HANDS ON GUIDE

2 - Terminology and terms

The term ‘architecture’ is overused these days almost to the point of being meaningless, but in technical terms, its definition is precise and very simple.

An instruction-set architecture is simply the list of instructions a processor knows and can perform, and the way in which these instructions are used. It’s no more an architecture than the directions on the side of a packet of rolled oats are an architecture for making porridge.

However, many people, and therefore marketing departments, like impressive-sounding words, so it ends up being referred to as the x86 architecture, rather than a more helpful name such as the x86 instruction format.

The individual instructions that make up an instruction set have mnemonic names that describe their function. In the original 8086 specification, these were pretty self-explanatory; the ADD instruction, for instance, adds two numbers together. There are now hundreds of x86 instructions, as the list has continued to grow, but they’re all at least vaguely descriptive. ADD is a good example of a common feature of most (but not all) practical instructions – they contain the instruction itself and some data. In machine-language terms, the name of the instruction is known as the opcode (sometimes the operator), and the data is known as the operand. An instruction only ever has one opcode, but can have more than one operand. The formal definition for the x86 ADD instruction is as follows:

ADD src, dest

When an x86 processor encounters an ADD opcode, it expects two operands: a source value and destination value. It will then fetch the source value from its location and add it to the destination value. These are called source and destination because, at the machine-language level, data is manipulated using one of the processor’s local registers, which are small, dedicated areas of memory that contain just one number. The ADD instruction fetches the value of one register, adds it to the value of the second register, and then stores the result in the location of the latter.

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