A Lot Of Small Things: Tag RAM, Cache And COAST Module
The Asus P55T2P4 was shipped in different versions. Certain revisions are lacking the tag RAM component, limiting the memory expansion to 64 MB cacheable area. The tag RAM IC is nothing but a memory component that was used on old boards with Intel's 430HX chipset. Using a 16 bit component increased the cacheable area to a considerable 512 MB. With the new AMD K6-2+, a tag RAM IC is not needed anymore. This CPU is able to address 512 MB system memory, which is also cached.
Before we start talking about L2 cache we would like to clarify the role of motherboard cache. Old CPUs such as Pentium, Pentium MMX or K6-2 from AMD do not have any on-die L2 cache. This feature is actually implemented on the motherboard. When AMD first introduced its K6-III processor, full-speed L2 cache was integrated on-die of this CPU. However, AMD did not disable the "former" L2 cache modules on the motherboard, but used it as so-called L3 cache. This table shows which CPU has on-die L2 cache:
|CPU||on-die L2 Cache||Cache on Motherboard becomes...|
|Intel Pentium||none||L2 cache|
|Intel Pentium MMX||none||L2 cache|
|AMD K6-2||none||L2 cache|
|AMD K6-2+||128 kB||L3 cache|
|AMD K6-III||256 kB||L3 cache|
|AMD K6-III+||256 kB||L3 cache|
Owners of 430HX motherboards find themselves with three options:
- COAST extension slot, no onboard cache
- 256 kB L2/L3 onboard cache and COAST extension slot
- 512 kB L2/L3 onboard cache, no COAST extension slot
A COAST (c ache o n a st ick) extension slot can be used with additional cache memory modules. Owners of boards without COAST socket (as shown in the picture below) should consider themselves lucky: These models already have a factory-installed 512 kB onboard cache in form of SRAMs.
Only with a tag RAM component, the cacheable area could be increased to 512 MB in the good old days. The AMD K6-2+ does not need such a tag RAM chip anymore.
Most Asus P55T2P4 boards come with pre-installed 512 kB L2 cache. Otherwise the board contains a COAST socket (COAST = Cache On A Stick) that can accommodate a memory module with a maximum capacity of 256 kB.
Final Words: Power Upgrade Of An Old Motherboard
The Asus P55T2P4 still has a lot of fans. Over the last four years its owners have always been able to upgrade this board with up-to-date components. With the AMD K6-2+/500, the old board with Intel's 430HX chipset receives its last performance boost. At this point it should be noted that the overall performance does not depend on the processor alone. Other components like the graphics card and hard disk have a significant impact on the performance of a system. Originally the P55T2P4 was designed for the Pentium 233 MMX in 1997. Now, using an AMD K6-2+/500, the performance of this old veteran is quite astounding.