Z80 fig-FORTH 1.3

<h2><i>figForth Refuses to Die¹</i></h2>

<br/> A fig-FORTH² implementation for the Z80 that can be built using TASM³:

$ tasm -80 -b figforth.asm forth.com forth.lst

Alternatively, non-Windows users can use the uz80as⁴ Z80 compiler.

The resulting forth.com executable can be run in CP/M. For example⁵:

A>FORTH ↵

Z80 fig-FORTH 1.3c
: CUBE ( N -> N.  CUBE A NUMBER ) ↵
   DUP DUP  ( NOW THERE ARE THREE COPIES ) ↵
   * * ↵
; ↵ ok
5 CUBE . 125 ↵ ok
-28 CUBE . ↵ -21952 ok
HEX 17 CUBE BINARY . DECIMAL ↵ 10111110000111 ok
BYE ↵
A>

Custom Words

This fig-FORTH implementation includes the following custom words⁶:

(OF)    n1 n2 --- n1 (if no match)
      n1 n2 --- (if there is a match)

<ul> The run-time procedure compiled by `OF`. See the description of the run-time behaviour of `OF`. </ul>

CASE    --- addr n (compiling)

<ul> Used in a colon definition in the form: `CASE...OF...ENDOF...ENDCASE`. Note that `OF ... ENDOF` pairs may be repeated as necessary.

At compile time CASE saves the current value of CSP and resets it to the current position of the stack. This information is used by ENDCASE to resolve forward references left on the stack by any ENDOFs which precede it. n is left for compiler error checking.

CASE has no run-time effects.

</ul>

ENDCASE    addr1...addrn n --- (compiling)
      n --- (if no match)
      --- (if a match was found)

<ul> Used in a colon definition in the form: `CASE...OF...ENDOF...ENDCASE`. Note that `OF ... ENDOF` pairs may be repeated as necessary.

At run-time, ENDCASE drops the select value if it does not equal any case values. ENDCASE then serves as the destination of forward branches from all previous ENDOFs.

At compile-time. ENDCASE compiles a DROP then computes forward branch offsets until all addresses left by previous ENDOFs have been resolved. Finally, the value of CSP saved by CASE is restored. n is used for error checking.

</ul>

ENDOF    addr1 n1 --- addr2 n2 (compiling)

<ul> Used in a colon definition in the form: `CASE...OF...ENDOF...ENDCASE`. Note that `OF ... ENDOF` pairs may be repeated as necessary.

At run-time, ENDOF transfers control to the code following the next ENDCASE provided there was a match at the last OF. If the was no match at the last OF, ENDOF is the location to which execution will branch.

At compile-time ENDOF emplaces BRANCH reserving a branch offset, leaves the address addr2 and n2 for error checking. ENDOF also resolves the pending forward branch from OF by calculating the offset from addr1 to HERE and storing it at addr1.

</ul>

FILE    cccc

<ul> Close the current .FTH file, and opens the given file. Note that a file can be loaded automatically on startup by specifying its name on the command line, e.g. `FORTH SCREENS.FTH`. Startup will be aborted with a `No File` error message if the file cannot be opened. </ul>

FTYPE    --- addr

<ul> A constant containing the three character file type used by `FILE`. Defaults to **.FTH**. </ul>

OF    --- addr n (compiling)
      n1 n2 --- n1 (if no match)
      n1 n2 --- (if there is a match)

<ul> Used in a colon definition in the form: `CASE...OF...ENDOF...ENDCASE`. Note that `OF ... ENDOF` pairs may be repeated as necessary.

At run-time, OF checks n1 and n2 for equality. If equal, n1 and n2 are both dropped from the stack, and execution continues to the next ENDOF. If not equal, only n2 is dropped, and execution jumps to whatever follows the next ENDOF.

