ZX65SIM

This file documents the updates made to the 6502 simulator
for CP/M developed by R.M Kruse, and released to the public
in an article published by Doctor Dobb's Joural, August, 1981
issue.

---------------

   As distributed, ZX65 was difficult to "bring up", due to
its hard-wired linkages to the CP/M BIOS, along with the customized
loader modification necessary to relocate the package to high mem-
ory.  Also, the simulator contained a relatively large amount of
code to provide a built-in DOS, to allow the page-zero CP/M area
to be overwritten by the 6502 code being simulated (restricting the
use of page 0  extracts a cost in efficiency, since the 6502 has such
"cheap" page-zero addressing modes).

   It occured to me that the memory limitations could be circumvented
by remapping all 6502 memory references into Z80 memory.  This required
major modification of the simulator (basically, I had to find all of the
code that addressed 6502 memory, and change these to external subroutine
calls), as well as a new module (ZX65HOST.MAC) that could be linked in
with the simulator.  This new module may be rewritten for different users
that may have different needs (in fact, I have three locally for hardware
development I'm doing).  The distributed version simply remaps 6502
references into the free space above the program (using the L80 $MEMRY
variable to locate the free area); the initialization entry point is
used to load a binary file (see the source file for syntax); this could
be modified, however, to provide such things as hardware initialization
for, say, a hardware in-circuit-emulator to be used in conjunction with
the simulator program.

   I also added a crude "traceback" feature, that displays the last n
pc's executed ('n' is specified via an equate in ZX65SIM.MAC); these are
stored in a circular buffer within the simulator, and accessible via the
'H' command.

   I removed the built-in DOS, since binary files may now be loaded via
the initialization subroutine within ZX65HOST.  I know that this circumvents
the author's original SAVE FILE command, but I never really saw a need for
such a command.  Perhaps someone will add it back in using the CP/M BDOS...

   Finally, I modified the simulator to assemble using Microsoft's M80/
L80 combination (instead of the obscure SDS assembler formerly used).  Since
not everyone is likely to have this wonderful set of software tools, I've
included the linked object code in the distribution set.  To reassamble and
link, use the following sequences:

		M80 =ZX65SIM		 (to assemble the main module)
		M80 =ZX65HOST		 (to assemble the remapper)
		L80 ZX65SIM,ZX65HOST,ZX65HOST /N/E
					 (to link the two, and produce
					  ZX65SIM.COM)

   I should note that the 6502 fetch and store routines within ZX65HOST
must NOT modify any registers (of course, FETCHA must return the fetched
byte in the accumulator).  AINIT (the initialization routine) may modify all.

   Here is a breakdown of the files included with the package:

	ZX65SIM.COM	-- the assembled and linked object code
	ZX65SIM.MAC	-- source for the simulator
	ZX65HOST.MAC	-- source for the remapper
	ZX65SIM.DOC	-- the file you're reading
	ZXART.DOC	-- the DDH article as originally published in DDJ
	ZXTABLES.DOC	-- tables that were published with the article
	ZX65HINTS.DOC	-- original author's post-publication comments

   Note that none of the original documention has been modified; therefore
you should be aware that some of it may be inapplicable in this new release
(such as those files that reference the built-in DOS).

   If you have any questions regarding this package (at least insofar as
concern the changes that I've made), I may be reached via the FORT FONE FILE
FOLDER, an RCPM system located in Fort Atkinson, Wisconsin (phone number:
(414) 563-9932).

			Happy simulations!

			Ron Fowler
			Dec 17, 1982