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Zapple-II

Tools for building and running Z80 code under ProDOS on an Apple II with a Z80 Softcard (or clone)

  • Z80 cross assemblers running under ProDOS on the Apple II
  • CP/M BDOS emulation to allow CP/M programs to run under ProDOS
  • Sample program: Processor Technology's SOL-20 BASIC/5

Z80 Cross Assemblers & Tools

I didn't fancy writing a Z80 assembler from scratch, and I wasn't able to find any existing Z80 cross-assemblers for 6502. In order to get something up-and-running quickly, I searched for small Z80 assemblers written in C.

Z80Asm

The first suitable candidate I found was Z80Asm, which was developed by Udo Munk back in the 1988 to 1990 timeframe. (Hr. Munk is still active and has repositories here on GitHub.) I am using an older version of Z80Asm (v1.1) which is written in K&R (pre-ANSI) C, so it was easy to get it to compile on Aztec C for the Apple II.

The program is rather large for Aztec C's compiler, and the resulting binary was also too large to run in the available memory. I made a few minor modifications:

  • I split the largest source file z80arfun.c into eight pieces because Aztec C can't handle files larger than around 500 lines.
  • Slimmed down a few buffers to save some RAM.
  • Reduced the number of supported files from 512 to 10.
  • I had to compile the code to Aztec C VM code using cci rather than to native 6502 code using cc. The natively compiled version creates an executable which is too large to run.

The resulting assembler runs but it is quite slow due to the use of the Aztec VM and can't assemble large programs with lots of symbols without running out of memory. However I have been using it successfully to develop the BDOS emulation.

Z80Asm also builds and runs on Linux which allows larger files to be assembled and is much faster than running on 6502 at 1Mhz.

Z80as

Udo Munk pointed me towards an alternative assembler, Z80as, which was originally developed by the Mark Williams Company and which ran on PDP-11 under Coherent. This assembler has the advantage of small size, and is also written in K&R C.

Z80as compiled 'out-of-the-box' under Aztec C on the Apple II, without any modification.

This assembler generates Intel HEX files rather than BIN files, so I wrote a simple converter called HEX2BIN.

Z80as also builds and runs on Linux which allows larger files to be assembled and is much faster than running on 6502 at 1Mhz.

HEX2BIN

This is a quick-and-dirty conversion program for converting the HEX files generated by Z80as into BIN files that can be loaded on the Apple II using BLOAD.

HEX files can have 'holes' in them, and HEX2BIN takes care of zero-filling the holes.

CP/M BDOS Emulation

I have started work on a CP/M BDOS emulation layer. The plan is to add support for all the CP/M 2.2 system calls, which should allow a CP/M program to run on the Softcard Z80 CPU and have all the system calls routed to the 6502 and serviced using the Apple II ROM monitor routines and the ProDOS MLI.

This is at an embryonic stage at the moment as it only provides three system calls:

  • BDOS call 01h: C_READ - Console input
  • BDOS call 02h: C_WRITE - Console output
  • BDOS call 0Bh: C_STAT - Console status

There are two parts to the BDOS emulation:

  • softcard80.asm - This is the Z80 code to handle BDOS calls and send them to the 6502 to be processed. Written in Z80 assembler. I am currently assembling this using Z80asm (but will probably switch to Z80as when it grows too large.
  • softcard65.asm - This is the 6502 back end code. Written in Merlin8 v2.58.

Sample Programs

BASIC/5

This is one of the BASIC interpreters from the Processor Technologies SOL-20 system. The source code was provided as an example with z80as.

I assembled this code under Z80as on Linux, since it defines too many symbols to assemble natively on the Apple II in the available memory. I plan to take a look at the Aztec C build configuration to see if it is possible to find more memory for dynamic allocation (ie: malloc()).

It is a 5K BASIC, so it is rather primitive. However it does have a floating point package and trig functions.

BASIC/5 only uses three system calls: C_READ, C_WRITE and C_STATUS.

There is currently no support for loading or saving BASIC programs, but I may add this later.

The manual for BASIC/5 is included in this GitHub repo, in PDF format.

How to Run The Code

TODO: Write this!