L1, JX30
Summary:  Explains the issue of write-back versus write-through cache
          on the Pentium Overdrive CPUs.

The manuals that ship with the Intel 63MHz and 83MHz P24T Pentium Overdrive CPU chips mention the write-back/write-through issue in Appendix A, where the cache is described in the feature list as "16KB Code Cache and 16KB Write Back Capable Data Cache**". The footnote at the bottom of the page for the double-asterisk says -

 " Support of the internal write back data cache is system-dependent.

If the system does not support a write-back internal cache, the
cache operates in a write-through manner, similar to a standard
Intel 486 processor."
Many people are confused about the difference between internal CPU cache (also called L1 or Level 1 cache) and external cache on the motherboard (also called L2 or Level 2 cache). And of course there are other types of hardware cache such as the caches built-in to hard drives or on some controller cards and various kinds of software caches such as Smartdrive or the caching systems built-in to Windows 3.1 and WIN95.

The external cache (L2) has nothing to do with the internal CPU cache (L1). So whether or not a system has external cache or whether or not that cache is set up as write-through or write-back has nothing to do with whether a system-board can support the P24T internal cache in write-back mode.

This also is not normally a BIOS issue. It could be possible for a board to be designed such that this could be controlled by the BIOS but this is probably not the normal practice and it is not possible to write anything into the BIOS to "fix" a board that is not designed to support the P24T properly to begin with. The write back cache support is controlled by a pin on the CPU and as such is normally going to be purely a hardware issue, having to do with how the board is physically designed.

The issue of whether a motherboard properly supports the P24T seems to arise from the fact that these chips were announced several years before they were available and apparently Intel changed some of the specifications during the years between the initial anouncement and the time when the chips became available (late 1995). Micronics has stated to us that this is in fact the case on the JX-30 series. They have told us that these boards do not support the P24T and that the reason is that Intel changed the specifications from those originally provide to the industry. We do not yet know if the write-back cache support is an issue on this board. The problems that people are experiencing with the P24T on the JX30's are not related to whether the board supports internal or external write-back cache.

We have had numerous calls from customers and non-customers using the P24T on the JX30 and JX30G reporting various problems under both the old BIOS and our upgrade BIOS. We should not assume that our BIOS will fix any odd behavior caused by installing the P24T.

Also the regular JX30 does not support external write-back cache, according to Micronics. Our BIOS does have a setup option for write- through or write-back. We implemented this because it is included in the original BIOS. But there is no mention of write-back cache in any of the documentation for this board and we have been told by Micronics that write-back cache is not supported on the JX30. The JX30G does support external write-back cache if the system has the appropriate cache chips installed. But again, this has nothing to do with the issue of the write-back cache on the P24T.

We do not currently know of any problems with the P24T on the Intel Classic R or the Anigma motherboard, although these boards also do not support the internal CPU write-back cache.


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While every attempt is made to ensure the accuracy of our support documents, Micro Firmware provides these documents on an AS-IS basis for information purposes only.