ST20C2 datasheet(25/212 Pages) STMICROELECTRONICS

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Instruction representation

The instruction encoding is designed so that the most commonly executed instructions

occupy the least number of bytes. This chapter describes the encoding mechanism

and explains how it achieves this.

A sequence of single byte

instruction components is used to encode an instruction.

The ST20 interprets this sequence at the instruction fetch stage of execution. Most

users (working at the level of microprocessor assembly language or high-level

languages) need not be aware of the existence of instruction components and do not

need to think about the encoding. The first section (4.1) has been included to provide

a background. The following section (4.2) need only concern the reader that wants to

implement a code generator.

4.1

Instruction encoding

4.1.1

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 b yte are a

function code, and the four least significant

bits are used to build an

instruction data value as shown in Figure 4.1.

Figure 4.1 Instruction format

The representation provides for sixteen instruction components (one for each

function), each with a data field ranging from 0 to 15.

There are three categories of instruction component. Firstly there are those that

specify the instruction directly in the function field. These are used to implement

primary instructions. Secondly there are the instruction components that are used to

extend the instruction data value - this process of extension is referred to as

prefixing.

Thirdly there is the instruction component

operate (opr) which specifies the instr uction

indirectly using the

instruction data value. opr is used to implement secondary

instructions.

4.1.2

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

instruction. Secondary instructions interpret it as the operation code for the instruction

itself.

function code

data

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