What type of Agnus chip do I have?

[Ral-Clan]

Hi, in going through my parts bin I found the following Amiga chip:

CSG 318069-17
8375

According to http://www.amigahardware.com, this is an 2MB Agnus chip for an A500/A2000: http://www.amiga-hardware.com/showhardware.cgi?HARDID=1448

But wait!  I thought the A500 / A2000 could not address 2MB of chip RAM without special 3rd party add-ons!

So is this a typo on the page?  Is this really a 1MB Agnus from the Enhanced Chip Set?

[Ral-Clan]

Are you sure about the A500+ (plus) being able to access 2MB Chip RAM out of the box?  Wikipedia's entry on that machine says it can only address 1MB.  So does every other site I've found that has info on the A500+.
http://en.wikipedia.org/wiki/Amiga_500_Plus

Confusingly, Wikipedia's entry for the Agnus chip indicates that the 2MB version of the Agnus was shipped in (the/some) A500+, but Wikipedia's own page on the A500+ states the computer itself could only address 1MB Chip RAM.

http://en.wikipedia.org/wiki/MOS_Technology_Agnus#Chips_by_capability

Such contradictory information!

Wikipedia's entry for the chip I have says the 318069-17 is a 1MB chip for a Rev 6 A500 or A2000, not 2MB like the other page - this conflicts with the info on amigahardware.com, which says it is a 2MB chip, so I am looking for a definitive answer.

[Ral-Clan]

I'm a little more confused now.  Wikipedia's entry on the A500 also says:

The Amiga 500 has a "trapdoor" slot on the underside for an upgrade of 512 kB of RAM. The extra RAM is classified as "fast" RAM, but is sometimes referred to as "slow" RAM since due to the design of the expansion bus it is actually on the chipset bus. Such upgrades usually include a battery-backed real-time clock. All versions of the A500 can have the additional RAM configured as Chip RAM by a simple hardware modification, which involves fitting a later model (8372A) Agnus chip. Likewise, all versions of the A500 can be upgraded to 2 MB Chip RAM by fitting the 8372B Agnus chip and adding additional memory.

Is it really this simple?  The whole reason I started this thread is that I was thinking of installing a DKB MegaChip that I have into my A500 to get 2MB chip RAM, but if the A500 can be upgraded to 2MB CHIP without extra hardware, then I'd rather save the DKB for another Amiga.

I've used Amigas for a couple of decades now and never knew that the A500 could "easily" be upgraded to 2MB Chip RAM this way.

[Ral-Clan]

Okay, NOT as easy as it seemed at first (a lot of hardware modifications required).

Probably easier to just install the DKB MegaChip into the A500.

Still, back to this extra Agnes chip I mentioned at the top post. Is it 1MB or 2MB? I'm guessing the former.

The reason I'd like to know for certain is that I'm taking the DKB out of an Amiga (A2000) I'm going to sell, and will need to install a "regular" 1MB Agnes into it before selling so the computer is fully functional - so if this extra Agnus I have is a 1MB version, I will put it in to the A2000 donor board.

[Ral-Clan]

I installed the Mini Megi Chip (NTSC) Memory Expansion from AmigaKit which STILL only gives non + machines 1.5MB chip ram. I then did the mod to direct the 512KB ram in the trap door to chip which then gave me a full 2MB chip ram. Of course, then you have no fast ram unless you add it internally from an expansion board or from the Zorro II connector, which is why I have an A590.

Oh Crap!  I hadn't thought of that.  I'm so used to operating an A2000, which has 1MB on the motherboard that I never thought about the fact that the A500 only has 512K on the motherboard, and the rest is in the trapdoor slot.

This is what I'm going to put the DKB Megachip into: an A500 ECS modified for WB2.1 ROMs with 512K on the motherboard, 512K in the trapdoor slot and another 2MB RAM on an sidecar hard drive (Supra 500XP).  Total of 3MB RAM.

So, does anyone know if the DKB Mega-Chip can automatically pick out 2MB from all that scattered RAM, or will I need to conduct some motherboard mods to get "see" and use a full 2MB of chip RAM?

Perhaps the "mod" you mention in the quote above is the one that I've already done to get the A500 to use the ECS chipset (this Amiga already "sees" 1MB of ChipRAM with the Fat Agnus)

[Ral-Clan]

The MegaChip doesn't need to "pick out" its RAM. It uses your existing Chip RAM and has 1MB onboard to give you the full 2MB.

Ah, I thought the MegaChip merely allowed the Fatter Agnus onboard its riser to address more memory on the A500.  I didn't realise it actually had 1MB onboard.  I took a good look at the hi-res photo of the MegaChip on the BBOAH and I can see a Hitachi chip on it labelled "HM514800JP8".

A datasheet search says that this is:

80ns V(cc): -1 to +7V 50mA 1W 524,288-word x 8-bit dynamic random access memory

I'm assuming that Greek stuff about the word x 8-bit means it is a 1MB RAM chip? I wonder if this 1MB of RAM on the DKB contributes to the 8MB limit the Amiga can address (i.e. I wonder if when using the MegaChip with its 1MB, and with the additional 512K+512K of the A500 and A501, I would have to limit the Supra 500XP to 6MB of RAM to avoid going over the 8MB total).

Perhaps this Mini Medi chip being mentioned doesn't have any on-board RAM like the DKB does?