How to do internal RAM mod for 8-16K area ?

[Bean]

I have the 62256 32K RAM chip and I have found schematics for 16-32K and 48-64K areas.
I would like to have RAM from 8K thru 32K so that the 8-16K area doesn't get erased when resetting. (I will do the battery backed option).
I believe the ROM repeats in the 8-16K area, so do I need to pull "ROM CS" high during access to 8-16K area ?
I was thinking I would use A13 NOR A14 and feed that to "RAM CS" then invert that and feed it to "ROM CS". Does that sound like it would work ?
Do I have to factor in A15 in any way ?

Thanks for any help you can provide,
Bean

[mrtinb]

This circuit gives you 64k battery backuped memory.

The 8k ROM can be replaced with RAM with a switch.

https://web.archive.org/web/20210224003 ... mories.htm

[1024MAK]

Yes, to use the 8K to 16K area, your circuitry has to prevent the ROM from being activated. In an unexpanded ZX81/TS1000 there is actually no part of the memory map where there is no memory… This is because of incomplete address decoding.

The /ROMCS line has to be be taken logic high on the ROM side of resistor R28 to disable the ROM.

Have you read this topic?

Also, because of the way that the system gets the video data from RAM and the pixel data from ROM, you should also read this description of how it works.

Mark

[Paul]

I would like to have RAM from 8K thru 32K so that the 8-16K area doesn't get erased when resetting. (I will do the battery backed option).
I believe the ROM repeats in the 8-16K area, so do I need to pull "ROM CS" high during access to 8-16K area ?

Yes you have to pull ROMCS high when RAM in 8-16K is activated.
If you have a 32k RAM (62256) the you can have RAM in 8-40K.
I have a schematic for this that I can post tomorrow.
I'm not at home at the moment.

[Paul]

I will do the battery backed option

RAM in 8-16K is always reset resident.
Battery is only necessary if you want to preserve the content when power is switched off.

[Bean]

Thanks for all the info.
I will give it a try in the next couple days.

I am clear now on the ROMCS.
So I don't have to do anything with A15 if I only want 8K to 32K ?

Thanks again,
Bean

[1024MAK]

If A15 is not included in the decoding, but A13 is, the ROM will appear in the Z80 memory map in two places: 0x0000 to 0x1FFF and 0x8000 to 0x9FFF.

If you include both A13 and A15, the ROM will appear in the Z80 memory map only one place: 0x0000 to 0x1FFF.

For the RAM, you can use full address decoding for 0x2000 to 0x3FFF. But for the area 0x4000 to 0x7FFF, for the video system to work, the Z80 must be able to execute the very same RAM when the address bus is in the range 0xC000 to 0xFFFF. Meaning either A15 is not considered in the address decoding for these ranges or your circuitry includes the /M1 signal from the Z80 (sometimes known as “M1 NOT”).

Mark