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Implementation Restrictions

The name of a variable or label, and the length of a literal string may not exceed 250 bytes. More characters than 250 will be truncated. (Can be changed from rexx.h)

Numbers follows C restrictions, thus integers are long and real numbers are held as double.

The FOR and simple counts on a DO instruction, and the right-hand term of an exponentiation may not exceed maximum long number.

The control stack (for DO, IF, CALL, etc.) is limited to a nesting level of 256 and from the internal stack of the Operating system. For MsDos REXX the maximum recursive level will be around 70-80.

Functions and subroutines cannot be called with more than 15 arguments (Can be changed from rexx.h).

Input and Output cannot be redirected for commands executed through INT2E.

Variables

Variables are held in a binary tree, where the tree is balanced when one branch starts to become very big. Even though the variables are stored as a bintree there is an internal cache system for the faster access.

Each variable in rexx is a length prefix string, and it is kept in memory in 3 different types, long, real, string according the last operation that affect that variable.

ie.a = 2/* will be kept as string (Length-prefixed) */
 a = 2 + 1/* will be kept as integer (long) */
 a = 2 + 0.1/* will be kept as real (double) */

The advantage of the above scheme is that numerical operations are performed much faster than the other algorithms. The main disadvantage is on the integer operations. 32 bit integers have a maximum of 2billion, so if you try something like this 

	factorial = 1
	do i = 1 to 50
		factorial = factorial * i
	end
will result to 0 instead of the factorial of 50! To find the correct result you have to fool the interpreter to think that factorial is real and not integer, this can be done if you write factorial = 1.0 ....

You can easilly translate a variable to any format you like with the following instructions 

	a = a + 0.0   /* will translate a to real */
	a = trunc(a)  /* will translate a to integer */
	a = a || ''   /* will translate a to string */

Sometimes it is very important to know how a variable is kept in memory (usually for the INTR function) so there is an extra option in DATATYPE function "TYPE" that returns the way one variable is hold. 

	DATATYPE(2,"TYPE")     -> "STRING"
	DATATYPE(2+0.0,"TYPE") -> "REAL"
	DATATYPE(2+0,"TYPE)    -> "INT"

C routines are used for the translation of string to number, so a string like '- 2' will be reported by DATATYPE as a NUMber when rexx tries to evaluate it as a number it will return a value of 0 instead of -2, because of the spaces between the sign and the number.

Stems

substitution to stems may be anything including strings with any character. No translation to uppercase is done to subscripts 
	lower = 'ma'  ; stem.lower   -> 'STEM.ma'
	upper = 'MA'  ; stem.upper   -> 'STEM.MA'
Stems can be initialized with a command like stem. = 'Initial value'

Functions

Even though the depth nesting limitation is 250, stack in the 16bit-DOS version it will crash with nesting near 90.

If a program or a dos command is used as an external rexx function the interpreter will call COMMAND interpreter to execute the command with its output redirected to a dummy file. The dummy file will be created in the current directory or in a temporary directory that is pointed by TEMP environment variable.

TRANSLATE sometimes wont work properly for strings with characters above ASCII 127. Works OK for Greek character set.

VARTREE wont work properly with variables with non-printable characters

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