Location information in a memory
use without knowledge of system of commands is absolutely impossible, after all the Assembler language represents symbolical record of digital codes of computer language, codes of commands of the processor. Certainly, for development of the software there are every possible software. To use them it is usually possible and without knowledge of system of commands of the processor. More often programming languages of high level, such as Pascal and SI are applied. However the knowledge of system of commands and the Assembler language allows to increase several times efficiency of some most important parts of the software of any micropro-cessor system - from the microcontroller to the personal computer. For this reason in this chapter we will consider the main types of commands which are available for the majority of processors, and features of their application. Each command, chosen (read) of memory the processor, defines algorithm of behavior of the processor on the next some steps. The code of command speaks about what operation should be executed to the processor and with what operands (that is codes of data) where to take initial information for command execution
and where to place result (if it is necessary). The code of command can borrow from one to several bytes, and the processor learns about that, how many command byte he should read, from the first byte read by it or a word. In the processor the code of command is deciphered and will be transformed to a set of the micro operations which are carried out by separate knots of the processor. But to the developer of microprocessor systems this knowledge isn't too important, to it the result of performance of this or that command is important only. 2.3.1. Addressing of operands The most part of commands of the processor works with codes of data (operands). One commands demand entrance operands (one or two), others give out target operands (one operand is more often). Entrance operands are called still as operands sources, and days off are called as operands receivers. All these codes of operands (entrance and target) should settle down somewhere. They can be in internal registers of the processor (the most convenient and fast option). They can settle down in system memory (the most widespread option). At last, they can be in input-output devices (the most exceptional case).