Rekonstrukt - A "New" Forth Machine

I have been a Fan of Forth since the 1980ies, and when I picked up my FPGA stuff a few weeks, I decided to put some more effort into getting a complete Forth based FPGA system to run before turning back to the SECD reimplementation project. This new Forth machine is called "Rekonstrukt", and I have created a Google Code project to publish the source code.

My last FPGA productivity rush ended with Maisforth running on the Spartan-3E Starter Kit. Maisforth is a ANS-like 8 bit Forth originally written by Albert Nijhof for the somewhat obscure MC6809 based computer called "Maiskastje" built by a dutch computer club a few years ago. Porting Maisforth to System09 was rather easy; all that was needed was a little tweaking of the serial I/O routines so that the System09 MC6850-lookalike serial port was properly handled.

My next goal is to support most of the Hardware that the Spartan-3E Starter Kit has to offer by Forth. This will require some VHDL hacking in order to implement the low level interfaces as well as implementing Forth libraries.

Implementing a SPI controller

I started by implementing an SPI interface. SPI is a serial bus protocol for moderately high speed connections between devices inside one system. On the Spartan-3E board, the analog capture unit, analog preamplifier, digital to analog converter, serial flash and platform flash chips are all connected to one shared SPI bus. It would be possible to operate this bus by bit-banging in software, but this would be rather slow. In hardware, it is easily possible to operate the SPI bus at a bit rate of 10 Mhz.

While other open source VHDL SPI controllers exist, none of them easily interfaced with the "proprietary" bus protocol of System09. I took this as an opportunity to freshen up my VHDL and FPGA design skills. Again, I learned that it does not make sense to synthesize to the real hardware before simulation has shown that the design basically works, but that successful simulation does not automatically mean that the design works in the real hardware. With the help of some regulars in the #fpga IRC channel on freenode.net, I got the SPI controller to run.

Implementing vectorized I/O in Maisforth

System09 comes with a VGA controller that provides for a 80x25 text console, named vdu8. vdu8 is register based, i.e. to write a character onto the screen, the host CPU needs to write the screen position where the character should go into the cursor address registers, then write the character to be shown into the data registers. In order to make vdu8 useable as I/O device for Forth, a small terminal emulator is needed that interprets standard control sequences like Linefeed, Carriage Return and Backspace properly so that it can be used to handle EMIT calls.

The VGA console should be useable as an alternative to the serial console and switching between the two should be possible at run time. The common technique to achieve that is to make the words that need to be switched between alternative implementations be vectored, or "deferred" words. What this means is that a pointer to the word that actually implements the word is stored in the word that is being called by the system, and this pointer can be modified at run time in order to switch to the desired implementation.

Normally, the Forth compiler performs a name lookup when compiling a word, not when executing it. Thus, once a word has been compiled, the addresses of the words that are being called are fixed, and the implementations of these words cannot be changed in retrospect.

For normal, RAM based operation, implementing deferred words is quite simple:

: alias ( xt -- ) create , does> @ execute ;
: noop ( -- ) ;
: defer (  -- ) ['] noop alias ;
: is ( xt  -- ) ' >body ! ;

alias is used to set up an alias for a word. defer is used to set up an alias for the do-nothing word alias. The implementation of such a word can later be changed using the is word.

This works well for RAM based Forth in which the , word can be used to compile a number into the dictionary and change that later. For a ROM based Forth like Maisforth, the default dictionary is read only and as we are interested in vectorizing words that are used by the system, the vectors need to be put somewhere else. Maisforth provides for a "user space" for this purpose. The "user space" is located in RAM at the beginning of the address space, and the IVEC word can be used to allocate a cell from that space in the cross compiler. When adding new user vectors, the USERBYTES constant needs to be increased so that the cross compiler knows the changed memory layout. I did not find out how I could get the vectors initialized automatically by the cross compiler, so I added a new 'EMIT vector initialized to 0 which would be called in the (EMIT word, and changed the COLD word to intialize the 'EMIT vector when rekonstrukt starts.

More information about vectorized functions can be found in Leo Brodie's excellent book Starting Forth in the Chapter Under the Hood.