DOCUMENTATION FOR XDRIVE.ASM Version 1.0

           by Herb Shore and Paul Grupp Aug. 24, 1983

     This program will allow extra memory beyond the usual 64K to
function as though it were a very fast disk drive.  You may want to 
change the name of the installed program to indicate the name of the 
new drive, i.e. CDRIVE.COM for drive C:.  The extra drive (drive M: 
as distributed) can be used in the same way as all the other drives, 
though of course all data on this drive is lost when the power is 
turned off.  One way to use the system is to PIP over all working 
files to drive M: after activating the XDRIVE, working on the files 
while logged onto M:, and then PIPing them back to a floppy before 
shutting down the computer.  For repetitive operations such as 
assembling, compiling, using a word-processor with overlay files, 
etc., the increase in speed is fantastic.

     The system is especially easy to use if you have replaced the 
standard CCP with ZCPR or ZCPR2.  With ZCPR, the standard utility 
files (PIP, STAT, MAC, etc.) can remain on drive A:, since these 
files are not accessed after they have been loaded.  ZCPR will 
automatically search A: for these files if they are not present on 
M:.  Only those files which are repeatedly accessed need be on the 
fast drive.

     To implement the system, edit XDRIVE.ASM setting the system 
dependent equates, then assemble using MAC.  Note that the Maclib 
Z80.LIB is needed.  Produce the COM file from XDRIVE.HEX using LOAD 
or DDT.  Then to activate the XDRIVE run XDRIVE.COM from CP/M.  This 
program will ask if you wish to initialize the directory area of M:.  
If the computer has just been turned on, answer YES, since the 
directory area contains garbage.  However, if your system has crashed 
due to a program runaway, it is very likely that extended memory is 
still OK.  In that case you can do a cold boot, run XDRIVE, and 
answer NO when asked if the directory should be initialized.  When 
XDRIVE signs on it reports the available file space for the RAM drive.

     SOFTWARE REQUIREMENTS: The XDRIVE system requires about 1K bytes 
of global RAM above the BIOS.  To be exact, the actual amount is 410 
bytes plus one byte for every 8 blocks of disk space.  One block is 
1Kbyte for systems with 256K or less of disk-RAM and 2Kbytes for 
systems with more than 256K.  These details are only necessary if one 
is attempting to conserve every byte, since providing 1K of space 
above the BIOS is more than adequate. You may have to reduce the size 
of your CP/M system to provide the space. Other than providing this 
space, there are no special requirements for the size of the CP/M 
system or the location of the BIOS; this is determined by the program.

     HARDWARE REQUIREMENTS: As described above, approx. 1K of global 
RAM must be provided.  Global RAM is memory that responds to the 
standard 16 bit address bus independently of the setting of the 8 bit 
extended address bus.  If your global RAM is larger than 1K, the 
system will still work, but the memory available for the extra disk 
is reduced.  The system must provide a way of setting the extended 
address bits by outputting the extended address to an output port.  
If your present system does not have this capability, it can be 
easily implemented.  Turning 1K of RAM into global RAM is also not 
difficult.  One way is as follows: connect a 6-input AND gate to 
monitor address lines A10-A15.  When all six lines are high, the 
upper 1K of memory is being addressed.  When this happens, tri-state 
the circuit (output port) driving the extended address bus and force 
all 8 bits of this bus to be low.  Thus, the RAM at extended address 
00H will become global for the upper 1K of the 64K address space.  A 
word of caution: If your system uses DMA devices, these devices may 
also attempt to drive the extended address bus.  Make sure that any 
extra circuitry you add is disabled during DMA operations in order to 
avoid bus conflicts.  Also note that if DMA devices do write into the 
extended address space, that this will overwrite the data on the M: 
drive.

     HISTORY: XDRIVE replaces and obsoletes an earlier memory-disk 
program by one of us (HBS) called RAMDRIVE.  The prime objective in 
going from RAMDRIVE to XDRIVE was to produce a "universal" program 
that would work under a wide variety of hardware and software 
environments.  The program has run without apparant bugs on a number 
of different systems, with/without hard disk, with/without 
interrupts, with small/large amounts of extra RAM.

     The major differences between the two programs are as follows:

     1) RAMDRIVE used a separate loader program and needed a 
complicated procedure to produce a COM file; XDRIVE is self- 
relocating and is simple to install.

     2) RAMDRIVE used a 1K block size, which will not work with more 
than 256K of RAM-disk.  XDRIVE sets the block size to 1K or 2K 
automatically depending on the amount of extra RAM, and will work 
with any amount of RAM.

     3) XDRIVE is much more memory efficient than RAMDRIVE in that it 
relocates to high memory only those routines that must be there -- 
less than 512 bytes.

     4) RAMDRIVE did not work in a system using interrupts.  XDRIVE 
senses if interrupts are enabled and, if so, disables them during 
accesses to extended memory.  This allows operation with an interrupt 
driven system.