What is a 48k with 128k RAM?

[1bvl109]

An oxymoron of course.

But apart from this, as I have a 48k in a keyboard case with too much space viewtopic.php?f=22&t=1723 and I have to solder around on that thing anyway the thought crossed my mind, what could be done else. Inspired by this mem viewtopic.php?f=24&t=739&start=10#p10540 I looked up https://www.worldofspectrum.org/faq/ref ... erence.htm I wonder what 128k software would run, if I provided some memory accessible by bank switching through bits 0-2 on 0x7ffd in a 128k compatible way. Let's say I'm ok with no sound, no joystick ports and the 48k ROM for the moment.

What worries me most is this

"Bit 3: Select normal (0) or shadow (1) screen to be displayed. The normal screen is in bank 5, whilst the shadow screen is in bank 7. Note that this does not affect the memory between 0x4000 and 0x7fff, which is always bank 5."

While the other bank switching stuff should be implementable on top of the 48k motherboard, this seems to require a whole new design.

[1024MAK]

Converting a 48k to a 128k has been done
There is/was a page in the web describing it.

As to your questions, most games write to the sound chip assuming it will respond, so if it is not present, most games don’t know or care.
The joystick ports on the Amstrad machines are mapped to the same I/O port as keys 1 to 9 and 0. So that’s not a problem. And any external joystick interface that responds like a Sinclair Interface 2 (their joystick interface) will be fine. Obviously including the Interface 2

To get the whole 128k of RAM, all 16 existing DRAM chips have to be upgraded to 64K bit DRAM chips (or 256K bit DRAM chips).

Most 128K games don’t use the extra ROM(s) in the 128K machines.

Now, the shadow screen problem. Well, I’ve not investigated or read about how many games use this. So I don’t know how much this was used. I suspect not as much as you would expect given the limitations.

But if you do want to add this feature, it’s a case of adding the appropriate circuitry. This is just extra ‘address decoding’ that takes an input from the bank select latch and then alters the control signal from the ULA to select a different memory range in the contended RAM. The ULA does not know, or care which bank of contended RAM supplies the data. As it is all from the same DRAM chips.

Mark

[1bvl109]

Converting a 48k to a 128k has been done
There is/was a page in the web describing it.

The more I think about it, the more it seems out of my league. I'm struggling with the keyboard repair already. But thanks for your thoughts.

To get the whole 128k of RAM, all 16 existing DRAM chips have to be upgraded to 64K bit DRAM chips (or 256K bit DRAM chips).

I thought I could get away with only replacing the upper 32k.

Now, the shadow screen problem. Well, I’ve not investigated or read about how many games use this. So I don’t know how much this was used. I suspect not as much as you would expect given the limitations.

It seems useful for this gigascreen flicker stuff, giving a trival implementation for static images and you can change one image, while the other is displayed.

But if you do want to add this feature, it’s a case of adding the appropriate circuitry.

As is upgrading it to a ZX Evolution, I guess. "appropriate circuitry" indeed

This is just extra ‘address decoding’ that takes an input from the bank select latch and then alters the control signal from the ULA to select a different memory range in the contended RAM. The ULA does not know, or care which bank of contended RAM supplies the data. As it is all from the same DRAM chips.

It seems to make changes to the motherboard itself necessary instead of just deactivating upper RAM and adding some outside stuff.

[1024MAK]

Yes, lots of changes are required to the original PCB. Doing the upgrade is a big job.

If you have a look at the memory map and bank switching arrangements, you will see that half the banks are listed as contended, this is the clue that the actual memory chips that are used for this RAM are actually the 64k chips which also contain the screen data. The equivalent to the “lower” RAM in a 48K machine.

Mark

[Seven.FFF]

There’s a different number of Ts per line and lines per frame too. You’ll never get a really authentic 128K.