ScuzzBlog: Diaries December 2023

Entry 27th December 2023: Post 1: A500 Plus - It's all CHIP RAM.

A500 Plus - It's all CHIP RAM.

I recently made the post below discussing the various options with
respect to AGNUS upgrades and RAM on the A500. I kinda dodged the
main solution which was to get an Amiga 500 Plus. Not only do you
get the much enhanced and larger 2.04 ROM, but you get the hard
drive controller and way better icon/tooltype management. ECS is
standard with the Super Denise and much fatter AGNUS.

But the best feature and one rarely discussed is the fact that
memory stuffed in the trapdoor becomes CHIP RAM. It's all CHIP RAM
up to 2MB. The fat lady lets you use even your old A501 and boost
your CHIP RAM.

Is there no limit to the benefits of the Amiga 500 Plus ?

Anyhoo, here is that post I made regarding the RAM options for the
Amiga 500 ....

[ Recent Post ]

It's worth first addressing fast/slowfast over chip. The original
boards basically were the 512K chip and could add 512K fast on
the trap door. With the 6A boards came the 1MB AGNUS 8372A which
gave 1mb available on the CHIP. The original 500s only had the
onboard 512K though often came with an A501 RTC and 512K Fast off
the trapdoor. The Plus basically moved the A501 onboard so you
have 1MB of CHIP plus the dreaded battery, though some were still
only supplied with 512K but still had the 2.04.

To enhance your CHIP options on an A500 you need to make modifications
to the board and have fitted the 1MB 8372A. There are expansion
boards that can be configured to work as CHIP but need the adaptor
on the GARY. Note that the onboard memory is on the slow bus.

Read this ...



The 2MB CHIP using the 8375 works very much like the MegaCHIP see below.


Interestingly older accelerators like the ViperCD that I have provide
8MB FAST and an 020 accelerator but still only has 512K CHIP. The
GVP sidecars I use have the 8MB memory also.

The ROM also needs to be upgraded to the 2.04 though its very useful
having a ROM switcher to dodge between 2.04 and 1.3. Interestingly
the greater benefit of the A500+ upgrade was the use of the 2.04
simply cus of hard drive control. Most upgraded so they could use
the sidecars and the 8MB RAM. Acceleration wasn't really the issue
with the 500, it was a better OS that had full icon control, a
better Workbench, hard drive controls and more AmigaDOS functionality.
The 1MB upgrade plus the Denise to get ECS was really to feed the
2.04 ROM.

By the way to utilise the CHIP off the expansion having done the
mod you would normally call upon one of these.


Plugs into the GARY. Its all about timing really cus the CHIP uses
the alternate clock cycle to power the custom chips. The GARY as
I understand it deals with the timing so the trapdoor memory can
access the CHIP cycle. Hence the mod to the jumper 7A if I recall
next to the trapdoor slot. Don't quote me on that.

Modern solutions no doubt have more advanced ways of dealing with
things but old skool you have 512K RAM full stop on an original
A500 that can expand with a 512K Fast in the trapdoor.

6A has the 1MB AGNUS which can access up to that with a modification
to the motherboard to use the trapdoor plus possibly the GARY mod/
trapdoor memory card.

There is the 2MB AGNUS with the use of the 8375 or an 8372B but I
have only used that with a MegaCHIP in the Checkmate


Gives me 1.5MB CHIP with a suitable memory expansion card.


Check out the memory across the top of the screen using 2.04 on the Checkmate.



Sorry for the splash of images. Difficult to explain otherwise.

Trying to get the older kit now is pretty much impossible I would
say, so you are looking for a modern solution. The AGNUS is key
really and I would think 2MB is the barrier. Personally with a 6A
I would be looking maybe just to enhance my RAM via a side expansion
unit. I have never found the speed on the A500 a problem. I just
got use to how they function. But 2.04 is a must on a switcher.
CHIP is overrated.

[ Clear as mud ]

A500 Plus - It's all CHIP RAM.

The very splendid Amiga 500 Plus.

The ROM switcher switch.

1 MB of actual CHIP RAM.

The ROM switcher flicking between 2.4 and 1.3.

ECS Denise and the 8375 2MB Agnus.

Here is an 8375 in another A-500 Plus.

The 2.0 ROM signals use of the 2.04 WB.

Note that memory is all graphics mem.

ECS AGNUS 2MB and ECS Denise.

2Mb of CHIP RAM.

Over to Workbench 1.3 on the same machine.

Note the memory at the head of the Workbench.

Only 1.7 MB this time .. interesting.

New RAM boards for the Plus .

1 MB CHIP RAM from the trapdoor.

The original 8371 Agnus 512K.

The Rev 6A 1MB 8372A.

The 2MB MegaCHIP on the Checkmate.

An A500 motherboard with graphics and other memory.

With the MegaCHIP giving 1.5MB CHIP RAM.

GARY adaptor to use trapdoor RAM as CHIP RAM.

RAM on an A500 with the GARY mod.

Seven of my eight Amiga 500 Plus machines
running the health check on a black and
white monitor. All graphics memory only
of varying sizes.

Same goes for the A600 with just
the graphics memory showing.

This A600 has a 1MB trapdoor memory card
adding to the 1MB onboard giving 2MB CHIP.

One of the benefits of the A600 was
the internal hard drive facility.

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Last updated 27th December 2023

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