Commodore Amiga 500 1Mb Chip Ram Mod

Introduction

There are many games which require at least 1Mb of chip ram. This became apparent when trying to play Kickoff 3 with only 512k

After installing a 0.5Mb Ram expansion in the trap door the game still refused to work. Using the Kickstart Memory scan shows the expansion is detected as Slow ram. The Amiga 500 plus automatically configures trap door ram as additional chip ram but not the standard Amiga 500.

This (HOW TO) is primarily based on a Rev 6a A500 motherboard, with an 8372a Agnus. There is instruction for the Rev5 motherboard but I didn’t have one to hand to test.

The A500 motherboard shown here is a Revision 6A which allows for a straightforward modification to get a 512K trapdoor expansion Ram to be seen as additional Chip RAM. The 6A motherboard comes with the Enhanced Chip Set (ECS) variant of the Fat Agnus chip 8372A(PAL) which is able to support up to 1MB total Chip RAM. This 1MB Chip RAM Mod will combine the 512k onboard RAM with the 512k Trapdoor RAM to give 1MB total Chip RAM as is in the A500 Plus models.

Original A500s contain the 8370 (NTSC) or 8371 (PAL) Agnus. Commonly known as OCS or Original. Subsequent A500 versions used the ECS or Enhanced Chipset which allows the Fat Agnus to access upto 1Mb of Chip RAM. Some Rev 5 boards come with the older OCS Fat Agnus. See here for a more detailed description of the Fat Agnus chips.

Disclaimer Warning

If you perform these alterations it is at your OWN RISK; that means if you kill your A500 or hurt yourself or others its YOUR FAULT, it is your responsiblity if you decide to follow this guide.

Doing The Checks

You can check if you have the correct ECS Agnus 1Meg chip in Sysinfo as shown top right in the image bewlow.

Check your motherboard and Fat Agnus chip versions here

Chip RAM is used for such tings as graphics and DMA operations as it is the only memory that the custom Amiga chips are able to access and 512k is very limited. Opening Workbench windows, increasing the screen resolution, adding a wallpaper all gobble up precious Chip RAM. Using WHDLoad can be a problem for 512k machines as most games require 1MB of Chip RAM.

Modification

Tools required:

Sharp Knife(craft)
Soldering Iron

There are two very small cuts to be made and a small solder blob to bridge two contacts

The areas to be worked on are JP2 and JP7A located here

JP2 will need the bottom two pads trace cutting and the top two pads soldering together as follows

JP7A just needs the trace cutting betweern the bottom two pads as follows

And thats all for the 6A version motherboard.

Rev 5 Motherboards

On Rev 5 Motherboards the JP2 pads exist however the JP7A pads are missing. Instead the trace identified below replaces JP7A and just needs cutting with a sharp knife.