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{{fbdoc item="title" value="Differences from QB"}}----
Since version 0.17, ""FreeBASIC"" introduced a -lang command-line option, that is used to change the language compatibility mode. Use the -lang qb option when compiling to select the most QB compatible parser. All differences listed below assume that -lang qb was used.

==Architecture/Platform incompatibilities==
- ""FreeBASIC"" is written for 32-bit operating systems and a 32 bit DOS extender, and cannot utilize code which depends on 16-bit DOS, 16 bit assembly or memory model (segment & offset, XMS/EMS, ...).
- ##DEF SEG## is no longer necessary and will not work - any code which ##POKE##s to video memory this way will no longer function, however, for DOS it can be easily rewritten using DPMI features.
- ##CALL INTERRUPT## no longer functions, as it relies on 16-bit DOS. DOS interrupts can be called in the DOS32 version by using the DPMI library, but they might work slowly because of the 32bit-16bit-32bit mode changes the processor will have to perform.

==Changed due to ambiguity==
- A scalar variable and an array with the same name and suffix can no longer share the same name.
- ##SHARED## can't be used inside a ##SUB## or ##FUNCTION## as it resulted in shared variables not defined in the main program. A proper ##DIM SHARED## in the main program must be used.
- ##COMMON## declarations do not depend on the order they are made, variables are matched by name and for that reason named COMMON is no longer supported. All COMMON arrays are variable-length arrays in FB.
- If a single line ##[[KeyPgIfthen|IF]]## has an (unnecessary) colon directly after the ##THEN##, it has to be terminated by an ##[[KeyPgEndif|END IF]]## in FB. If that unneeded colon is removed, FB will behave as QB.

==Design differences==
- Graphics support was internally redesigned, see [[GfxLib|GfxLib overview]]
- ##CLEAR## is no longer used to reset all variables and set the stack. Variables must be reset one by one, and stack size can be changed in the compiler command line. The keyword ##CLEAR## is used to do memory fills in FB.
- String ##DATA## items must be enclosed in quotes in FB, in QB this was optional
- All functions must have been declared, even with a ##CALL## in ""FreeBASIC"". With ##CALL## it was possible to invoke prototype-less functions in ""QuickBASIC"". (to be supported in future with -lang qb)
- In ""FreeBASIC"" all arrays must be explicitly declared. (Interpreted ""QuickBASIC"" arrays are automatically created with up to 10 indices.)
- Strings use a null (char ##0##) terminator to be compatible with C libraries and the Windows API, fixed-length strings can't contain ##chr$(0)## chars for now.
- When ##INKEY##[##$##] reads an extended key (Num Pad, Arrows...) it returns a two character string. In FB the first character is ##CHR##[##$##]##(255)##, while in QB this first char is ##CHR$(0)##.
- With fixed length strings ""FreeBASIC"" gives the real length as ##[[KeyPgLen|LEN]]## plus one (null-char), even on ##[[KeyPgType|TYPE]]## fields.
- In ""FreeBASIC"", unused characters of fixed-length strings are set to ##0##, regardless of what "-lang" compiler option is used. In QB, unused characters are set to ##32## (space, or " ", in ASCII).
- When a fixed-length string is declared, but still not initialized, all characters are set to 0, both in ""FreeBASIC"" and QB.
- The arrays are stored in row-major order in FB, they were stored in column-major order in QB by default. Row major order: data with consecutive last index are contiguous. Column-major order: data with consecutive first index are contiguous. For example, if you have ##DIM A(1 TO 6, 1 TO 8)##, in row-major order the elements are stored such that ##A(3,5)## is followed in memory by ##A(3,6)##; in column-major order ##A(3,5)## is followed in memory by ##A(4,5)##.
- Programs don't stop anymore on runtime errors unless //[[CompilerOpte|-e]]// or //[[CompilerOptex|-ex]]// option is used in the command line. Using these options allow the use of QB style error handling (##ON ERROR##, ##RESUME##...).
- Octal numbers are written ##&o...##, whereas in QB they could be written as ##&o...## or ##&...##.
- In FB ##FOR## loops in subs/functions do not accept arguments received ##byref## as counters. A local variable must be used.
- FB's ##[[KeyPgLocate|LOCATE]]## does not respect the fourth and fifth arguments for cursor shape.
- FB's ##[[KeyPgScreengraphics|SCREEN]]## does not allow switching the visible or the work-page. Use ##[[KeyPgScreenset|SCREENSET]]## instead.
- FB's ##[[KeyPgColor|COLOR]]## does not allow a third argument for border color.
- FB's ##[[KeyPgTimer|TIMER]]## returns the number of seconds since the computer started, while QB's ##TIMER## returns the number of seconds since midnight. (""Win32"" and Linux only: No more wrapping at midnight! :))
- In QB a ##chr$(13)## inside a string did a ##CR+LF## when ##PRINT##ed. In FB the ##CHR(13)## prints just at what it is, a ##CR##.
- ##EOF## can no longer be used to detect an empty comms buffer. Empty buffer should be tested comparing ##Loc## with ##0## in FB. Also, for files opened in ##RANDOM## or ##BINARY## mode, ##EOF## returns non-zero already after reading exactly the file size, see ##[[KeyPgEof|Eof]]##.
- Integer variables do not signal overflow errors in FB, even with the //[[CompilerOptex|-ex]]// option on. Any QB code relying in catching integer overflow errors will not work in FB.

==Archaic commands==
- ##BSAVE## and ##BLOAD## can be used in FB only to save and retrieve screens or graphic buffers. They will work only if gfxlib is linked, this is, if a graphics screen mode is requested somewhere in the program. The console can't be saved with ##BSAVE## or retrieved with ##BLOAD##. The other use of ##BSAVE##-##BLOAD##, saving and loading full arrays, can be achieved easily with ##GET## and ##PUT##.
- ##FIELD## statement (for record definition at runtime) has been left aside. The keyword ##FIELD## is used in FB to specify field alignment in ##[[KeyPgType|TYPE]]## variables.
- PC Speaker commands no longer function: Any references to ##SOUND## or ##PLAY## statements will result in an error message. There is a third party library available to emulate this functionality, but it's not included with ""FreeBASIC"".
- Fake event-driven programming (##ON KEY##, ##ON PEN##, ##ON STRIG##, ##ON TIMER##) no longer works. They could be emulated by a separate library.
- ##MKSMBF$## and all the ##""MKxMBF$""## commands supporting the pre-QB4.0 Microsoft proprietary floating point format (MBF) are not implemented.
- The use of parenthesis in the arguments passed to a function to emulate by-value passing is not permitted. The ##CALL## quirk resulting in all arguments being passed by value, no longer works. The proper ##[[KeyPgByval|BYVAL]]## and ##[[KeyPgByref|BYREF]]## keywords must be used.
- ##FILES## is not implemented. Instead, PDS 7.1-compatible ##[[KeyPgDir|DIR]]##[##$##] can be used.
- ##IOCTL##, ##ERRDEV## and ##ERRDEV$##, low level functions to access hardware are not implemented as they were OS-dependent.
- ##CALL ABSOLUTE## to run inline machine code is no longer supported. Instead you can use ##[[KeyPgAsm|ASM]]...END ASM## blocks to insert inline assembler commands. Or use the ##ASM ...## one line command.

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