</ul>

RomWBW extensions

Support for RomWBW HBIOS features⁷ is included when fig-FORTH is built with the -DROMWBW flag:

.B    n --

<ul> Print a BCD value, converted to decimal. No following blank is printed. </ul>

AT    col row ---

<ul> Position the text cursor at the given position. Both column and row positions are zero indexed, thus `0 0 AT` will move the cursor to the top left. Note that `AT` does *not* update `OUT`. </ul>

CLS    ---

<ul> Clear VDU screen. </ul>

KEY?    --- c t ¦ f

<ul> Check if a key has been pressed. Returns false if no key has been pressed. Returns true and the key's ascii code if a key has been pressed. </ul>

STIME    addr ---

<ul> Set the RTC time. addr is the address of the 6 byte date/time buffer, YMDHMS. Each byte is BCD encoded. </ul>

TIME    --- addr

<ul> Get the RTC time and leave the address of the 6 byte date/time buffer, YMDHMS. Each byte is BCD encoded. </ul>

fig-FORTH Editor

The fig-FORTH EDITOR⁸ is included in the SCREENS.FTH file:

FILE SCREENS ↵ ok
7 12 INDEX ↵

  7 ( fig-FORTH EDITOR V2.0 SCR 1 of 6)
  8 ( fig-FORTH EDITOR V2.0 SCR 2 of 6)
  9 ( fig-FORTH EDITOR V2.0 SCR 3 of 6)
 10 ( fig-FORTH EDITOR V2.0 SCR 4 of 6)
 11 ( fig-FORTH EDITOR V2.0 SCR 5 of 6)
 12 ( fig-FORTH EDITOR V2.0 SCR 6 of 6)ok
7 LOAD ↵ 2DROP MSG # 4  R MSG # 4  I MSG # 4  ok
EDITOR ↵ ok
1 CLEAR ↵ ok
0 P ( EAT MORE PIES! ) ↵ ok
1 LIST ↵
SCR # 1 
  0 ( EAT MORE PIES! )
  1 
  2 
  3 
 ... 
 14 
 15 
ok
FLUSH ↵ ok

fig-FORTH 8080/Z80 Assembler

The fig-FORTH assembler enables the creation of both full and defining words using assembly language.

The assembler is invoked using CODE, which creates a dictionary entry with given name and then assembles the mnemonics following. The mnemonics are based on the 8080 instruction set with Z80 specific extensions. The assembly code must end with PCIX to ensure control is returned to Forth.

Note that Forth postfix notation applies between opcodes and operands.

CODE FOO  ( n1 n2 -- n3 as sum of n1 + n2 )
  H POP   ( get first number from stack   )
  D POP   ( get second number from stack  )
  D DAD   ( add hl and de, result in hl   )
  H PUSH  ( push result to top of stack   )
  PCIX    ( jump to NEXT                  )
C;  ( end of definition, return to FORTH  )

The following rules apply when creating words using the assembler:

1. The BC register pair must be preserved across words.
2. Avoid use of the Z80 alternative register set.
3. Do NOT use the IY or IX registers, which are used by fig-FORTH to keep track of the inner interpreter routine.
4. Definitions must end with PCIX (cf. JNEXT in figforth.asm) or PCIY (cf. JHPUSH).

Additionally, the assembler performs minimal checks which means that it is easy to create illegal instructions, resulting in systems hangs or crashes.

References

1. C. H. Ting, Systems Guide to figForth, 3rd Edn (San Mateo, CA: Offete Enterprises, 2013), p. vi
2. William Ragsdale, 'fig-FORTH INSTALLATION MANUAL' (San Carlos, CA: FORTH INTEREST GROUP, 1980)
3. Thomas Anderson, The Telemark Assembler (TASM) User's Manual (1998), Vintagecomputer http://www.vintagecomputer.net/software/TASM/TASMMAN.HTM [Accessed 14 December 2020]
4. Jorge Giner, uz80as - Micro Z80 Assembler, Github https://github.com/jorgicor/uz80as [Accessed 19 January 2021]
5. John James, ‘What Is Forth? A Tutorial Introduction’, in BYTE, 5.8 (1980), 100–26
6. Charles Eaker, 'JUST IN CASE' in FORTH DIMENSIONS, II/3 (1980), 37-40
7. Wayne Warthen, RomWBW Architecture, (RetroBrew Computers Group, 2020)
8. Bill Stoddart, 'EDITOR USER MANUAL', (London, UK: FIG United Kingdom, ND)