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Notes for updateing and maintaining Basic.
                (c) P. (Rabbit) Cockcroft 1982.

1) Variables and their uses.

char    *point    - This points to the next character that will
                    be interpreted.

typedef struct lin *lpoint

lpoint  stocurlin - Points to the start of the line structure
                    of the current line. Is null in direct mode.

unsigned curline  - Linenumber of current line being executed.
                    Zero in direct mode.

lpoint  errortrap - Pointer to start of line where error trapping will
                    go to. Null if no error trapping.

char    inserted  - Flag set if program has been changed. Used to clear
                    data space if program text changed.

char    trapped   - Set if cntrl-c has been hit. Will cause program to
                    exit next time through the execute routine.

char    elsecount - Set if elses are legal terminators. (After an if
                    statement only).

char    runmode   - Set if in runmode. Zero in direct mode.

char    vartype   - Type of variable. Set by getname() - get a variable,
                    getop() - get a number and several of the other
                    functions.
                    Values:-   0) Floating point variable
                               1) Integer variable
                               2) String variable.

char   *filestart - Pointer to start of file buffers. If you want to
                    add more dynamic buffers for other purposes. Put
                    them in before filestart is initialised. For semi
                    permanent buffers. Or place them after estarr and
                    change data space by using xpand (Fairly unstable).

char    *fendcore - End of file buffers start of basic text.

char    *ecore    - End of basic program.

char    *vvend    - Very end of allocated data. Changes very quickly
                   ( by for-next - gosub stack ).

typedef value  union {
                double f;
                int     i;
                };

value  res        - General purpose register for maths. Result always in
                    here. (else return value from evalint() ).

long  overfl      - Used when integer maths overflows. Value is
                    converted from long to double with vartype set
                    accordingly by over().

int     cursor    - Current cursor location across screen.

char    line[]    - Input line. Place where edit and readfi() put the
                    input line. Where the editor works. Input to compile
                    function.

char    nline[]   - Compiled line is here. What is executed in direct
                    mode. Sqirelled away by insert().

int     (*functb[]),
        (*functs[]),
        (*commandf[]),
        (*strngcommand[]) - Maths , Command, and string function jump tables.

char    *ermsg[]  - Table of error messages. To add other error messages
                    put them at the end of this table and increment MAXERR.

Other pointers and variables are used for various purposes probarbly
not needed for adding functions. Don't change them if you don't know
what they do.

2) Useful functions.etc.

object *getname() - If point points to a valid name then returns a
                    pointer to it. If variable does not exist create it.
                    (Will not create arrays).
                    Only ever returns with a valid pointer.

int     getch()   - Get the next character on the line. Ignores spaces.
                    ( Care must be used at end of line so don't run off).

int     check()   - calls error if not at the end of a statement.
                    Use when got all arguments and want to see if and
                    garbage is at end of command.

int     error()   - Call error routines. Will always tidy up. Sets error
                    code to parameter. (NO checking of parameter is done).
                    THIS ROUTINE NEVER RETURNS. WILL ALWAYS GET YOU OUT
                    FROM WHERE YOU ARE BACK TO COMMAND MODE. (Useful).

int     eval()    - Will evaluate any mathematical expression. The
                    result will be in res with vartype set accordingly.

int     evalint() - Will call eval() and then returns an integer if
                    possible. Otherwise returns -1. (Negative values are
                    usually thought of as an error).

int     stringeval(f) - Will evaluate string expressions. f is the
                    destination it must be a pointer to a character
                    array of at least 256 characters. (Usually gblock or
                    on stack or allocated by grow() -actually on the stack).
                    gcursiz will contain the length of the string.
                    N.B. gblock (can) will be corrupted by stringeval
                        and the input routine ( edit() ).

int     edit(f1,f2) - Input routine from the terminal. (Full editor).
                   f1 is number of characters to be used as a prompt.
                   (in line[] ), f2 is the number of characters to be
                   printed out at the start. (so can be used to edit the
                   given string. The string must already be in line[].
                   f2 must never be less than f1.

object  *grow(v) - Will allocate v bytes on the stack. V must be even.
                   Should be used for maths functions which need a lot
                   of stack space for their local variables. It will
                   call error if it runs out of stack space. (Will only
                   use one segmentation register for the stack. Rest is
                   for program and data). Returns a pointer to the space.
                   (This means that 75 or so brackets can be used).
int   istermin(c),
      isletter(c),
      isdigit(c) - Macros that return true if c is a terminator, a
                   letter or a digit.

3) Adding More functions, commands. etc.

The basic idea is to put an entry in the corresponding table and to add
the function name and token value into the tokenising table at the
correct spot. Commands need to return a value all others do not.
Commands that need to change the direction of the program return -1
otherwise use 'normret'. - Most commands will return by using 'normret'.

Maths and string functions that require brackets fit perfectly into this
scheme but the exceptions are as follows:-

commands:-
        These require a zero to be deleted and the function to be
added in it's place this is so that mid$ can be used on the left hand
side of a statement. Nothing else need be done.

Maths functions without brackets or optional brackets:-
        These are nasty because the value of rnd is used for various
uses. So :- Decrement the defined constant RND and then decrement the
entry in the tokenising table. Place the new entry where RND used to be.

( Maybe you might like to make brackets not as tightly bound to
functions - currently must be tight against the keyword (No particular
reason but thats how I like functions to be)).
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