Contents

• Introduction
• Which Am5x86 do you like?
• The CPU Socket
• The motherboard jumpers
• BIOS Compatibility

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 CCT 586 Upgrade Kit
• The Evergreen 586 Upgrade
• The Kingston TurboChip 133
• The PNY Quickchip 133
• The Powerleap PL/586-133
• The Trinity PowerStacker 5x86
• The VisionTek - ExtremeCPU

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

• Original 486 socket
  This 168-pin PGA (Pin Grid Array) socket was designed to allow the 486SX/DX or 486SX2/DX2 CPUs to be "plugged-in". This requires a lot of force however, even if it is a LIF (Low Insertion Force) socket. If this is the only CPU socket on the motherboard, you need tools to extract the old, and insert the new CPU. Without these tools, you will easily break the CPU.
  I recommend one of the Am5x86 upgrade kits for these motherboards. Select one with the proper tools.

• Socket 1
  This socket was designed as an extra (empty) upgrade socket on 486SX/SX2 motherboards. It is a 169-pin PGA socket and can be either LIF or ZIF. It is meant for the 169-pin 487SX upgrade CPU, but will also fit 168-pin upgrade CPUs like the 486DX2, DX4, and the Am5x86. As boards with Socket 1 normally have no DX4 support and therefore lack the necessary jumpers for the raw Am5x86, I also recommend one of the Am5x86 upgrade kits here.

• Socket 2
  This socket was designed to fit most 486 CPUs from Intel, AMD, and Cyrix and the Intel 486 Pentium Overdrive (P24T). It is a ZIF socket in a 238-pin PGA layout. 486 CPUs and the Am5x86 just use the inner 168 pins of this socket. The outer pin rows are only used by the Pentium Overdrive. As this socket supports 5 Volt CPUs only, one of the Am5x86 upgrade kits is the recommended choice here. If the motherboard has a "clock multiplier" jumper (for the Intel DX4), the raw Am5x86 CPU can be used as well, provided you add a Voltage-regulator Adapter between the socket and the CPU.

• Socket 3
  This is the most widely used CPU socket on 486 motherboards. It is a ZIF socket in a 237-pin PGA layout and is, apart from some different keying pins for the Pentium Overdrive, identical to socket 2. The big difference however, is the support for both 5V and 3.3V CPUs. If your motherboard supports the Intel DX4 directly, it will have a socket 3 and all the necessary jumpers for the Am5x86. This socket is therefore ideally suited for the raw Am5x86 CPU. Just add a (clip-on type) heatsink with fan and your upgrade is complete.
  

       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 motherboard jumpers

• Clock Multiplier jumper
  The CPUs internal clock multiplication factor is controlled by the CLKMUL pin R-17. If this pin is left floating (or connected to +3.3V), both the DX4 and the 5x86 will operate in the clock-tripled mode. To get the Am5x86 in clock-quadrupled mode, this pin has to be grounded . As this would have put a 486DX4 in clock-doubled mode, you have to look for the clock-multiplier jumper and change it from x3 to a x2 setting.
  You can use my CPU Identification utility to confirm the correct setting.
  If you can't locate the clock-multiplier jumper and if you have a multimeter, you can use the following procedure to find it.
  1. Unplug and then open the system.
  2. Lift the ZIF socket handle and take the CPU out. Store it in an anti-static bag or box or place it with it pins on a piece of conductive foam.
  3. Now insert a wire in the CLKMUL pin R-17 and close the ZIF socket handle to connect the wire.
  4. Connect this wire to one lead of the multimeter and set it to a low Ohm range.
  5. Use the other lead to search the jumper pins for a connection.
  6. When found, check if an adjacent pin is connected to the chassis. Placing a jumper over these pins will ground the CPU's CLKMUL pin and put the Am5x86 in x4 mode!
  7. Restore the system to working order.
  Caution: Anytime before you touch any component inside your system, place one hand on the chassis, such as the power supply case, to prevent electrostatic discharge damage.

• CPU Voltage
  The Am5x86 needs a supply voltage of 3.45V + 0.15V. Setting the voltage jumpers as for the 3.3V 486DX4 will normally take care of this. Note that most motherboards actually supply 3.38V when set for 3.3V so that both 3.3V and 3.45V CPUs can be supported.
  If you can't find voltage jumpers, it is possible that your motherboard can autodetect the required CPU voltage. New CPUs like the 486DX4 and the Am5x86 have a VOLDET (Voltage Detect) pin that causes compatible motherboards to switch to 3.3V automatically. The motherboard manual should be clear on this. If you're not absolutely sure and if you have a multimeter, you can use the following procedure to check this out.
  1. Unplug and then open the system.
  2. Lift the ZIF socket handle and take the CPU out. Store it in an anti-static bag or box or place it with it pins on a piece of conductive foam.
  3. The VOLDET pin is pin S-4. It is tied to ground (Vss) internally in the CPU. Simulate this by inserting a wire from pin S-4 to Vss pin R-4.
  4. Now insert two wires for the multimeter. The ground wire goes in Vss pin S-10 and the live wire in Vcc pin R-10.
  5. Close the ZIF socket handle to connect the wires.
  6. Switch the system on and check for 3.45V + 0.15V DC on the multimeter. If you see 5V, then your motherboard is not autodetecting.
  7. Restore everything as it was.
  Caution: Anytime before you touch any component inside your system, place one hand on the chassis, such as the power supply case, to prevent electrostatic discharge damage.

