elks-enhanced - XT-Emporium

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ELKS Basic
==========
From Robin Edwards' ArduinoBASIC (https://github.com/robinhedwards/ArduinoBASIC).

A complete BASIC interpreter for your 80's home computer! This BASIC supports almost all the usual features, with float and string variables, multi-dimensional arrays, FOR-NEXT, GOSUB-RETURN, etc.

This interpreter uses some of the commands from Sinclair the original BASIC, but string functions are the newer RIGHT$/LEFT$ variety, rather than A$(2 TO 5) Sinclair-style slicing.

To exit BASIC use CTRL-D. To quit a basic program use CTRL-C. You can have a host x86-64 version of BASIC by:
```
$ make host
./hostbasic
```
`hostbasic` allows you to identify BASIC language errors before executing under ELKS. `hostbasic` does not support graphics, but the native version does.

BASIC Language
--------------
Variables names can be up to 8 alphanumeric characters (no special charactes such as "_") but start with a letter e.g. a, bob32
String variable names must end in $ e.g. a$, bob32$
Case is ignored (for all identifiers). BOB32 is the same as Bob32. print is the same as PRINT

Array variables are independent from normal variables. So you can use both:
```
LET a = 5
DIM a(10)
```
There is no ambiguity, since a on its own refers to the simple variable 'a', and a(n) referes to an element of the 'a' array. For string arrays, this is incompatible with some older verisons of BASIC where ordinary string variable's lengths needed preallocation, e.g. DIM A$(80) for an 80 character string. Here, all string variable values are managed dynamically.

```
Arithmetic operators: + - * / MOD
Comparisons: <> (not equals) > >= < <=
Logical: AND, OR, NOT
```

Expressions can be numerical e.g. 5*(3+2), or string "Hello "+"world".
Only the addition operator is supported on strings (plus the functions below).

Commands
```
PRINT <expr>;,<expr>... e.g. PRINT "A=";a
LET variable = <expr> e.g. LET A$="Hello"
variable = <expr> e.g. A=5
GOTO lineNumber
REM <comment> e.g. REM ** My Program ***
STOP
CONT (continue from a STOP)
INPUT [string prompt,] variable e.g. INPUT a$ or INPUT a(5,3)
IF <expr> THEN cmd e.g. IF a>10 THEN a = 0: GOTO 20, "ELSE" is not supported
FOR variable = start TO end STEP step
NEXT variable
GOSUB lineNumber
RETURN
DIM variable(n1,n2...)
READ var
DATA NumberOrString [,NumberOrString...]
RESTORE [lineNumber]
CLS removes all text and graphics
PAUSE milliseconds
POSITION x,y sets the cursor
NEW clears the current program from memory
LIST [start],[end] e.g. LIST or LIST 10,100
RUN [lineNumber]
LOAD "filename" loads filename.bas
SAVE "filename" saves filename.bas
SAVE+ "filename" saves filename.bas, sets auto-run on load
DELETE "filename" deletes filename.bas
DIR list all *.bas files in current directory
MODE number (set graphics mode, e.g MODE 1 to use PLOT/DRAW/CIRCLE)
    MODE 0  # return to text mode
    MODE 1  # default graphics (EGA if EGAMODE= in environment, otherwise VGA)
    MODE 2  # VGA 640x480 16 color
    MODE 3  # EGA 640x350 16 color
    MODE 4  # CGA 320x200 4 color
    MODE n  # BIOS mode number n (decimal).
        Modes 5+  dont't support CLS and also position 0,0 at top left
COLOR fg,bg
PLOT x,y
DRAW x,y
CIRCLE x,y,r
INPB(port) (IO read byte from `port`)
INPW(port) (IO read word from `port`)
OUTB port, value (IO write byte `value` from `port`)
OUTW port, value (IO write word `value` from `port`)
POKE offset,segment,value (Memory write byte `value` to `segment:offset`)

Architecture-specific
PIN pinNum, value (0 = low, non-zero = high)
PINMODE pinNum, mode - not implemented

Not yet implemented
RANDOMIZE [nmber]
```

"Pseudo-identifiers"
```
INKEY$ - returns (and eats) the last key pressed buffer (non-blocking). e.g. PRINT INKEY$
RND - random number betweeen 0 and 1. e.g. LET a = RND
PI - 3.1415926
```

Functions
```
LEN(string) e.g. PRINT LEN("Hello") -> 5
VAL(string) e.g. PRINT VAL("1+2") runtime evaluated
INT(number) e.g. INT(1.5)-> 1
ABS(number) e.g. ABS(-1)-> 1
CHR$(number) e.g. CHR$(32) -> " "
CODE(string) e.g. CODE(" ") -> 32
STR$(number) e.g. STR$(2) -> "2"
LEFT$(string,n)
RIGHT$(string,n)
MID$(string,start,n) extracts a substring from a given string
COS(x) cosine
SIN(x) sine
TAN(x) tangent
ACS(x) arc cosine
ASN(x) arc sine
ATN(x) arc tangent
EXP(x) e exponential
LN(x) natural logarithm
POW(x,y) e.g. POW(2,0.5) -> 1.414 square root of 2
HEX$(number) e.g. HEX$(25923) -> "6543"
PEEK(offset,segment) Memory read byte from `segment:offset`

Architecture-specific:
PINREAD(pin)
ANALOGRD(pin) - not implemented

Not implemented, but alternatives exist:
x^y x to the y power, use POW(x,y) instead
SQR(number) square root, use POW(number,0.5) instead
SGN(number) get sign, use IF number < 0 etc instead

Not yet implemented:
SCREEN$(line,col) - retrieves the ASCII code of the character displayed at (line,col)
ATTR(line,col) -  returns the attribute byte for this screen position. The byte contains information about the color attributes of the pixel block at (line,col)
POINT(x,y) - checks whether the pixel at screen coordinates (x, y) is ON or OFF
```
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