CPU Upgrade:
Give your 486 PC a kick with the AMD 5x86 - 133!
The whole story.

by Jan Steunebrink
Version 4, July 1999

AMD 5x86 picture



With this story, I like to tell what I have learned by upgrading my 486 PC with the Am5x86 CPU.
I will try to provide as much practical detail as possible.

The Am5x86 is actually a 486DX (the fastest 486 around) that delivers performance superior to the Pentium-75 on a modern 486 motherboard.
That's why AMD named it Am5x86-P75.
It is a clock-quadrupling CPU so it runs internally at 133 MHz on the usual 33.33MHz bus-speed. The 16 Kb Level 1 (on-chip) cache supports both the Write-Through (read cache) and the Write-Back (read and write cache) mode.

AMD has put a lot of information about the Am5x86 on their web-site which I will not repeat here.
You can even download the complete datasheet there. (file: 19751.pdf 1.5Mb!)
Start at www.amd.com/products/cpg/5x86/5x86.html and then come back here.

Which Am5x86 do you like?

The major CPU-upgrade companies have all put the Am5x86 in their program. Take a look at:
These kits will give you a hassle free upgrade and come with instructions, tools, heatsink or cooling fan, 5V to 3.3V regulator, their own jumpers, and warranty. They sell for around $100 and I recommend them if your 486 doesn't have a ZIF (Zero Insertion Force) CPU socket, or lacks DX4 support.

If, on the other hand, you have a more recent 486 motherboard, with ZIF socket, and support for clock-tripling CPUs like the Intel DX4 (P24C) or Am486DX4, or the 486 Pentium Overdrive (P24T), I would go for the raw Am5x86 CPU. It sells for about $50, however stocks are getting low now AMD stopped producing the chip. So you may have to try the second-hand marked to get one.
Using the raw Am5x86 involves adding a (clip-on type) heatsink with fan, and setting all the motherboard jumpers yourself. But this is what this story is all about!

The CPU Socket

On top of the original 486 CPU socket, Intel has specified several 486 and Pentium upgrade sockets. Upgrade-ready 486 motherboards will have a CPU Socket number 1, 2, or 3.
This is the layout of socket 2 or 3 as viewed from above.

       1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17
  / O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O |
A | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | A
B | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | B
C | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | C
D | O  O  O  O  -------------------------------  O  O  O  O | D
E | O  O  O  O |                               | O  O  O  O | E
F | O  O  O  O |                               | O  O  O  O | F
G | O  O  O  O |                               | O  O  O  O | G
H | O  O  O  O |           Socket 2            | O  O  O  O | H
J | O  O  O  O |              or               | O  O  O  O | J
K | O  O  O  O |           Socket 3            | O  O  O  O | K
L | O  O  O  O |                               | O  O  O  O | L
M | O  O  O  O |                               | O  O  O  O | M
N | O  O  O  O |                               | O  O  O  O | N
P | O  O  O  O  -------------------------------  O  O  O  O | P
Q | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | Q
R | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | R
S | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O | S
  | O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O  O |
       1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17

The sockets 2 and 3 have a 19x19 pin layout but I have only indicated the pin-numbers for the inner 168 pins in a 17x17 layout. The Am5x86 does not use the outer pins, so skip the first and last row and column when counting for a pin location.

The motherboard jumpers

If you install the raw Am5x86 CPU, but you can't get the Am5x86 jumpers setting from the motherboard or system manufacturer, then start by setting all jumpers as for the Intel 486DX4-100 or, even better, the AMD Enhanced Am486DX4-100. See below for further details. Note that the Am5x86 upgrade kits do not use the motherboard Clock Multiplier and L1 cache WB jumpers. They have these settings on the unit, except for the Kingston Turbochip 133. This kit is fixed in the x4 clock mode and the L1 cache is always Write-Through!

BIOS Compatibility

Most older BIOSes do not detect AMD CPUs and display the Am5x86 as an 80486DX2or a DX4. This should be no problem, as the Am5x86 has no model specific registers that need to be programmed by the BIOS.
However, a consequence of this incorrect CPU detection can be that the BIOS assumes a higher then actual busspeed. This can result in a degraded performance if the Auto Configuration option in the BIOS Setup is used, because the BIOS programs the chipset with more timing margin than necessary. If this is the case, disable the Auto Configuration feature and set the Cache and DRAM timings to their optimum value. Also check the AT clock setting. It should be CLK/4 for a busspeed of 33MHz.
In addition, the CPU clock can be incorrectly displayed as 100 or 120MHz by the BIOS. Use my CPU Identification utility to detect the true internal CPU speed.

In rare cases, the PC will not boot at all because the BIOS regards the CPU signature as invalid. You can identify this problem by placing the Clock Multiplier jumper and the L1 cache WB jumper back to their DX4 positions (x3 resp. WT). With this setting, the Am5x86 will behave exactly like a 486DX4 and the PC should boot and detect the Am5x86 as a DX4.
You will need a BIOS upgrade to solve this problem, or you can stay in clock-tripled mode and operate the Am5x86 as a (fast) DX4-120 by increasing the motherboard clock to 40MHz.

L1 cache WB mode
If your chipset supports the L1 cache WB mode and you operate the Am5x86 in WB mode, there can be a problem when accessing the floppy drive. This is the case if the chipset does not "see" the HITM# signal from the CPU. If you have the jumpers setting correct ;-) the problem is the BIOS who did not program the chipset for the L1 cache WB mode. This should normally be done automatically when the BIOS detects a WB capable CPU, or manually via an "Internal Cache WB/WT" option in the BIOS Setup. If this option is missing and you have this problem, there are three solutions:

  1. Set the L1 cache jumper to WT mode and accept the ca. 8% performance penalty.
  2. Get a BIOS upgrade.
  3. Use a utility to program the chipset directly in WB mode.
    E-mail me if you need this solution and indicate your chipset details.
    I have utilities now for the OPTi802G(A) and SiS471 and I'm presently working on utilities for the Intel Saturn II, SiS496/497, and UMC498F chipsets.
Power Management
If the motherboard and BIOS support Power Management, there can be a problem. The Am5x86 supports all PM features of the Intel SL-Enhanced (Green) CPUs which includes the DX4. But if the BIOS does not detect the Am5x86 as a "Green" CPU, it will not load the SMI (System Management Interrupt) handler in memory. When the chipset wants to go to "Green" mode and issues an SMI, the system will hang because the Am5x86 responds but the handler is not in place.
To avoid this problem, just disable all Power Management features in the BIOS Setup.
If your BIOS is from 1995 or later, you should not have this problem.

Any comments? Please send a message to J.Steunebrink@net.HCC.nl


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