Getting more than 1MB chip mem from my A500

[FabFr]

Hello,

I am trying to find a way to get more than 1MB chip memory on my A500.

My setup is the following : (I think I'll take some pictures during the week)

Motherboard 6A, soldered sockets with 512Kb on the free places.
A501 Expansion, replaced battery
68010 CPU
3.1 ROM
CFIDE68k mod with 4GB WB3.1 Bootable card
SupraRAM 500RX fully populated with 8MB Fast
Denise replaced by SuperDenise

Some WHDLoad games fail to start, because the lack of Chip memory.
I did the JP2 / JP7A mods, but it will only allocate 1MB chip mem and not 1.5MB.
I got my hands on a 2MB 8375 Agnus, but after installing it, it won't bootup.
I heard that the pin layout is different on this one, and except doing a very tricky modding it will not work.

The other solution I looked for was to get a DKB MegAChip, unfortunately it seems to be a rare pearl too, so I wanted your advices to know which solution should be possible to get more than 1MB chip memory?

Thanks in advance!!

Fab

 

[Zetr0]

Hello Fab,

Lets see if I can help.

A500 and the elusive 2MB CHIP
The A500 in its natural state will only allocate 512KB of CHIP RAM (8370 Agnus), all other RAM that is attached to the trap door is seen as OTHER RAM, some classify this as Ranger RAM.

Agnus Evolution - 8370 to 8371
Through the development of the A500, Commodore updated the Agnus (8371) to be able to allocate 1MB of CHIP RAM, however this was not enabled on the revisions of A500 motherboards but Commodore did provide provisions to enable it for the end user.

The easiest of these is the Rev6A A500, however with a little more effort the earlier revisions can be modified also.

The Lonely 8375 Agnus
The 8375 Agnus was developed for the later version of the A500+ and sadly is not pin compatible with other versions of Amiga. Well not without creating an interface for the Agnus.

The 8372AB and 8372B Agnus
However you could use an 8372AB or 8372B Agnus (with a little additional routing and multiplexing) to achieve up to 2MB of addressable CHIP RAM, this does require some more serious modifications to the motherboard and a 74LS32 logic chip to multiplex between memory bank address locations.

Sadly the 8372AB and 8372B Agnus are only found in A3000's and because of which are quite converted and or prized in repairing damaged A3000's, however I have heard that some Chinese manufactures do have a cache's of these, but your search and results may vary.

In a nutshell there is a limited options for getting 2MB of CHIP RAM on the A500, options really as follows

1. Replace the A500 motherboard with an A500+
2. Modify the A500 with an 8372AB or 8372B Agnus, 74LS32 + RAM
3. Create a 2MB CHIP Adaptor based on the 8375 Agnus

Option 1 the downside is that this may break compatibility for games / software written to exploit the 512KB 8370 Agnus, but that is very few early games IMHO.

Option 2, is a lot of work and will require some technical knowledge of electronics and logic gates - not to mention some inner working knowledge of the Amiga.

Option 3 is not that difficult and if you have the skills to achieve Option 2, then in my opinion then this is what I would do instead - however it does rely on an 84 pin PLCC plug-in component that is quite hard to source.

 

[kerkyra2007]

I need your help regarding this matter also .

My hardware A500+ motherboard revision 8A.1 in a case of A500.
Has already fitted a memory A502/16 with a Gary adaptor attached like the photos below :

But unfortunately this is what I get with ACA 500 enabled.

Any help????

 

[Zetr0]

@kerkyra2007

Γεια σου φίλε μου,

Lets see if I can help.

Now the A500 comes equipped with the 8375 Agnus that can address up to 2MB of RAM in a passive manner as the address arbitration is different compared to the original A500 and A2000 systems.

You do not need a Gary Adaptor to add a further 1MB of CHIP RAM, in fact all you need is 8x (4bit by 256kbit) memory chips as they use a different RAS / CAS to access them.

Using the A502/16 adds 2MB (16x 256k*4bit) of OTHER MEMORY to the system at a higher address [ multiplexing A20 from the motherboard, A21 and A22 from GARY ] - this means you only have 1MB of CHIP RAM and 2MB of OTHER (SLOW FAST).

Unfortunately due to the nature of the card's method of adding memory to the RANGER ADDRESS SPACE, I don't think its modifiable to split into 4 banks (each bank is 256k * 16 bits ) using the lower two banks as CHIP RAM. If I knew more of the nature of the cards method of multiplexing I could offer a possible modification but it would be a stab-in-the-dark at best and would only really yield results through trial and error.

For the moment, I would try the following - remove the modification to the board and the Gary Adaptor - unless there is a locked logic level this may add an additional 1MB of RAM to the CHIP RAM path giving you 2MB of CHIP (but you will lose the use of the additional 1MB in OTHER).

While using additional memory on the CHIP PATH is a cheap way to add additional RAM, unless its been developed specifically for the A500+ it will be both much easier and cheaper to get a CPU socketed FAST RAM card - like Kipper2k's 8MB FAST RAM CARD.

Unlike SLOW CHIP RAM, CPU based RAM doesn't have to wait for the custom chips, and hence will improve the processing speed of the system by 30%.



In the case you have here, I would look to source a 1MB CHIP RAM upgrade (www.vesalia.de had some a little while ago) and look at buying Kippers 8MB FAST and IDE combo adaptor alongside Kickstart 3.1

This will give you an all in one delight of 2MB CHIP, 8MB FAST and CF IDE action for lots of fun and games.

