Internal 64K upgrade for the CPC464

[Zetr0]

Hello there fellow CPC'ers

I have been reviewing some of the collected motherboard schematics of the both the CPC464 and CPC6128, and noted that the onboard memory differs only slightly.

In fact it is so slight a difference I think that it would be possible to make a small PCB to add a further 64K of RAM.

Looking at the 6128 the upper 64K is used by MUX'ing A6 and A7 AND CAS1 from the GateArray.

Now, the Multiplexing of A6 and A7 can be achieved with a simple Logic IC, and tapping up the CAS1 signal from the Gate Array isn't a problem - so why - when it looks so simple is there no information on performing this modification - clearly I must of missed somthing, but I wonder what that might be.

[TFM]

Take a look here:

http://8bit.yarek.pl/upgrade/cpc.cpc4mb/index.html

In case you need a couple of bits more ;-)

[Bryce]

Have you decoded the GAL? Are you sure that's all that's being done?

Bryce.

[MacDeath]

easier on a 464Plus... 

[Zetr0]

Thanks for all the input,

Indeed, I had missed a slight trick, that J106 present on the 6128 that arbitrates the upper 64K of RAM, allowing the Video Chip to access it as well!

looks like I am on the hunt for PAL/HAL J106 from a CPC6128

Its about time I did some truth tables and PAL equations.... its been a while

Other than butchering a CPC6128, anyone got a spare I could borrow (I wont break it) =D

[fatbob]

Hi Zetro,

I have a 6128 you can borrow since I am busy playing with my 6128+ at the moment.

Small world isnt it 

[Bryce]

I don't think anyone has reverse-engineered the 6128 RAM PAL yet, so feel free to do it I'm sure many people would be happy if we could produce new ones.

Bryce.

[Zetr0]

Hey, I know that I am new here but can I report FatBob for being most AWESOME!?

@FatBob

*lol*  Indeed a very small world my friend, but you do seem to collect everything!!!  truly a man after my own heart!

I would love to take you up on your offer, please hit me up in PM and we can sort some postages ( I promise not to damage her) - if luck holds out.... I may be able to build a compatible GAL - thus a simple 64k upgrade for 464.

While it would be simple enough to add 64K as a pure CPU extension, the trick here is to allow the Video Chip to access it.

I have high hopes as they say =)


@Bryce

Please... wish me luck LOL

If I could ask for anything at the moment would you be able to recommend / link any 128k Memory testing programs and or any programs that MUST HAVE 128k to run.

[SyX]

While it would be simple enough to add 64K as a pure CPU extension, the trick here is to allow the Video Chip to access it.

Sorry, i didn't see this before, the expansion RAM is not used for the Video Chip , the important to be 6128 compatible is being able to use the special paging modes &C1-&C3.

Take a look to the end of this thread to get a better explanation from TFM, Johnny Olsen and me

If I could ask for anything at the moment would you be able to recommend / link any 128k Memory testing programs and or any programs that MUST HAVE 128k to run.

For software that make use of expansion ram, look here, here and here. For example, the wonderful Edge Grinder needs 128KBs to run

For memory testing, if you need an special program, only said what would you like to do the memory test program? But an easy and fast test from basic is to write a byte in &4000 (poke &4000,1), then enable an expansion ram page (for first expansion ram page OUT &7F00,&C4), poke another value in &4000 (poke &4000,255), disable the ram expansion that was paged (OUT &7F00,&C0) and read the byte in &4000 (PRINT PEEK(&4000)), if the value printed in screen is 255, the expansion ram wasn't paged; if appears an 1, it looks correct... of course you would need more test, but you can use a program of the previous lists for that

[Zetr0]

Thanks Syx!

soo much to learn =)