About Zapple:

   Zapple was a popular "Monitor" program,
written for the Z80 back in the 70's.  It's intended
target was Z80-based S-100 computers.  It was not free.
(When I bought my S-100 CPU board, it did not come with any
code for the EPROM.)  These computers would often have 
some sort of monitor stored in EPROM.  The computer might 
boot into the monitor and you would have to type G then 
some address to start up CP/M.

   The idea of the monitor is to have some software
that relies on only a serial port, the CPU, and ROM/RAM,
which can be used to test out other parts of the system.
Zapple can read and write to I/O ports, display memory,
move memory, test memory both destructive and no-destrucively,
fill memory with a value, read in an Intel Hex file (a program)
and jump to it, with up to two breakpoints.  It is not an 
operating system, just a useful program to have running 
when you are trying to figure out why some new hardware that
you added isn't working, or to see if your memory is reliable.
That sort of thing.  It was designed to do as much as possible
while still fitting in a 2K (2716) EPROM, as that was the 
common size monitor EPROM for S-100 systems in the late 70's.

   I found a scan of the Zapple source code on the internet 
a few years ago and typed in the code and comments, then 
spent a little time debugging it as I had mistyped a few things.  
I was working on getting an old S-100 system up and running.
I removed some code that didn't seem useful and added the checksum
command, to test the reliability of reading from EPROM.

   The baud rate at which you run Zapple seems to be limited by
the speed at which it can parse Intel hex files when using the
R command.   I have found that 2400 baud works well when using
a 4 MHz Z80.  For higher speed Z80's, you could set the baud rate 
faster, but if you have a 4 MHz Z80 and set the baud rate to 4800
baud, it may drop bytes when you are loading an Intel hex file.
Using a newer UART that has a receive FIFO should help with this
issue.  I haven't tried to push it, as 2400 baud is OK for me.

   Bob Grieb