A Fig-Forth Utility

A Fig-Forth Utility

Jürgen Pfeifer

There are several versions of Forth. The most popular is the Implementation of the Forth-Interest-Group (FIG), the well known fig-Forth.. But there exists an improvement, the 79-Standard Forth, which is very close to fig-Forth.

The 79-Standard describes a very useful word; which doesn't exist in fig-Forth. It is the word "roll." The Forth stack notation for roll is:

roll n–

This word extracts the nth stack value to the top of stack, not counting n itself, moving the remaining values into the vacated position, n must be strictly positive.

Examples: 3 roll = rot (a fig-word). 1 roll = no operation.

The screens here contain a low-level definition of "roll" for a 6502 fig-Forth, using the Forth 6502 macroassembler.

As an application, the screens contain the definition of the signed double-integer multiplication operator "d*". Try to define it without roll!

SCR  # 106
  0  ( ROLL
  1  CODE ROLL ( N --- )                  JPF   JUL82 )
  2    1 # LDA, SETUP JSK, XSAVE STX, N LDA, SEC, 1 # SBC, CS
  3    IF, 0= NOT IF,
  4                  TAY, .A ASL, CLC, XSAVE ADC, TAX,
  5                  BOT LDA, PHA, BOT 1+ LDA, PHA,
  6                  BEGIN, DEX, DEX,
  7                         BOT LDA, SEC STA,
  8                         BOT 1+ LDA, SEC 1+ STA,
  9                         DEY, 0=
  10                UNTIL, PLA, PUT JMP,
  11             THEN,
  12   THEN, NEXT JMP, END-CODE
  13   (ROLL :  EXTRACT THE N-TH STACK VALUE TO THE TOP OP STACK, NOT
  14           COUNTING N ITSELF, MOVING THE REMAINING VALUES INTO THE
  15           VACATED POSITION. N>0 )

SCR  # 107
  0  ( ROLL APPLICATION                      JPF   JUL82 )
  1  : PICK 2* SP@ + @ ;
  2    (N1 --- N2  : RETURN THE CONTENTS OF THE N1-TH STACK VALUE,
  3                  NOT COUNTING N1 ITSELF. N>0 )
  4
  5  : 2SWAP ROT >R ROT R> ;
  6    ( D1 D2 --- D2 D1 : EXCHANGE THE TOP TWO DOUBLE NUMBERS
  7                        ON THE STACK. )
  8
  9  : D* OVER 5 PICK U* 6 ROLL 4 ROLL * + 2SWAP * + ;
  10    ( D1 D2 --- D3 : LEAVES THE ARITHMETIC PRODUCT OF THE
  11                     DOUBLE PRECISION INTEGERS D1 AND D2 )
  12
  13
  14
  15