ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 25

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

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
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.
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 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. Secondary 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.
An instruction component
The instruction data value and prefixing
pfix
nfix
Table 3.4 Prefixing instr uction components
mnemonic
n
n
Figure 3.7 Instruction format
function code
7
prefix
negative prefix
4
3
data
name
0
3 Architecture
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