Scor­pio News

The test program (listing no. 1) is not an exhaustive test but it does verify that the mapping works. In it all the forty eight 4k blocks from addresses 10000H to 3FFFFH are mapped into the 4k block starting at 1000H and the block number is written into the first byte of the block. The process is then repeated mapping the blocks beck and checking that the first byte is still what had been written. It outputs ‘Success’ or ‘Failed’ depending on the result. I didn’t have any problems so I haven’t bothered to extend the program to explore which blocks failed.

Having all this additional memory, what did I do with it and was it worth it?) The first thing I discovered was that it was no use for programs as there is no way of linking sections of code or data at a particular address. Therefore the first thing I did with the extra RAM was to use it as a 192k byte RAM disk (see listing no. 2). The sector size was made 128 bytes to avoid the need for deblocking and the track size was made 4k bytes so that the track number for a read or write gave me the 4k block number to map.

To make full use of it I changed SUBMIT and the CCP to use the RAM disk for the “$$$.SUB” file. Then I reduced the disk size by 4k and reloaded the CCP and BDOS from it at each warm boot. Finally I patched Wordstar to look on drive M: for its overlay files. What an improvement (provided I remembered to copy then across first), particularly the speed up in editing. But I do regret that the disk will never be big enough to use as a source disk.

For some time I considered the possibility of writing disk cache software to use the memory more efficiently, but eventually opted to try a banked version of CP/M Plus which already includes all the disk cache software. This meant that I had to allocate 4k blocks of memory to banks, a 4k block to common memory and then write my own bank switching routine (listing no. 3).