Jaguar 5-way BIOS Modification


When modifying my Jaguar CD into a Developer CD I found the need of having multiple BIOS versions in my Jaguar. The Developer CD needs the Stubulator '94 BIOS but I also wanted to use the BJL BIOS of Bastian Schick. The solution: Look at the excellent description of the BJL installation by Matthias Domin but use a bigger EPROM, program it with multiple BIOS versions and make the upper address lines switchable.

I used a 512Kb EPROM so I can have 4 different BIOS versions. My Jaguar now has Stubulator '93, Stubulator '94, BJL 1.06 and JagOS 1.03 and of course the original boot ROM.



Adding a new BIOS chip to your Jaguar is not easy. To do this you need a lot of soldering experience and a lot of patience. Do this modification at your own risk and don't blame me if something goes wrong.




Simplified schematics

The modification works by soldering a Flash ROM with the new BIOS versions on top of the original BIOS ROM. Most pins are soldered on the original ROM except for pin 1, 22 and 30. A toggle switch is used to switch between the two ROM chips. A 4-bit rotary switch is used to select the BIOS version in the Flash ROM. The schematics below show the connections of the ROM chips that are not connected directly on the PCB anymore.


Step-by-step guide

  1. First take your Jaguar apart and remove the shielding so you only have the PCB. Locate the BIOS chip which is the big 32 pin chip at the upper right part of the PCB (near the power connector).
  2. Cut pin 22 (/CE) as close to the PCB as possible. Bend pin 22 of the chip up. Be careful not the break it.
  3. Get the IC socket. Bend pin 1 (A18), pin 22 (/CE) and pin 30 (A17) up to the outside of the socket.
  4. Solder the IC socket on top of the ROM chip. Make all connections except for pin 1, 22, and 30.
  5. Solder a 47K resistor between pin 22 (/CE) of the ROM chip and and pin 32 (VCC).
  6. Solder a 47K resistor between pin 22(/CE) of the IC socket and pin 32 (VCC).
  7. Solder three wires to the toggle switch. The common contact (The middle one) goes to /CE on the PCB (there where you cut the pin 22 of the chip). Easiest is to solder the wire at the bottom of the PCB.
    The other two wires go to respectively pin 22 of the BIOS chip and pin 22 of the IC socket.
    With this switch we can now switch between the original BIOS chip and the new BIOS chip.
  8. Solder a 47K resistor between pin 1 (A18) of the IC socket and pin 16 (ground).
  9. Solder a 47K resistor between pin 30 (A17) of the IC socket and pin 16 (ground).
  10. Solder three wires to the rotary switch. The common contact goes to pin 32 (VCC) of the BIOS chip. Bit 0 of the rotary switch goes to pin 30 (A17) of the IC socket. Bit 1 of the rotary switch goes to pin 1 (A18) of the IC socket. Bit 2 & 3 of the rotary switch remain unconnected.
    Now we can use the rotary switch to select one of the 128Kb banks of the Flash ROM chip.
  11. Find a nice place in the Jaguar's housing for the toggle switch and rotary switch. I placed them at the upper right of the housing.
  12. Program the Flash ROM with the BIOS versions you want. Program in each 128Kb block a BIOS:

$00000 - $1FFFF: First BIOS
$20000 - $3FFFF: Second BIOS
$40000 - $5FFFF: Third BIOS
$60000 - $7FFFF: Fourth BIOS

  1. Insert your freshly programmed Flash ROM in the IC socket. If everything went well your Jaguar will look similar to this (Note: the blue wire in the pictures is part of a reset switch and not part of the BIOS upgrade):


Finally reassemble your Jaguar. The toggle switch will switch between the original BIOS and an extra BIOS. The rotary switch determines which extra BIOS to use.