Help to identify this A500 mood.

[Zetr0]

This is not a chip ram mod, this is an additional 512KB as OTHER memory, I suspect that this wires go to a switch.

here is the method of adding 1MP chip to an 8371 Agnus equipped A500




Interestingly JP7A and JP2 are still configured in a default mode - such that Agnus cannot address A19 as CHIP RAM and JP7A (exram) is still configured on the path.  

I have noted that that there is pin lifted from GARY (pin 35) this goes off to as switch I suspect to toggle it either HIGH or LOW (ground) - this will assert the memory ON as other mem and respectively OFF.

[Zetr0]

@arttu80

Actually the 8371 Agnus can address 1MB of CHIP RAM, if you note the image I posted it explains how this is done on a Rev6a A500.

8372A/B is often cited as a 2MB CHIP Agnus, sadly there are not enough Address lines on the A500 Chip Path - you would need to add Address 20 and 21 and mux these signals with EXRAM and A22 A23 A24 on the GARY adaptor.

Its a lot of work but it is possible.

Sadly the 8375 Agnus is pin-incompatible and would require a socket replacement to work on any 8370 / 71 and 72 Agnus.

[Zetr0]

Ah, Zetr0 was faster than me. Good quality response, Zetr0! Still not sure how you can get 1 Mb chipRAM from 8371 Agnus...

LOL, seems we are posting at the same time.

In a nut shell the modification I have posted goes a little something like this.

JP2 is tied to A19 on the 8371 - with this set to in its default configuration this sets A19 to address only 512KB of CHIP RAM, with it cut and shifted this will allow addressing of a further 512KB of RAM as CHIP memory - up to 1MB of the lovely stuff =)


JP7A is the EXRAM signal and this is tied to GARY, if you are installing memory on the motherboard you don't need to cut this signal, however the trapdoor expansion is not going to work.


With the mod descriptive above you can use an A501 as an addition 512K of CHIP RAM.

[Zetr0]

What's the circuit board down near the trapdoor port, then?  Perhaps OP could try to lift up on it, or post a pic of the bottom?

This small PCB is a factory installed unit that was replaced later by the addition of U35: 74F244 and U32: 74F139 on the Rev 6a motherboard.

The two chips arbitrate amongst most BCASU 1 BCASL 1 and mux (multiplex) signals from RAS1, RAS0, BCASL0 and BACSU 0.

[Zetr0]

Hmm... Very interesting. Not meaning to steal this thread with this topic, but I always was in belief that 8371 Agnus is 512k OCS chip. Actually even SysInfo will support this statement. Ofcourse I believe your words, seems you know little more about HW than I, but still little confused. I actually always replace 8371 with newer when possible (A2k and 500s). Is it possible, that PAL machines are different to NTSC in Agnus chips? I deal with PAL stuff only...

I gots to reading..... .....  a lot =)


an OCS chip set is just that, it comprises of DENISE, and 8370 Agnus.

an ECS chip set seen on the A500 is a Super Denise and an 8375 Agnus, this is also an A600 as well.

Now this will bake your noodle a bit, what makes the chip set OCS or ECS is in fact the Denise.  If you have a Super Denise installed you essentially have an Enhanced Chipset, as you gain extra screen modes.

Agnus is just an (Address Generator Unit) - depending on what you have 8370, 8371, 72, 74 will determine the address it can generate, it also handles some aspects of video in regards to PAL / NTSC.

the 8370 can address 512KB of CHIP
the 8371 can address 1MB of CHIP
the 8372 can address 2MB of CHIP*
the 8375 can address 2MB of CHIP*

*I should mention that there are some revisions of 8372 and 8375 that can only address 1MB of CHIP - most notably a particular revision in the early rev5.3 A2000's

Sadly I don't think Commodore had any idea when it came to name convention and there are some crazy revisions within each iteration - its quite difficult to get a right answer as well as very easy to be complete confused.

Alexh, I believe a member here, he probably knows more about Agnii than anyone outside the designers at commodore to be honest

[Zetr0]

Just found this Agnus resource thread started on EAB

(click here)

In that we go into the revisions and pin outs of various flavours of Agnii, again, Alexh's contribution is invaluable

[Zetr0]

@CSXLAB

Personally I would modify the motherboard for a full 1MB of CHIP RAM.


What is CHIP RAM?
CHIP RAM means what is says, all the co-processing CHIP's ( Denise, Paula, Garry, CIA's and Blitter) can access this pool of RAM.

To do any native graphics on an Amiga, or native sound, all of this data has to go through CHIP RAM.  So the more CHIP RAM you have the better!

CHIP RAM is very precious on any Amiga, and most programs will make use of it, adding enhanced sound or features.


What is OTHERMEM ?
Now, OtherMem is just that - this is memory that cannot be directly used by the Amiga CHIP set.  OtherMem is short for Other Memory, this can be FAST MEMORY, which can only be used by the CPU or something that is known as SLOW (FAST) Memory.

The latter means that the memory is located on the motherboard and the CPU has to wait until its turn to fetch / put data in this memory area.  Back in the day this was a cheap way of adding memory to an Amiga, while it didn't improve processing performance it dose provide more RAM for the CPU to use.


Modifying you A500 for 1MB of CHIP RAM
So in your case I would remove the wires and put the GARY pin back in the socket.

I would then using the image I posted earlier, cut the track of JP2 (pins 2 and 3) and solder a bridge between pins 1 and 2.

I would then cut the trace between pins 2 and 3 of JP7A and thats it... you will then have 1MB of CHIP RAM.


CPU Accelerators and CPU FAST RAM
Once you get your accelerator or additional CPU Memory upgrade, this will appear as Other memory (FAST RAM).  FAST RAM alone will improve the performance of your CPU by nearly 33%.  Obviously an Accelerator will drastically improve performance considerably.

For example, an A600 is very much like an A500+, more often than not it is referred to as an SMT version of the A500+ with IDE.

Like an A500 the A600 can output about 0.54 MIPS of processing power, however with a simple ACA620 - this give you 3.3MIPS of processing power - about 600% increase!!

also the ACA620 will also provide up to 10.5MB of FAST RAM as well as a MAP ROM feature which I find is a most delicious cherry on the cake.


Sadly I do know the specification of the ACA500, my hope is that it includes a CPU and with some FAST RAM (and an IDE solution) this will be a MUST HAVE card for A500 / A500+ owners.... and to be fair even A2000 / A1000 owners too.

A500 RAM upgrade
Currently there are a couple of options available to use,  Kipper2k on http://www.amibay.com makes and sells 8MB FAST RAM adapters for approximately £40 - these are auto-config (which means plug and play) and it will boost your CPU performance by about 33%.  It will also give you lots of ram and you can create RAD disk's and store data to be buffered to CHIP ram.


I hope all this helps, to the non technical this stuff can get quite hairy quite quickly.