# i8080 (software) emulator and CP/M for the MEGA65

Foreword: this project of mine is from 2017! Now I'm trying to catching up

again.

Goal: to try to provide a CP/M for the MEGA65 with software emulation of the

i8080 CPU initially, Z80 later and writing a CP/M v2.2 compatible BIOS so

the original DR BDOS (or some replacement like ZSDOS - though it will require

a Z80 emulation) can work.

However CP/M application runs with the 8080 emulation using some "trap opcode"

to dispatch BIOS and BDOS call into native mode. File system based (only file

level, not disk block!) would be translated to MEGA65 hypervisor DOS calls,

thus the native SD-Card formatted for FAT32 would be usable by orginal CP/M

applications as well.

This project contains the following major "tricks" (not so much tricks, and

maybe not as good as it can be, but still some ideas and examples on M65):

* C65 basic-stub loader (so you must start the program in C65 mode! Not C64!!)

* Usage of MAP opcode, C65/M65 memory model, etc

* M65 speed control

* "32 bit linear addressing" opcodes

* M65 direct character "WOM" (write-only memory) access

* Usage of some 65CE02 opcodes (but some of them exists on 65C02 or 65816 as

well, though the opcode bytes can be even different!), like TRB, TSB,

BBRx, BBSx, SMBx, RMBx, Z-reg pased ops, INW, 16 bit branches, and such.

* As a bonus: one time it already helped to find a bug in Xemu/MEGA65 with

rarely used CPU feature (ADC opcode with 32 bit addressing)

## Compile

As usual, you need quite decent CA65 (from cc65) assembler with "4510" CPU

support (currently, maybe only git version is good enough for this?). You

also need an average UNIX-or UNIX like (Linux, OSX, even maybe Windows with

cygwin, or some Linux-box solution) development environment, with GNU make,

wget, and other standard utilities. You also need Python to be installed.

You can say 'make' to compile everything, or 'make test' to test the result

in Xemu MEGA65 emulation (you need only the binary to be installed somewhere

in your search path, it will fetch things for you, if needed with wget).

## Current problems and limitations

* i8080 emulation is farily complete as the emulated opcodes, though there

can be huge amount of bugs, not so much tested. Also now only "DAA"

is not emulated, since the half-carry flag is not emulated at all, it's

really a pain in the ass to do :(

* Also, some i8080 features are not implemented, like the "half-carry" flag,

which may cause problems (surly with DAA but there can be other cases too)

* "Terminal" handling is a big mess of unoptimized quickly written code,

this is not the major point for now

* i8080 emulation has problems with cases when operands/etc would cross the

64K boundary. Surely it can be handled, but emulation would be slower to

do this. I think, this case shouldn't be used anyway by a regular CP/M

program too much ...

* Of course there can be TONS of bogs on opcode emulations ...

* Usage of M65 character-WOM is neat idea to demonstrate the technique, but

it has a serious problem: after "exiting", screen will be messed up, because

my i8080 emulator installs ASCII-compatible character generator layout for

more comfortable usage of I/O routines.

* Currently not so much BDOS and BIOS calls work, what would be the major

intent here, besides the 8080 emulation.

* "Console" emulation if fairly incomplete, screen scrolling should be done

with DMA, control codes should be implemented, there is no keyboard yet

at all (only output), etc ...