Many people misuse these terms that have quite specific meaning (at least
in the computer systems world). Often the terms are defined using the
other words!
Command
User input to an operating system to request some service.
e.g. ls is a Unix command
Directive
A request, to an assembler, from the programmer. Directives are
interspersed with machine instructions in an assembly language program.
Some MIPS directives are listed on pages A-51 through A-53 of our text.
e.g. .ascii "315" is an assembler directive.
Instruction
A word, interpreted by the CPU, that tells the control unit what operation
to perform.
A sequence of instructions, designed to perform some task, is called
a program.
The description of the complete instruction set for a CPU is called its Instruction
Set Architecture (ISA) .
The MIPS instructions are listed on pages A-55 through A-75 of our text;
also listed there are pseudoinstructions which are assembler
directives, not instructions.