gas for x86 GAS assembly, gcc for GNU C, gas64 for x86_64 GAS assembly, clang for clang, llvm for LLVM IR.

The compiler will emit GAS assembler code to a .asm file which will then be compiled to an object file using 'as'. This is fbc's original x86 code generation backend.

The compiler will emit GAS assembler code (64-bit) to a .asm file which will then be compiled to an object file using 'as'.
On Darwin, use gcc, clang or llvm. GAS64 does not currently emit native Mach-O sections.

The compiler will emit C code to a .c file which will then be compiled to an .asm file using 'gcc' as a high level assembler. The C backend is intended to make FB portable to more platforms than just x86. This requires gcc to be installed so that fbc can invoke it to compile the C code, also see Installing gcc for -gen gcc.

The compiler will emit C code to a .c file which will then be compiled to an .asm file using 'clang' as a high level assembler.

The compiler will emit LLVM IR code to a .ll file which will then be compiled to an .asm file using 'llc'. The LLVM backend is still a work in progress. It is intended for the same purpose as the C backend, and could theoretically solve some of the C backend's problems, such as debugging meta data support.