 

[roy_bates]

i think it would be best to have a full picture of that motherboard as some logic is missing.

let me explain.
the motherboard might be a revision 8 but its set up to be used as a 500 not the plus.

you will have to modify the motherboard to 500+ spec to increase the chipram.and then it WILL access 2mb of chip if installed.
this is relatively easy as the spaces are there for the missing parts.

the downside is you wont be able to use that card with the gary adaptor.

if your reading this zetro,you can see the missing part in the second picture.the part that should be there if it was a 500+ would be a 74f139

a full picture of the whole board would help.

 

[FabFr]

Thanks for reply.

I tried to look around for Megachip, MiniMegi and similar, but no luck.

My A500 is a Rev6A mobo coming with a 8372A Agnus (I did put some pics in my profile, can upload bigger ones if needed)

Howhever, if I switch to a A500+ mobo, will the CFIDE mod and the SupraRAM still work with it?

Thanks in advance!

 

[roy_bates]

Thanks for reply.

I tried to look around for Megachip, MiniMegi and similar, but no luck.

My A500 is a Rev6A mobo coming with a 8372A Agnus (I did put some pics in my profile, can upload bigger ones if needed)

Howhever, if I switch to a A500+ mobo, will the CFIDE mod and the SupraRAM still work with it?

Thanks in advance!

yes, it will still work.

 

[Zetr0]

Roy does raise a very interesting point,

Some A500+ Rev8A motherboards have a 74F139 in U32 where as other do not, this is because U32 is only used for 512K systems and is bypassed by JP4A and JP4B - hence why most A500's 512K A500's would be installed with them, where the 1MB A500+ is not.

 

[Zetr0]

@FabFr

The 8372A Agnus can only address up to 1MB of CHIP RAM I am afraid to say. As Roy has pointed out the A500+ is compatible with CFIDE mod and SupraRAM

 

[demolition]

Megachip and similar expansions are not very common, so it is usually easier and cheaper to just get a 500+ MB with a 1MB trapdoor expansion to get the 2MB chip mem (hope it is not price driving to write this?).

I have kipper2k's 8MB RAM with CF installed in my A500+ with 2MB chipmem, so that's the solution I would recommend. And yes, any RAM expansions using Gary adapters will not work in a 500+ AFAIK.

 

[FabFr]

Thanks for your advices!

Yes, I definitely think I'll opt for getting an A500+ with 1mb expansion. Other solutions may be kinda difficult to find, or risky.

I'll hunt for A500+, will keep you informed when I'll get my hands on it.

Thanks again

 

[demolition]

If you have trouble finding the 1MB expansion, it should also be possible to use the very common A500 512kB trapdoor expansions in the 500+ and get 1.5 MB chip. It might be good enough until you find the real deal.

 

[kerkyra2007]

Greetings my friends.
@zetro is nice greeting you in your mother lanuguage
Let me explain : my quest started when I purchuased an Amiga 500 rev6a to play with the new ACA500. This board with an expansion of 512KB amd the hack for 1MB CHIP RAM worked verey good with the ACA and the only "problem" was loading whdload games (Eventually I will have to make another thread).
Trying I came to the idea that with a little more fast ram things would be better and so I have this motherboard 8A.1 which has the following
Agnus Chip : 328069-10 8375
The gary adapter connects to
jp2 blue , jp8r green , jp8l yellow and white to the expansion.
U32 is housing no chip
U34 is 74F373pc
U35 is 74f244n
This is the missing part of the photo

Now since Amiga is seeing 1MB chip and 2 MB slow fast memory why when with the ACA500 can see only the 1MB Chip and only 512 extra trapdoor memory and specially when whdload loads nothing happens and the system is almost crashed (CLI/shell is working)?
I tried to take away the gary adapter but I do not have neither the tools or the skills to remove the modes on the motherboard and of course is not booting.
A question more : is there a possibility that A502/16 do not have 2MB onboard? On the memory chips writes : zmd u61256d c08 9142
Why I came to this idea : When whdload is trying to load a game the Chip ram comes to zero and is dissapearing the 512 trapdoor memory. So if the system is trying to allocate something that do not exist is hanging.
Thank you in advance for your further help
Yiannis

 

[demolition]

Using a Rev 8a MB I don't think you use any fast mem with the ACA500, so you will get 2MB chip + 2MB fastmem in total.
Using a Rev 6a you can get 0.5MB chip and 3.8MB fastmem or possibly 1MB chip and 3.5MB fastmem.
This is just based off the specs of the ACA500, so I can't be sure how it works.

The specs says it will work with existing trapdoor expansions up to 1.8MB fast.

 

[kerkyra2007]

The motherboard has 512kb chip and with this board with the gary adapter gives 1MB Chip and 1,5 MB Slow fast ram.
With the ACA500 gives 1 Mb chip and 1.8 MB fast.
Is this the limitation you are referring?

Update 17/01
Unfortunately it seems that I was framed because this memory expansion actually has only 512 KB on board.
I put it on the rev6a A500 and the amiga is reading only 512 KB
What was the funny thing : I put the old unamed 512 KB memory expansion on the rev8A.1 A500 and still reads 1 Mb chip and 1.5 MB "Fast" ram.
So just the modding of the mobo with the gary adapter is giving the extra memory and not the board.
Thank you all for your help