• WB Level 1 Cache
  With the motherboard jumpers set for the Intel 486DX4, the Am5x86 will operate its 16Kb Level 1 (on-chip) cache in the Write-Through mode. For improved performance, the L1 cache should operate in the Write-Back mode but this is only possible if the motherboard chipset supports it. This support is completely separate from the WB support for the larger 128Kb or 256Kb Level 2 cache on the motherboard.
  As the Intel 486 Pentium Overdrive (P24T), the Intel WB Enhanced 486DX2 (P24D), the AMD Enhanced Am486 Microprocessor Family, and the Cyrix Cx486DX(2) (M6/M7) are the only other 486 CPUs with WB support, you need a chipset with support for one of these CPUs to have any chance on WB support. Be aware that most 486 motherboards don't support the L1 cache WB mode, but if yours does, the motherboard manual will clearly state this.
  Popular 486 chipsets that do support L1 cache WB mode are:
  
  • ALi 1439/1445/1431
  • ALi 1487/1489
  • Intel 420ZX (Saturn II)
  • OPTi 82C802G(A)
  • OPTi 82C895
  • SiS 85C471
  • SiS 85C496/497
  • UMC 498F (UM8498F/8496F)
  • UMC 881 (UM8881/8886)
  • VIA VT82C486
  • These are the most common but there are others. E-mail me if you have one.
  If you know your motherboard has L1 cache WB support, there are three ways to get it in case you don't have the Am5x86 jumpers setting.
  Also, check your BIOS Setup for an "Internal Cache WB/WT" option. If available, set it to Write-Back.
  1. Set the jumpers as for the AMD Enhanced Am486DX4-100 except for the Clock Multiplier jumper (see above).
  2. If the Am486 is not supported, set the jumpers as for the Intel WB Enhanced 486DX2-66 (P24D) except for the CPU Voltage and Clock Multiplier jumpers (see above).
  3. The hard way! If you know your electronics, grab your multimeter and read on. :-)
     The WB Enhanced CPUs use four extra signal pins to support the Write-Back operation of the internal cache. These signals are: WB/WT#, HITM#, INV, and CACHE# and are only used in WB mode. With the multimeter you have to check which jumpers connect to these signal pins on the CPU socket.
     The details for the Am5x86 are:
     • WB/WT# is pin B-13. This pin is an input for the CPU and must be connected to Vcc (+3.3V or +5V) to get the L1 cache in WB mode. If this pin is not connected, an internal pull-down resistor will keep the L1 cache in the WT mode. The jumper for this signal can connect it directly to Vcc or to a WB/WT# output of the chipset.
     • HITM# (Hit Modified line) is pin A-12. This pin is an output for the CPU and must be connected to the HITM# input of the chipset. This signal is essential to maintain cache coherency when the WB policy is used.
     • INV (Invalidate) is pin A-10. This pin is an input for the CPU and is used to invalidate a cache line under specific conditions. The jumper for this signal can connect to the INV output of the chipset or to the W/R# output of the CPU (pin N-17).
     • CACHE# is pin B-12. This pin is an output for the CPU and indicates the internal cacheability of the current bus cycle. If there is a jumper for this signal, it will connect to the CACHE# input of the chipset, but there are WB chipsets that don't use this signal.
     Caution: While working with the multimeter, take the usual precautions to prevent electrostatic discharge damage. Also, don't place the motherboard on a conducting surface because you may short-circuit the CMOS battery which may result in an explosion!
  You can check the L1 cache WT/WB mode setting of the Am5x86 with my CPU Identification utility. This program indicates the L1 cache mode in plain English.
  If you use another utility that can do a CPU Identification, look under CPU-ID "Model". An "8", a "14", or an "E" indicates WT and a "9", a "15", or an "F" the WB mode.

  With the CPU in WB mode, you can test the correct functioning of the CPU/Chipset WB interface (via the HITM#, INV, and CACHE# signals) by booting from a DOS 5 or 6 floppy. (Do not use a Windows 95/98 floppy for this.) If this works, the L1 cache WB mode is set up correctly. If not, see the BIOS Compatibility section below.

Boot-up
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.

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