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TurboBasic XL v1.5 disassembly

This is a disassembly of the TurboBasic XL version 1.5 basic interpreter for the Atari XL computers, in MADS assembler format.

To assemble you need MADS from http://mads.atari8.info/ and my mkatr tool, from https://github.com/dmsc/mkatr/.

The source is assembled into two binaries:

The original, unmodified interpreter as TB15.COM,
A version with some fixes as TB.COM.

This is a work in progress, but most labels have proper name and the code has many comments.

This code was tested to assemble at different address than the original binary without breaking.

Relocation

The provided Makefile generates a version of the interpreter that relocates itself to the lowest address posible, by reading MEMLO and copying the code to just above the value. This gives more memory to the BASIC programs, depending on the DOS version.

To assemble this version, the procedure is:

The source is assembled at two different MEMLO values, $3000 and $2000.
A C program (get-reloc.c) is included that reads the two binaries and builds a table with all address that need to be patched in runtime, and includes the table inside the binary.
Code is included in the binary that at runtime, reads the MEMLO value and patches all addresses in the relocation table, then copies the code to the final memory address.

To assemble this version, use the tb_lowmem define, or use the included Makefile.

Fixed Bugs

The source has fixes for a few interpreter bugs present in all other TurboBasic XL versions:

When adding or deleting lines when inside a FOR loop, the runtime stack is not correctly adjusted, for example this program:
```
10 ? "START"
20 FOR J=0 TO 10
25 ? "J=";J
30 IF J=5 THEN DEL 10,10
40 NEXT J
50 END
```
The interpreter exits the FOR on the iteration 5, instead of counting up to 10.
A bug with the parsing of IF statements without ENDIF, the end of statement is incorrectly checked. This is an example program:
```
10 ? 1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1:? 1;1;1;1;1;1;1;1;1;1;;;;;;:IF 1
20 ENDIF
```
If you remove one ";", the program runs correctly, but as shown it prints "ERROR 12".

In the PRINT statement, if the last token printed ends in $12 or $15, for example a CONTROL-R or CONTROL-U character on a string, the interpreter omits the new-line at the end, as it incorrectly assumes that the statement ended in a "," or ";".

This program shows the problematic statements:

20 ? "LINE 1:x" : REM   Ok, prints a new-line at end
30 ? "LINE 1:─" : REM   BUG, does not print the new-line
40 ? "LINE 1:▄" : REM   BUG, same as above
60 ? "LINE 1:";1.23456713 : REM   OK, prints a new-line at end
50 ? "LINE 1:";1.23456712 : REM   BUG, does not print the new-line
70 ? "LINE 1:";1.23456715 : REM   BUG, same as above

Detection of PAL/NTSC. The original TurboBasic XL assumes PAL ANTIC for the TIME$ function and the TIME$= statement, this means only 50 jiffies per second, so the functions return an incorrect value in NTSC computers.

This version includes code at startup that counts the number of scan-lines in a screen to detects the ANTIC type. If NTSC is detected the code is changed to return the correct values assuming 60 jiffies per second.

Note that both values are not exact, the real values are 49.86 and 59.92 jiffies per second in PAL and NTSC respectively, but for the intended usage the given values are close enough.
The DEC() function accepts lower-case hexadecimal digits.