Thanks Lee.
I used parted to examine the uSD and discovered it was basic fat 16.
I come from a time when a ST506 (5MB) seemed like more space than usable..
That lasted maybe a month.? Most systems with disk I found more than 60mb?
was more than excess and a 128MB CF being largely unused.
Two limits for that...
One is CP/M 2 max file system size is 8MB (65535 sectors max).
The other is each drive eats about 256bytes for the allocation map and with 64K ram, one word, ouch.
The common solution was some utility to allow for one drive as system and up to
three mapped (MOUNTED) in as needed.
For larger file systems CPM3 or the clones P2dos, Novados, Zrdos, or Zsystem
fixed an internal BDOS bug (and enhanced many things) and allows for up to
262144 sectors (logical 128 byte) or 33MB but the solution came with a directory
that could take a while to parse.? A penalty of a flat file system.
Generally for large media (IDE, CF and SD) that is LBA addressed
I use:
?Sectors= (alloc/128)/4 as, or 8 physical sectors per track
?8MB logical disk? (8mb/4096 alloc size) for 2048 Tracks.
CP/M actually passes a 16bit value for track and sector passed
in register pair BC.? Sector is typically less than 256 and track
can be as high as needed to address 8MB total.
That allows for a new logical partition spaced every 16384
physical sectors ((2048 * 4096= 8MB))/512= 16384 physical
sectors.?
Also CHKS is a checksum for directory to lock out possible
floppy changes mid operation.? For most systems with fixed
media (SD qualifies) the value is 0 (zero).
The only headache for CP/M Z80 is track and sector (logical 128)
is 16bit values, aka one track of 1 local sector * 65536 tracks, or?
256 sectors * 256 tracks, or and combination that multiplies
out to 16bit (65535) without overflow.? So for devices larger?
the offset is applied per drive number with 24 or 32 bit math
added to create a LBA value large enough for the device in use.
The application of this is for a 128MB CF that's 262144
512 byte physical sectors. or 16384 physical sectors
per 8MB logical disk. since they can be addressed from
LBA 0000 each logical disk starts every 16384 logical
sectors for a total of 16 possible logical drives.
ITs then a simple matter of adding (32bit) 16384 (0x004000)
For each logical drive (at the Bios level) or 0x004000 * drive
number. Its easily handled using a BIOS internal table that
the external BDOS does not access.
I keep it simple as since the whole initial system loads from
Eprom to ram that leaves out "system" tracks completely.
A simplification.? If that system needs upgrade the easy
way load a file and overlay the existing system, a handy
way to load improved CCP, BDOS or BIOS.
What I can't imagine is using a large device (more than 8mb)?
as a bunch of 8"SSSD floppies (241KB useable space)?
The acid test for disk adequacy is:
ASM cpm22.BCD? ? ; assemble a large version of CPM fully commented at 104KB
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? : source on B, hex on C, and PRN on D
FYI the resulting PRN file barely fits on a empty 8"SSSD floppy.??
That's why everyone from then want the largest disks possible.
Also why I started down the hard disk path in 1980 as even 2SDD 8" (1mb)
was still slowing me down and a 4 drive box was heavy, huge, and loud!
Allison
