[wit]

is this instruction valid: ADD [BX], 25 ??

[jaeusm]

Well, that depends on the processor. Assembly instructions are specific to processors. So it's impossible to know with the information you've given.

[wit]

say an intel processor for example.

[jaeusm]

For future reference, you need to be more specific than giving a company name. There are thousands of processors, with each having unique (or overlapping in some cases) instruction sets. For instance, the Motorola 68HCS12 has some different instructions than the Motorola 68HC11.

Intel makes many different processors, like the IXP400 series network processors, the IOP300 series I/O processors, the IXC1100 control plane processors, etc. When you have decided what the processor is, Google that processor with the string "instruction set" and you should probably be able to find a complete listing with examples.

[codemeisterx86]

It all depends on the processor and assembler. I myself have mostly used FASM for assembly programming. This compiles to the x86 processor series.

Also, why do you need to know if "ADD [BX], 25" is valid? It would be easier to answer this question if you provided us with more info.