ST20CX datasheet(27/212 Pages) STMICROELECTRONICS

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4.1.4

Secondary instructions

The ST20 encodes all other instructions (secondary instructions) indirectly using the

instruction data value.

The instruction component

opr causes the instruction data value to be interpreted as

the operation code of the instruction to be executed. This selects an operation to be

performed on the values held in the integer stack. This allows a further 16 operations

to be encoded in a single byte instruction. However the prefix instr uctions can be used

to extend the instruction data value, allowing any number of operations to be

performed.

Secondar y instructions do not have an operand specified b y the encoding, because

the instruction data value has been used to specify the operation.

To ensure that programs are represented as compactly as possible, the operations are

encoded in such a way that the most frequent secondary instructions are represented

without using prefix instr uctions.

4.1.5

Summary of encoding

The encoding mechanism has important consequences.

• Firstly, it simplifies language compilation, by providing a completely uniform

way of allowing a primary instruction to take an operand of any size up to the

processor word-length.

• Secondly, it allows these operands to be represented in a form independent of

ajw n

adjust workspace

call n

call

cj n

conditional jump

eqc n

equals constant

j n

jump

ldc n

load constant

ldl n

load local

ldlp n

load local pointer

ldnl n

load non-local

ldnlp n

load non-local pointer

stl n

store local

stnl n

store non-local

mnemonic

name

opr

operate

Table 4.3 Operate instruction

mnemonic

name

Table 4.2 Primary instructions

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