參數(shù)資料
型號(hào): ST20Cx
廠商: 意法半導(dǎo)體
英文描述: Instruction Set Reference Manual
中文描述: 指令集參考手冊(cè)
文件頁(yè)數(shù): 131/212頁(yè)
文件大?。?/td> 647K
代理商: ST20CX
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4
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
0
3
4
7
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