Can somebody explain the differences between: masm, tasm, & nasm? Why can't I run tasm code on linux? Are they different languages? I thought that assembly language was unique for all systems.
TASM, MASM, and NASM are x86 assemblers.
Borland Turbo Assembler (TASM) and Microsoft Macro Assembler (MASM) are DOS/Windows-based, Netwide Assembler (NASM) is available for other platforms as well. TASM produces 16-bit/32-bit output, MASM and NASM also produce 64-bit output.
All of those assemblers take the x86 instruction set as input. That does not mean that assembler source files are identical and compatible, however.
Instruction syntax
Assemblers expect either the original syntax used in the Intel instruction set documentation - the Intel syntax - or the so-called AT&T syntax developed at the AT&T Bell Labs. AT&T uses mov src, dest, Intel uses mov dest, src, amongst other differences.
Windows assemblers prefer Intel syntax (TASM, MASM), most Linux/UNIX assemblers use AT&T. NASM uses a variant of the Intel syntax.
Assembler-specific syntax
Assemblers have their own syntax for directives affecting the assembly process, macros and comments. These usually differ from assembler to assembler.
Compatibility
TASM can assemble MASM sources in "MASM mode".
NASM can assemble TASM code in "TASM mode". So, in theory, you can take TASM code and assemble them using NASM on Linux using that mode. Of course, the code might still need adjustments. If the code have OS dependencies, these will require your attention as well as you move from Windows to Linux.