bit 0 SET – selects upper 32k block
RESET – selects lower 32K block
(not operative :f bit 7 is RESET)
bit 3) selects a4k page
brt 4) (bit 5 not used in 32k mode?
Dit & SET – select upper 32k of page 0 as permanent
RESET – select lower 32k of page 0 as permanent
(rot operative 2f bit 7 18 RESET)
bit 7 SET – select 32k mode
RESET – nelect 64% mode
There are 3 modes of operation:
1) ak paging – the memory is set up in 44k blocks which means
that to change pages, a program aust ensure that the Program
Counter holds the value of the next snstruction on the incoming
page after the U FE xX 12 executed. Not the easiest of things.
2) 32K paging – in this mode each page is divided into two 32k
pages and the user can then assign either the upper or lower 32k
© page 0 as permanent and then page in any other 32k half page.
This makes the controlling prageam much simpler and would be
very “us ful” for applications where large data files are
manipulated by small programs.
5) memory-mapping – thie is the mast powerful mode af ald and it
allows any 4k black from the total RAM to be ‘mapped’ into any
ak slot 1m the 200 menory map. Ihe ‘source’ block can be” mapped
into more than one “target” block.
The switching from page to page on the RAM cara can become:
very confusing and a memory management program would be very
Useful. This 1% exactly what M.A P.-80 have done. If you buy a
RAM board and send them your CP/M, they will modify the CP/M so
that 16 treats any RAM over Sak as a virtual disk.” This allows
CP/M to support its normal disks plus an additional disk’, Py
which is accessed in eiactly the same way as the real disks,
only much faster. ‘Disk’ P has al? the normal features af a CP/#
Gist, including a directory, but doesn’t cutter au much from
distette wear and tear or head alignment. It makes making
back-up copive of real disks easy when you can only afford ane
disk drive. Additional 32K blocks of RAM car be reserved from
the CP/M ta mnable the user to have more than Sak oF RAM.
Unfortunately, the board 15 incompatible with Nascom is but
I think this is its only negetive point on an athermiae well
designed and supported product. At £150 for the basic 4K board,
At must be well within the reach of most users. It is purely a
matter of plugging in the additional mesory chips to eave up to
the 256K and beyond barrier.
Many thanks to Me. M. Rothery for delivering the review
system and showing me how it worked and also to. Mr. P. Chance
and Mr. Ts Watkin far their technical help.