Z80 EMULATOR by Dave Higton

This is not yet complete, but I have uploaded it to gauge interest.  It
emulates a Z80, specifically in the environment of a Vector Graphic System
B computer.  The memory map is as follows:

0000-DFFF RAM
E000-EFFF EPROM (E000-E7FF is monitor, rest is extensions)
F000-F77F Video RAM, direct mapped to screen as 80 * 24
F780-F7FF RAM, not mapped to screen
F800-FBFF EPROM, sometimes used for disc boot EPROMs
FC00-FFFF RAM, used by monitor and basic I/O for stack etc.

All the RAM from F000-F7FF is accessible in the original machine during
vertical blanking only, therefore the area from F780-F7FF is very rarely
used in practice.

The original machine uses the PHANTOM line of the S100 bus to relocate
EPROM to locations 0000-0FFF for the first instruction following a hard
reset. The first three bytes in EPROM are a jump to a location in the
monitor EPROM.  Therefore I have made the reset jump to &E000 rather than
0000. This is directly equivalent.

The I/O map of the original cannot be exactly copied in the emulator since
this would require three serial ports, for instance.  However, the
keyboard status port is at 00, keyboard data at 01, and serial status/data
INPUT ports are at 05 and 04.  The original computer had Micropolis 5 1/4"
hard-sectored disc drives (this was state of the art when it was
designed!) and the emulation cannot reflect this either.  I'm working on
mapping the Arc's floppy disc hardware into the I/O map at &F0 to &F5, but
this isn't complete yet.  See later caution!

There is no concept of interrupts in this emulator.  However, none of the
normal system software requires them, so this shouldn't be a problem.

All single byte instructions are emulated except &76 (HALT) which would be
meaningless (would cause the machine to freeze because there aren't any
interrupts to un-HALT it), therefore causes an abort.  All the CB-prefixed
instructions are emulated.  The easier ED-prefixed instructions are
emulated, others aren't yet (e.g. RLD and so on).  The indexed
instructions using IX are complete except for the DD CB group
(shifts/rotates/bit test/set/reset). The IY instructions are not present
except for PUSH IY and POP IY.  When I have finished the IX group, it will
be easy to complete the IY group, largely by copying and global
search/replace.

The monitor has the following commands:
A ASCII Dump <start> <end>
B Boot Disc (this will cause an abort)
C Compare <start> <end> <2nd block start> Compares 2 blocks of memory
D Hex Dump <start> <end>
E Ext Com (copies serial port's input to screen)
F Find-2 <start> <end> <byte1> <byte2>
G Go to <address>
H Hi RAM (jumps to an address in the top 1K of RAM)
I Input <port>
J Jump to DOS (at 0000, to cause a CP/M warm start)
K Break at <address> Enter breakpoint
L Lo ram (jumps to an address in main RAM)
M Move <start> <end> <new start> (copies a block of memory)
N Non-destructive RAM test <start> <end>
O Output <port> <data>
P Program <address>
Q Checksum <start> <end>
R Registers
S Search-1 <start> <end> <byte>
T Test RAM <start> <end> (destructive test, i.e. overwrites old contents)
U User area
V Boot Dualstor (would boot some kind of disc, but won't - yet!)
W Boot Megastor (would boot a hard disc, but won't - yet!)
X Exchange <start> <end> <snd block start> Exchanges 2 blocks of memory
Y Echo keys (to screen)
Z Zero/fill RAM <start> <end> <data>

Commands such as N, T and Y are terminated by the Esc key.  Others return
immediately to the monitor main loop, except for the disc boot operations.

The character generator is faithfully emulated.  Printing characters from
&20 to &7F are standard ASCII.  Characters from &00 to &1F are block
graphic characters.  Setting bit 7 displays any character in inverse video
- this enables displaying the 6th block graphic.

The monitor has several entry points. &E000 is the cold start entry, &E003
is the warm start, &E009 is the video driver.  Executing opcode &FF (RST
38H) prints the register contents and enters the monitor.

Any opcodes which are not implemented cause the emulator to print out all
register contents and exit.  The emulator runs in system mode.  Interrupts
are enabled, but I'm not sure what interrupts are actually accepted and
serviced.  The monotonic timer appears not to run.  The emulator is an
application, i.e. it executes at &8000 and takes over the machine
completely.

I recommend that you don't output to ports &F0 to &F5 because of the
mapping to the disc hardware of the Arc.  If you do, you could manage to
write to a floppy disc, thereby damaging the information held on it.  You
have been warned!

The video driver handles various control characters, but they may not be
the ones you are expecting.  E.g. control-D clears screen, control-U is
cursor up, control-R is cursor down, control-W is cursor left, control-Z
is cursor right, control-P is erase to beg. of line, control-X is erase to
beg. of line, control-T toggles normal/inverse video.  There is no bell or
beep on the machine being emulated.

While executing, the Esc key does not cause an escape event.  This is
because the monitor programme requires use of the escape key.  It is reset
to normal when the emulator exits.  An abort (due to an illegal
instruction) is the only way to exit - like the PC emulator.

                              SPEED

Everyone asks this.  There isn't, however, a one-number answer.  Some
instructions execute fast (e.g. LD R, R'), others execute slowly.  The
final answer on speed has to come from any individual programme.  In
general, it executes on an ARM2 A3000 a bit slower than a 4 MHz Z80 with
no wait states. Since it is an interpreter, I don't know any way to speed
it up.

                            THE FUTURE

1) I do intend to implement all the instructions!

2) The floppy disc hardware will be mapped in properly.  This won't be
   simple because the emulator doesn't execute fast enough to read/write
   sectors/tracks directly; however, it should end up simple to use.

3) If someone can tell me how to write a keyboard handler, I'll make the
   Arc's cursor keys emit the codes expected by the monitor programme and 
   the other software written by Vector Graphic.

4) I want to customise CP/M for this hardware.  Clearly, the original 
   version was for a hard-sectored controller.

If I manage to get it to that state, then there will be a very good text
editor (SCOPE - SCreen Oriented Program Editor) and an excellent debugger
(ZRAID - Z80 RApid Interactive Debugger) available.  These programmes were
written by Vector Graphic.  I also have used, and can thoroughly
recommend, SLR Systems' assembler and linker - the virtual linker (SLRNK+)
being particularly useful since it can cope with a full 64K and more,
overlays, etc.  However, the SLR Systems programmes are current (as is the
company) and so these programmes would have to be bought by any user. 
Vector Graphic was an American company which went bankrupt some years ago
- can anyone advise me on what the legal status of their software might
be?  Clearly Digital Research are still in business, and own the copyright
of CP/M.  So if anyone knows anything about the legal requirements of
distributing a newly customised version of CP/M, I'd like to hear from
you.

Leave messages for me on The World Of Cryton BBS.

Dave Higton