thanks for your questions and I think I have to add this in documentation. So a section for programmers is needed.
I will answer it from last to first - is more easy for me.
Register $50
This is a special feature requested from developers of the german forum and as there was space in the CPLD I realized this. When ZXblast is attached to a standard ZX81 via expansion connector, /RAMCS is pulled high which disables all internal RAM. There maybe some hardware supplying own RAM (extension cards) which can not be switched off. To avoid collisions, ZXblast can be told to not serve a specific address area ($2000-$3FFF, $8000-$BFFF, $C000-$FFFF). If the corresponding bit is set, ZXblast does not react on read/write in these address areas - as long as RAM is chosen in general for the address area (see register $20, ROM/RAM selection to address area). If ROM is selected in one of these address areas, the ROM will NOT be switched off.
And by the way, there are some 19 inch ZX81 systems existing with a bus and several slots for hardware modules where this feature could be useful. These are named ZX96, ZX97 and maybe ZX2000 (not 100% sure, I don't own any of these systems.
Register $40 (page window address)
Here you can move the page area/window to any required location through memory. Choosing $50 will select the page to $8000-$9FFF if a 8k page size is used. If the page size is set to 16k, the page starts at $8000 and ends at $BFFF. You can not start a 16k page to address $A000-$DFFF for example as 16k pages begin on boundaries only (A13 is not used in this case).
And I did a small change for better programming
bit 4 is used as high bit of page number and bit 5-7 is used for page window address with following result:
$40=page window address
$00=$0000
$20=$2000
$40=$4000
$60=$6000
$80=$8000
$A0=$A000
$C0=$C000
$E0=$E000
bit 4=high bit of page select register to use page 16-31
Register $30 (page select)
$00-$F0 selects one of 16 pages while the other 16 pages are selected with setting bit 4 of register $40. The reason for this are restrictions to available address lines and macro cells in CPLD, requesting some compromises. So you have 32 pages all in all and have a memory hole when using 8k pages only (every second page is used). The 8k window size was a feature to have more fix memory available in your instance, only 8k pageable memory.
And the pages are used from bottom till down, page 0 is the highest page (instance 7, $C000-$FFFF) while page 29-32 would reference the ZXblast control instance - be carefully using this memory area. This has the advantage, that you can mix paging and multiple instances on you request. If you need only 256k memory, than only instances 4-7 memory is used for paging while you could use instance 1-3 for user programs. This mode is quite flexible.
page 0 = $7C000-$7FFFF
page 1 = $78000-$7BFFF
page 2 = $74000-$77FFF
page 3 = $70000-$73FFF
page 4 = $6C000-$6FFFF
... and so on
In fact this is not the true address in RAM as all page bits are inverted physically - but this is another story. From developers view you can use this model. And it is quite flexible as you could use the pages used or better blocked from the ROM of all other instances as well. So using page 3,7,11,15,19,23,27 you can have 7 instances with full memory and 7x16k pages as well. But need maybe more logic in your program.
Last but not least - all the features and memory access is available in parallel to the video display. The RAM is not mirrored (unless you set bit 5 of control register $10) and during video display the memory address of $4000-$7FFF is forced automatically even if the ZX81 does address the expected mirror at $C000-$FFFF. This allows to use this area for paging area as long as you do not force to execute code to this area. The extended M1NOT from the ZXmore (executable code anywhere in memory) is not available here. This might be realized in future, I think. Well - we will see.
Setting the mirror bit in register $10 is for compatibility mode only to specific programs requiring the (memory wasting) mirror technique.
So hope it is getting clearer now. Will draw some pictures for the manual for easier understanding.