Microprocessor 4

3Additional Concepts

The present chapter completes the study of ISA (Instruction Set Architecture) with additional concepts linked to the instruction set and to execution. The former means the concepts of illegal, invalid, reserved and trusted instructions, instruction alignment, the instruction set’s orthogonality and the symmetry and the concept of pure, re-entrant and relocatable code. The subjects of execution time, memory requirements, execution modes, portability and virtualization are then addressed. We conclude with some important aspects, which are hardware and software compatibilities, measuring execution performances and the criteria for choosing a microprocessor or MPU (MicroProcessor Unit).

3.1. Concepts associated with the instruction set and programming

This section addressed additional concepts linked to instruction sets. It completes § V1-3.5.

3.1.1. llegal, non-implemented, invalid, reserved and trusted instructions

An illegal instruction is an instruction that does not exist. It has not been implemented by the designer. In general, it is the first word of the instruction that does not correspond to the instruction set. For example, the MC6802 has 72 instructions of variable size and 192 valid machine codes out of 256, and so 59 are illegal. Modern MPUs (MicroProcessor Unit) generally raise a trap (cf. § 5.4), which will divert execution towards a routine for processing the exception (this is the case with the Arm^® family, for example). Some instructions ...