Micropower |
Volume 2 · Number 4 · September 1982 |
| Page 26 of 36 |
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| bit 0 | SET – selects upper 32k block RESET – selects lower 32K block (not operative if bit 7 is RESET) |
| bit 1 ) | |
| bit 2 ) | |
| bit 3 ) | selects 64k page |
| bit 4 ) | (bit 5 not used in 32k mode) |
| bit 5 ) | |
| bit 6 | SET – select upper 32k of page 0 as permanent RESET – select lower 32k of page 0 as permanent (not operative if bit 7 is RESET) |
| bit 7 | SET – select 32k mode RESET – select 64K mode |
There are 3 modes of operation:
1) 6K paging – the memory is set up in 64k blocks which means
that to change pages, a program must ensure that the Program
Counter holds the value of the next instruction on the incoming
page after the O FE XX is 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
of page 0 as permanent and then page in any other 32k half page.
This makes the controlling program much simpler and would be
very useful for applications where large data files are
manipulated by small programs.
3) Memory-mapping – this is the most powerful mode of all and it
allows any 4k block from the total RAM to be ‘mapped’ into any
4K slot in the Z80 memory map. The ‘source’ block can be mapped
into more than one “target” block.
The switching from page to page on the RAM card can become very confusing and a memory management program would be very useful. This is 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 it treats any RAM over 64K as a virtual disk. This allows CP/M to support its normal disks plus an additional ‘disk’, P, which is accessed in exactly the same way as the real disks, only much faster. ‘Disk’ P has all the normal features of a CP/M disk, including a directory, but doesn’t suffer as much from diskette wear and tear or head alignment. It makes making back-up copies of real disks easy when you can only afford one disk drive. Additional 32K blocks of RAM can be reserved from the CP/M to enable the user to have more than 6K of RAM.
Unfortunately, the board is incompatible with Nascom is but I think this is its only negative point on an otherwise well designed and supported product. At £150 for the basic 64K board, it must be well within the reach of most users. It is purely a matter of plugging in the additional memory chips to move up to the 256K and beyond barrier.
Many thanks to Mr. M. Rothery for delivering the review system and showing me how it worked and also to Mr. P. Chance and Mr. T. Watkins for their technical help.
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