Hi,
Using FAT16 as the underlaying model isn't as wasteful as you might think.
When booting up, the BIOS looks at the main boot partition (Sector 0) to see where the FAT16 Partition starts.
This becomes the base to further set two pointers, one for directory and the other for the FAT.
The directory is searched for the files that will become DISK-A to DISK-D.? Once there, the location of the first cluster then becomes the pointer to that disk.
Sectors are then added to that pointer and if it goes beyond a FAT16 cluster, then the code needs to look up the FAT table for the correct cluster.
SD Cards are 512 byte sectors, that's 4 CP/M sectors.? Clusters are usually like 128 sectors, so there's 512 CP/M sectors.??
But the real thing to keep your eye on, there's just 1 pointer to load (then add your sector) to find your sector for most of the time.??
If FAT16 was not used, it would be about the same, 1 pointer (because you want to offset all the drives on a single SD Card) and add.
Normal CP/M uses sequential reads/writes, this is not an issue and speed between FAT and raw-block would probably be about the same.
If you're running a CP/M program that needs random access to the disk, then FAT would definitely slow you down.
Even with the FAT hopping, it's still faster than real 8" drives (I know, because I've tried it).
Bit banging with Z80 is about 90% as fast as parallel processing.? That bit of loss was worth the gain of 4 chips and PCB Space.
This is because the Z80 is much faster (both speed and shorter cycle lengths).??
High up look:
raw-block:
-a bit faster 90% of time/use
-a lot faster 10% of time (when using programs that need full random access of CP/M Disk)
-needs extra firmware to format / load blank SD cards
FAT16:
-Entire disks can be duplicated using any PC that access FAT16
-Simple programming for CP/M File management of disks (CPM Disk Explorer tool)
-SD Card can hold many other files of use (like operating firmware, user manual, etc)
-Still not as slow as real floppy disks
In a nutshell, using FAT16 just made the file system easy to access on any modern computer.? I just figured, that's a win-win.
Cheers,
Josh
