Mapping the Atari Exclusive! (Part 3)

The table continues to store variable names, even those no longer used in your program and those used in direct mode entry. It is not cleared by SAVEing your program. LOADing a new program replaces the current VNT with the one it retrieves from the file. You must LIST the program to tape or disk to save your program without these unwanted variables from the table. LIST does not SAVE the variable name or variable value tables with your program. It stores the program in ATASCII, not tokenized form, and requires an ENTER command to retrieve it. You would use a NEW statement to clear the VNT in memory once you have LISTed you program.

Each variable name is stored in the order it was entered, not the ATASCII order. With numeric (scalar) variables, the MSB is set on the last character in a name. With string variables, the last character is a " $" with the MSB (BIT 7) set. With array variables, the last character is a "(" with the MSB set.

In array variables, bytes five and six contain the size plus one of the first dimension of the array (DIM + 1; LSB/MSB), and bytes seven and eight contain the size plus one of the second dimension (the second DIM + 1; LSB/MSB).

In string variables, bytes five and six contain the current length of the variable (LSB/MSB), and bytes seven and eight contain the actual dimension (up to 32767).

Following that is another count byte for the start of this line to the start of the next statement. These count values are set only when tokenization for the line and statement are complete.

Tokenization takes place in a 256 byte ($100) buffer that resides at the end of the reserved OS RAM (pointed to by locations 128,129; $80,$81).

Each GOSUB entry consists of four bytes in this order: a zero to indicate a GOSUB, a two-byte integer line number on which the call occurred, and an offset into that line so the RETURN can come back and execute the next statement.

Each FOR-NEXT entry contains 16 bytes in this order: first, the limit the counter variable can reach; second, the step or counter increment. These two are allocated six bytes each in BCD format (12 bytes total). The 13th byte is the counter variable number with the MSB set; the 14th and 15th are the line number and the 16th is the line offset to the FOR statement.

The minimum number of spaces between TABS is three. If you POKE 201,2, it will be treated as four spaces, and POKE 201,1 is treated as three spaces. POKE 201,0 will cause the system to hang when it encounters a PRINT statement with commas. To change the TAB key settings, see TABMAP (locations 675 to 689; $2A3-$2B1). PTABW is not reset to the default value by pressing RESET or changing GRAPHICS modes (unlike TABMAP). PTABW works in all GRAPHICS modes, not merely in text modes. The size of the spaces between items depends on the pixel size in the GRAPHICS mode in use. For example, in GR.0, each space is one character wide, while in GR.8 each space is one-half color clock (one dot) wide.

Locations 212 to 255 ($D4 to $FF) are reserved for the floating point package use. The FP routines ae in ROM, from locations 55296 to 57393 ($D800 to $E031). These page zero locations may be used if the FP package is not called by the user's program. However, do not use any of these locations for an interrupt routine, since such routines might occur during an FP routine called by BASIC, causing the system to crash.