Op code mnemonics:

Instructions  (statements)  in  assembly  language  are  usually  very  simple,  unlike  those that in  high-level languages. Usually, an opcode is a symbolic name for a single executable machine language instruction, and there is at least 1 opcode mnemonic defined for each machine language instruction. Typically each instruction consists of an opcode or operation plus zero or more operands. Most of the instructions refer to a single value, or a pair of values.  Operands may be either immediate (typically one byte values is coded in the instruction itself) or the addresses of data located elsewhere in storage. It is determined by the underlying processor architecture: the assembler simply reflects how this architecture works.