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… or I would like to think it is ultimate but it’s lame instead 😀

Intro

I needed to write a blog post showing different RAM hax/configs on Amiga 500 8A motherboard to serve as a reminder for future Me 😀

I took all pics for this post in early 2020 and I might not remember everything properly. If there are any mistakes, please leave some feedback in the comments section below the post.

Rev. 8A

When working with my stash of A500s, I’ve found two common motherboard revisions – 6A and 8A. I will focus on 8A here but some configs can be used in 6A too. The main reason is that 8A mobos are the latest, where 8375 AGNUS was used and it supports up to 2MB of ChipRAM. Also, I’ve already described the 6A ChipRAM mod in one of the previous posts.

Just to clarify it a bit, Rev.8 was used in A500plus AND in standard A500 but with unpopulated RTC, battery, less RAM, etc.

More info on AGNUS and A500 is HERE and HERE.

Revision details – HERE

Anyway, let me show you a star of today’s blog post with the default config 🙂

Quick start

We are interested in jumpers – JP2, JP3, JP4A, and JP4B(solder blobs under U32) and JP7A.

As seen in pics, these jumpers are set in default config – 0.5MB of ChipRAM and trapdoor port enabled.

You can also see there are four spots with unpopulated RAM chips – U16, U17, U18, and U19

For upgrades, I will be using 265kx4 DRAM MB81C4256-10P chips by Fujitsu.

In the next step, I soldered these chips onto the motherboard and booted to Amiga Test Kit which by the way, is an awesome piece of software! Thank you Keirf 🙂

Soldering chips without changing the jumper config is not enough, so we are still in a default config with 0.5MB of ChipRAM and trapdoor port enabled.

Below, are pics with 0.5MB trapdoor expansion plugged in which shows as 0.5MB SLOW – still default

Enabling 1MB ChipRAM

With chips soldered in place, now it is just a matter of setting a proper jumper config.

This is done by changing JP2(enable 1MB ChipRAM) and JP7A(disable trapdoor), the same as with Rev. 6.

Below, I’ve changed JP3 to check what are the results and obviously, it didn’t boot.

1MB ChipRAM + trapdoor expansion

Now I will show some examples with the above config and 0.5MB trapdoor expansion.

The tricky bit is jumper JP7A. Below is an example with solder blobs removed on JP7A and without anything connected to the expansion port.

The results are the same as before – 1MB ChipRAM

Now, even if I connect a trapdoor 0.5MB expansion and set the jumper in the following way, it still stays the same as if the expansion didn’t work at all.

To get 1.5MB of ChipRAM (1MB of mobo RAM + 0.5MB via trapdoor expansion) we have to remove solder blobs on JP4A and JP4B under U32 and install a 74F139 which is a dual 2-line to 4-line decoder/demultiplexer.

In my example, I used 74LS139 and it worked perfectly fine for short tests but I didn’t test it in games.

This is how I did it.

Below are results with trapdoor expansion disabled via a switch – 1: ON 0: OFF

2MB onboard ChipRAM hack

Now, I just wanted to emphasize it, this hack is only possible on Rev. 8 mobo and with 8375 AGNUS.

The idea here is to add 1MB of RAM by soldering it on top of the chips that are on the PCB already, a.k.a. piggy-backing method. However, every PIN 17 (CAS) of four RAM chips on the left has to be soldered to PIN 11 of U32 and four RAM chips on the right have to be soldered to PIN 5 of U32.

This is how it looked with a proper jumper config.

To make this a useful hack, I’ve added a switch on the1 back which would run two modes:

1. 0.5MB onboard ChipRAM + 0.5MB trapdoor expansion
2. 2MB onboard ChipRAM

I did this by soldering signals from JP7A and JP2 to the dual switch

Mode 1

Mode 2

Bonus config

There is one more config that I’ve tested which I think is interesting. It is a 1.5MB expansion that goes under the trapdoor and uses an add-on board that goes between GARY and Mobo. For this test, I had to temporarily remove the U32 hack.

This config results in 0.5MB onboard ChipRAM and 1.5MB SlowRAM.

Final words

As I’ve written at the beginning of the post, these pics are over 3 years old and I am writing it all from the top of my head so I may have messed something up. If so, please correct me in the comments section 🙂

Now you can safely watch our(Lamers) Demo 😉

Outro

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