On modern FPGA/CPLD based russian ZX clones is possible without hardware modification
implement ram memory paging also in low 16kB as rom replacement. This memory extensions
is often used on some interfaces for ZX Spectrum computers.
Can replace low 16kB rom with battery backup sram 512kB used as 32 memory pages.
On paging port is possible enable or disable writing to low 16kB (software write protect for
best protection/rom emulation)
paging port MB02+ (#17)
D0-D4 = memory page number 0-31 (sram or eprom)
D5 = write enable (1=write enable / 0=write protect)
D6 = sram enable (1=disable ZX rom and connect MB02 sram)
D7 = eprom enable (1=disable ZX rom and connect MB02 eprom)
(if D6=0 and D7=0 then MB02 memory is disabled)
(if D6=1 of D7=1 then MB02 sram or eprom is connect)
(if D6=1 + D7=1 then paging port is set to %10000000 (erom page 0+write disable)
After press NMI must CPU jump always to adress #66 in low 16kB (if MB02 memory is connect
then NMI jump to MB02 memory)
Some siminar ZX interfaces use also low 16kB memory paging with selectable writeprotect.
You need only external sram (eprom contain olny boot rom for loading DOS from hdd/fdd)
Also DIVIDE PLUS interface use MB02+ compatible memory. Or my MB02 memory interface for ZX:
If this (or any similar) memory paging will possible on russian ZX clone, then will possible use
system RESIDOS with full FAT16 support, BS-DOS system from MB02+ and more other software
(theoretically also FATWARE and DEMFIR systems form DIVIDE interface). Actually is ram in low
16kB used in ZX clones as CACHE sram in size 16 or 32kB. This is very small for implement any
big system and absent software writeprotect feature.
Is possible on russian ZX clones (Pentagon 1024/ATM turbo) ignore trdos paging and connect
external memory to low 16kB ?
- external interface connected to ZX-BUS disable internal roms in ZX clones
- low 16kB is replaced with external sram memory
Is it possible ? If on port 7FFD is set 48rom + is connect external sram in low 16kB + cpu jump
to adress #3Dxx ...... Will TRDOS connect ??