Similarly, the *NextBASIC return stack area* which was mentioned in *Chapter 4* maintains a record of your program's currently-active subroutine and procedure calls, loops and error handlers. The byte pointed by the RAMTOP variable shows the maximum address that is reserved for use by a *NextBASIC* program. We will visit this in more detail, in the section about the **CLEAR** command below. Finally the *User Defined Graphics area* holds all the definitions to the system's UDGs as discussed in *Chapter 13*. ### NextBASIC Data Structures *NextBASIC* stores numbers, strings, arrays, programming lines and **FOR...NEXT** loops in strictly defined forms called *data structures*. The following discuss all these data structures that are user accessible. Integer-based variables, arrays and control structures are not available to the user and are hidden by *NextZXOS* in protected memory areas so they're not covered here. Each line of *NextBASIC* program has the form:  ``` MSB LSB +---------+---------+-----------~ ~-----------+-----------------+ | 2 bytes | 2 bytes | ~ ~ | 0 0 0 0 1 1 0 1 | +---------+---------+-----------~ ~-----------+-----------------+ Line Text Text ENTER number Length + ENTER ``` Note that, in contrast with all other cases of *two-byte* numbers in the Z80N, the line number here is stored with its more significant byte (MSB) first: that is to say, in the order that you write them down (also known as *Big-Endian* order). A *numerical constant* in the program appears as ASCII text followed by its binary form, using the character **CHR$ 14** followed by *five bytes* for the number itself. The variables have different formats according to their features. The letters in the names should be thought as starting off in lower case. The available variants and their formats are: Number whose name is one letter only:  ``` +-----------------+----------+----+---------~ ~---------+ | 0 1 1 | Exponent |Sign| 4 mantis~ ~sa bytes | | | byte |Bit | ~ ~ | +-----------------+----------+----+---------~ ~---------+ [?] [?] ``` Number whose name is longer than one letter:  ``` +-----------------+-----------------~ ~-----------------+----------+----+---------~ ~---------+ | 1 0 1 | 0 ~ ~ 1 | Exponent |Sign| 4 mantis~ ~sa bytes | | | ~ ~ | byte |Bit | ~ ~ | +-----------------+-----------------~ ~-----------------+----------+----+---------~ ~---------+ Letter (≥60h) 2nd Letter Last Letter Value ``` Array of numbers:  ``` # of dimensions +-----------------+-------------+--------+---------~ ~---------+-------------~ ~-------------+ | 1 0 0 | 2 bytes | 1 byte | 2 bytes ~ ~ 2 bytes | 5 bytes each~ ~ | +-----------------+-------------+--------+---------~ ~---------+-------------~ ~-------------+ Letter (≥60h) Total length 1st dimension Last dimension Elements of elements and dimensions + 1 for # of dimensions ```