Oldie Tuning:
Asus P55T2P4 with AMD K6-2+/500
Редакция THG,  25 июля 2000

Socket7 Tuning: Asus P55T2P4 with AMD K6-2+


In the article 600 MHz with Socket 7: The AMD K6-2+ we thoroughly tested the AMD K6-2+ that turned out to be the upgrade hit for old Socket 7 systems. The feedback and enthusiasm among our readers was huge, and therefore we decided to follow up with a special that particularly addresses the legendary Asus P55T2P4. This board was introduced in 1996 and is based on Intel's 430HX chipset. The board had become very popular due to its stability, and because it allows overclocking of the Front Side Bus. In fact, the legendary 'T2P4' was the very first motherboard tested by Tom's Hardware Guide back in Fall 1996 and Tom based his historical Bus Speed Guide, which changed the whole motherboard industry, on results scored with the 'T2P4'.

Considering the age of the product it is very surprising that many users still work with the P55T2P4. One major reason why the owners do not want to give up their old board is its compatibility with the latest Socket7 processors. Equipped with a fast AMD K6-III or an AMD K6-2+ the board certainly reaches peak form. We tested the P55T2P4 with the fastest settings and the new K6-2+, and came to the following conclusion: This board does not belong on the scrap heap at all yet!

The popular Asus P55T2P4 is still up-to-date after four years. With the help of the .18 micron AMD K6-2+ it achieves the performance level of a modern PC system.

Prerequisites for Using the AMD K6-2+

Asus P55T2P4 Revision
Only revisions 3.x and later of the Asus P55T2P4 support the new AMD K6-2+.

An important prerequisite for running the AMD K6-2+ on the Asus board is the correct core voltage of the processor. Whether the P55T2P4 supports the necessary core voltage of 2.0 volts or not depends mainly on the actual board revision. Generally the motherboard must be equipped with two switching voltage controllers (split-plane design), one for the CPU core voltage and one for the I/O voltage. This feature was originally intended for the upcoming Pentium-MMX processors back then. Officially the board only offers core voltages between 2.5 and 2.9 volts. However, the controller is capable of supplying voltages between 2.0 and 3.5 in small increments. Our table lists the missing undocumented voltage values. In order to adjust to 2.0 volts all four pins between 2.5 and 2.9 volts must be closed with a jumper.

Undocumented feature: The old Asus board supports a core voltage of 2.0 volts. You have to close all four pins between 2.5 and 2.9 volts with a jumper.

Voltage Settings of the Asus P55T2P4 (Rev. 3.x)

Core Voltage Jumper Settings
2.0 v close pins 2.5, 2.7, 2.8, and 2,9
2.1 v close pins 2.5, 2.7, and 2.8
2.2 v close pins 2.5 and 2.7
2.3 v close pins 2.5 and 2.8
2.4 v close pins 2.7 and 2.8

Voltage Settings of the P55T2P4-S

Core Voltage Jumper Settings
2.0 v close VID1 and VID3
2.1 v close VID1 and VID2
2.2 v close VID2 and VID4
2.3 v close VID0 and VID4

These tables contain all undocumented settings for the core voltage. The bottom table shows the jumper configuration for the P55T2P4-S.

High Performance Thanks to 83 MHz System Clock

When the Pentium and Pentium MMX were popular, the maximum system clock was 66 MHz. For those CPUs the L2 cache resides on a Socket7 board and not on the die of the processor. As a consequence, the L2 cache and the system memory are accessed at system clock frequency. In order to run a modern K6-2+ at 500 MHz, it is necessary to set the system clock above the official specification. On most boards the jumper settings are noted for all clock rates up to 75 MHz. The highest possible clock rates of the P55T2P4 series are 83 MHz and with this frequency the highest performance is achieved. We must clearly point out here that in this case the PCI bus as well as the ISA interface are significantly overclocked. Because of the higher PCI clock of 41.5 MHz (instead of the default 33 MHz) some older graphics cards might run into problems. A Riva 128 graphics card in combination with two Voodoo2 cards (SLI mode) are an ideal solution. The undocumented settings for a system clock of 83 MHz are listed in the bottom table. The system clock value of 108 MHz, which is also listed, has no meaning in practice because the system refuses to boot at this frequency.

The jumpers JP8, JP9 and JP10 set the system clock. To operate the AMD K6-2+/500 successfully, the system clock should be set to 83 MHz.

FSB JP8 JP9 JP10 Note
50 MHz 2-3 2-3 2-3
55 MHz 1-2 2-3 2-3
60 MHz 2-3 2-3 1-2
66 MHz 2-3 1-2 2-3
68 MHz 1-2 1-2 1-2 turbo mode
75 MHz 1-2 2-3 1-2 runs very stable
83 MHz 1-2 1-2 2-3 runs stable
108 MHz 2-3 1-2 1-2 not working

The table shows all clock rates for the system clock. The 108 MHz value has no practical meaning, however.

Nothing Happens Without Multiplier!

The Asus P55T2P4 has two jumpers for adjusting the clock multipliers. As the board is only equipped with the jumpers BF0 and BF1, the maximum multiplier of 3.5 was just enough to operate a Pentium MMX 233 from 1997 with the setting 3.5 x 66 MHz = 233 MHz. There are two possibilities to achieve higher clock rates for the CPU frequency: One method is to increase the system clock to 75 MHz or even 83 MHz, as described above. Additionally AMD's processors with CXT core offer the option of chosing a multiplier of 6. To utilize this option the multiplier on the motherboard must be set to 2.0, which the processor then translates into factor 6. Therefore 66 MHz allow a maximum clock frequency of 400 MHz (6 x 66 = 400 MHz). Now the Asus P55T2P4 can accommodate an AMD K6-2+/500 if the system clock is increased to 83 MHz (6 x 83 MHz = 500 MHz). Real freaks could also drive the system clock to almost 90 MHz by exchanging the quartz of the clock generator (substitute the 14.83 MHz quartz with a 15 MHz model). In this case the PCI bus runs at 45 MHz, which is a problem for most components, however.

The Asus P55T2P4 only has the jumpers BF0 and BF1 allowing a maximum possible clock multiplier of 3.5. But the AMD K6-2+ interprets the multiplier "2" as 6.

Multipliers JP11 JP12
1.5 1-2 1-2
2.0 2-3 1-2
2.5 2-3 2-3
3.0 1-2 2-3
3.5 1-2 1-2
6.0 2-3 1-2

This table lists all possible clock multipliers.

CPU Clock FSB and Multiplier PCI Clock
500 MHz 6 x 83 MHz 41.5 MHz
450 MHz 6 x 75 MHz 37.5 MHz
400 MHz 6 x 66 MHz 33.3 MHz

These are the possible clock rates on the P55T2P4 in connection with an AMD K6-2+. The numbers are also valid for processors with CXT core

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 Mainboard 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:

  1. COAST extension slot, no onboard cache
  2. 256 KB L2/L3 onboard cache and COAST extension slot
  3. 512 KB L2/L3 onboard cache, no COAST extension slot

A COAST (cache on a stick) 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.

A feature unusual in conjunction with a 430HX chipset: The USB interface was already there in 1996.


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