ST20-C1 datasheet(25/205 Pages) STMICROELECTRONICS

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3 Architecture

programmers, working at the level of microprocessor assembly language or high-level

language, need not be aware of the existence of instruction components and do not

generally need to consider the encoding.

This section has been included to provide a background. Appendix C discusses

consequential issues which need to be considered in order to implement a code

generator.

3.4.1

An instruction component

Each instruction component is one byte long, and is divided into two 4-bit parts. The

four most significant bits of the byte form a

function code, and the four least significant

bits are used to build an

instruction data value as shown in Figure 3.7.

Figure 3.7 Instruction format

This representation provides for sixteen function code values (one for each function),

each with a data field ranging from 0 to 15.

Instructions that specify the instruction directly in the function code are called

primary

instructions or functions. There are 13 primary instructions, and the other three

possible function code values are used to build larger data values and other instruc-

tions. Two function code values,

pfix and nfix, are used to extend the instruction data

value by prefixing. One function code

operate (opr) is used to specify an instruction

indirectly using the

instruction data value. opr is used to implement secondar y instruc-

tions or operations.

3.4.2

The instruction data value and prefixing

The data field of an instr uction component is used to create an instruction data value.

Primary instructions interpret the instruction data value as the operand of the instruc-

tion. Secondar y instructions interpret it as the operation code for the instruction itself.

The instruction data value is a signed integer that is represented as a 32-bit word. For

each new instruction sequence, the initial value of this integer is zero. Since there are

only 4 bits in the data field of a single instruction component, it is only possible for

most instruction components to initially assign an instruction data value in the range 0

to 15. Prefix components are used to extend the range of the instruction data value.

mnemonic

name

pfix n

prefix

nfix n

negative prefix

Table 3.4 Prefixing instr uction components

function code

data

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