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# µEforth Internals

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µEforth (micro-Eforth) simplifies EForth even futher, by building just enough of the core of the system in C to allow the rest to be be built in proper Forth.

A handful of "tricky" words that involve internal loops or many steps are built in their own functions:

    FIND ( a n -- xt | 0 )
    PARSE ( ch -- a n )
    S>NUMBER? ( a n -- n f | 0 )
    CREATE ( "name" -- )
    EVALUATE1 ( -- )

This includes `EVALUATE1` which parses a single word and interprets or compiles it (reusing `PARSE`, `FIND`, and `S>NUMBER?`).

See core.h.

A few global variables connect parsing and compilation state between C and Forth (by way of a memory region accessed via `'SYS`):

    'TIB      --- Pointer to the Translation Input Buffer
    #TIB      --- Length of the Translation Input Buffer
    >IN       --- Number of characters consumed from TIB

    BASE      --- Numeric base for printing and parsing

    STATE     --- State of compiling, -1 for compiling, 0 for interpreting
    CURRENT   --- Pointer to pointer to last word of current vocabulary
    CONTEXT   --- Pointer to pointer to last word of context vocabulary

    'NOTFOUND --- Execution token of a handler to call on word not found

Error handling is routed via a deferred callback in `'NOTFOUND` used when a word is absent from the dictionary. This is eventually directed to an error routing that prints a proper error, once I/O and exceptions are available.

X-Macros are then used to build up a small set of core opcodes defined in 1-3 lines each:

    0= 0< + U/MOD */MOD   AND OR XOR
    LSHIFT RSHIFT   DUP SWAP OVER DROP
    @ SL@ SW@ C@ ! L! W! C!   SP@ SP! RP@ RP!
    >R R> R@   : ; EXIT
    EXECUTE BRANCH 0BRANCH DONEXT DOLIT
    ALITERAL CELL DOES> IMMEDIATE 'SYS

NOTE: Later to reduce the use of the RAM heap and improve performance, additional non-essential extra opcodes were added in place of high-level words.

See tier0_opcodes.h.

I/O and access to systems outside Forth are connected via a few per platform words. Typically this set of words should be minimal, while still allowing relevant parts of the host system to be available to Forth.