WIP: internal 512K expansion

[eto]

Update 464/664 version:

Since I don't want to have a version that does not support C3 but I also want to provide 576K at least, I finally went for a different approach which supports two SRAMs. This one will offer the option to add a third PAL. The first two PALs provide the logic for a standard 512K expansion without C3 support and the second PAL will then provide C3 support if a second SRAM is installed. (As for C3 shadowing is required, the secondary SRAM will provide 64K less of additional RAM than its total size.)

This will give us (hopefully) 3 different options with the same PCB and the same GALs:

• 576K (64 base + 512) of total memory without C3 support: 2 GALs, one SRAM (512K) 
• 1024KB (64 base + 512 + 448) of total memory incl. C3 support: 3 GALs, two SRAMs (512K)
• (probably) as a third option: 640K (64 base + 512 + 64) of total memory incl. C3 support: 3 GALs, two SRAMs (512K and 128K) 

The PCBs have just been ordered - more news will follow in 2-3 weeks after they arrived and have been assembled.

[Egg Master]

Very nice work. What I find interesting are:
- 512KB + 64KB (with C3 for 100% CPC usage)
- 512KB + 512KB (without C3 for SymbOS and above).

Both configurations can be set on the same expansion.

Perhaps it was possible to use a dedicated 64K SRAM chip for C3 so that it would never be disabled, regardless of the memory configuration.
It cost around 0,62€ each, including shipping, that is very interesting for this project: https://fr.aliexpress.com/item/1005006752389377.html

[eto]

- 512KB + 64KB (with C3 for 100% CPC usage)

What benefit would that have? A640K solution includes the 576K solution without any drawback. Most software will anyway only recognize 576K as they don't support more than 512K expansion RAM and SymbOS is perfectly happy with 640K. In both cases you need two SRAMs, 64K will not be cheaper than 128K and even worse - we would need a different PCB just for a 576K solution as the footprint of the SRAMs is different. 

- 512KB + 512KB (without C3 for SymbOS and above).

I implemented the 512K without C3 as there is software out there which needs the full 512K expansion (Mighty Steel Fighter) and would suffer if I implement C3 with shadowing (thus reducing the amount of memory by 64K). 

However that's no longer the case for 1024K. There's no software that is limited by "just" 1024K instead of 1088K. For SymbOS it doesn't matter at all as it can address a total of 1024K, so even if I would implement 512+512 without C3 you would "only" see 1024K of memory in SymbOS. 

[Egg Master]

I'm not talking about the base 64K RAM, because it's not on your expansion.
Obviously, it's in addition to the "512K + 64K" (640K) or "512K + 512K" (1088K).

A 128K SRAM IC costs twice as much as 64K IC and is also twice as large.
With some redesign, it can fit better with GAL ICs, and not affect the capacity.

[eto]

I'm not talking about the base 64K RAM, because it's not on your expansion.
Obviously, it's in addition to the "512K + 64K" (640K) or "512K + 512K" (1088K).

The 464 has 64K base ram. The iRAM adds 1024K (64 base + 2x 512) however from the second SRAM 64K are used for C3 emulation, thus reducing the available RAM of the second SRAM to 448K. This will give you a total of 1024K of RAM: 64K base + 512K SRAM1 + 448K SRAM2. And a total of 1024K is exactly the amount of RAM that SymbOS can address. So there is no benefit of having a solution without C3 as even with a full 1088K you would only get 1024K in SymbOS.

A 128K SRAM IC costs twice as much as 64K IC and is also twice as large.

Which one do you have in mind? I can't find an available parallel 64K SRAM. I can find 32K SRAMs but then you need two of them.

[sir_lucas]

Fantasic job!!  Keep us posted and thank you.

[Egg Master]

Which one do you have in mind? I can't find an available parallel 64K SRAM. I can find 32K SRAMs but then you need two of them.

I think you're skimming my posts.

In fact, I included a link to the 64K (65,536 words by 8 bits) SRAM IC into the message you're replying to.
You can find it at many place like here (10 pcs): https://fr.aliexpress.com/item/1005006752389377.html



This will permanently fix the SRAM capacity issue by dedicating it exclusively for the C3 functionality.

[eto]

I think you're skimming my posts.

Sorry, indeed I have missed that link. 

This will permanently fix the SRAM capacity issue by dedicating it exclusively for the C3 functionality.

I can not officially support this IC as one of the premises I made for the development of the expansion is that all components are still easily available (in large quantities) from reliable resources - if possible still manufactured. I don't think that's the case for this particular IC. It's of course fine to buy the stuff from AliExpress and especially when building prototypes I myself order at AliExpress to limit development costs but I would not base a design on an IC that I can only source from there. 

However I checked the pinout of this IC and it will be possible to use it if you want. You only need to make a tiny adapter e.g. with a strip board so that the UM61512 fits into the footprint of the second SRAM. Either an adapted logic for the third GAL or a tiny mod on the PCB are required and you are good to go. 

[Egg Master]

I have linked Aliexpress, because it is easy to order, but UTSource have close to 20,000 pcs available.
Of course, an adapter can do the job, but that wasn't the idea I originally set out.

Anyway, thank you for your answers.