MEMORY PAGING IN LOW 16kB

[Barmaley_m]

Of course it is possible, and it was done in many ZX clones since 1990! For example, such ZX clones as "Robic" (made at Selto-Rotor factory, Cherkassy, Ukraine), or "Orel BK-08" (made at DMZ factory, Dnepropetrovsk) all had a possibility to activate RAM in lower 16K of memory. All later popular russian ZX clones, including "Profi", "ATM-Turbo", possibly "KAY-256" and "Scorpion ZS-256" also have a possibility to activate RAM in lower 16K. Such ZX clones were therefore also able to run CP/M.

Unfortunately, there is no common standard for such "shadow RAM" system. The best way to design modern software is to make some hardware abstraction layer and let the people develop driver plugins with entry points to activate or deactivate lower 16K RAM. Some systems also had multiple RAM pages which could be paged into the lower 16K of Z80 address space. Or, in the simpler case, one could have just 4 values to type in the program configuration, such as:

Out(port1),val1 activates shadow RAM
Out(port2),val2 deactivates shadow RAM

One good example of system with hardware abstraction layer is IS-DOS which, under certain circumstances, uses shadow RAM and lets the user configure the above mentioned 4 values.

When the shadow RAM is activated, TR-DOS is usually disabled and does not activate its ROM or ports when code at location 3D00-3DFF is executed. This arrangement allows for proper functioning of CP/M.

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[VELESOFT]

This is ROMCS connection on real ZX Spectrum/Timex/Didaktik computers:

This connection enable connect external memory.



And this is russian connection on Pentagon 1024SL:


Is impossible connect to this ZX-BUS connector any old ZX interface with external memory. This interfaces must be modified for work on russian clones. (can connect external memory only if in low 16kB is set rom - external interface set signal /RDROM to log.1).

---------- Post added at 12:07 ---------- Previous post was at 12:05 ----------

Exist any russian interface with external memory mapped on rom area ?