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Dreamcast Bios Mod
Parts needed:

MX29LV160TMC-90 CMOS Flash Memory Chip
10k resistors (x2) -Dual Bios only
On-On Switch (SPDP or DPDT)-Dual Bios only
There are 3 different Dreamcast main board revisions.
These are:

VA0 (very early board with the metal heat sinks and pipes)
VA1 (The most common)
VA2.1 (the last revision with 4 sets of wires to connect the GD-Rom)
VA0
VA1
VA2.1
The first thing that you need to do is figure out what revision
Dreamcast you are working with. The main differences with
installing this mod is the VA0 uses a different voltage than the
VA1 and VA2.1.
The second thing to consider is are you piggy packing the Bios
(Dual bios) or are you doing a replacement? (Not recommended)
If you are doing a dual bios then you can purchase a blank chip
and program it after installation. For a bios replacement you
will need to get a pre-programmed chip.
Step 1. Flatten the legs of the MX29LV160TMC-90 Chip out:
Be careful, These legs break easily!
MX29LV160TMC-90
MX29LV160TMC-90 Flat
Step 2. Bend the legs back down as flat to the chip as you can:
Step 3. Bend legs 1, 12 and 44 out:
Step 4. Lift leg 12 of the onboard bios chip:
Step 5. Test fit and solder the MX29LV160TMC-90 chip to the
original bios: (Except the lifted legs).
Piggy Back (Dual Bios install) VA1 & VA2.1
See below for VA0 instructions.
Step 6. Connect pin 44  of the MX29LV160TMC-90 to Pin 9 of the
29LV002TC-90 Chip.
pin9.jpg
I like to use the via that connects to pin 9 (the “Or Here” in the above
left photo). Carefully scrape the oxidation layer from the pad on the
bottom of the board, then use kynar wire and push it through the top.
Solder on the bottom. See the photo below.
Step 7. Take one of the 10k resistors and connect it between
pin 23 and pin 12 on the original bios.