ST20-C1 datasheet(27/205 Pages) STMICROELECTRONICS

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3 Architecture

The mnemonics and names for the primary instructions are listed in Table 3.5.

3.4.4

Secondary instructions

The ST20 encodes all other instructions, known as

secondary instructions, indirectly

using the instruction data value.

The function code

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 evaluation stack, so that a further 16 operations

can be encoded in a single byte instruction. The

pfix instruction component can be

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

encoded.

Secondary 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 frequently used secondary instructions are

represented without using prefix instructions.

For example, the instruction

add is encoded by:

opr 4

The instruction

and is encoded by:

opr F9

mnemonic

name

adc n

add constant

ajw n

adjust work space

fcall n

function call

cj n

conditional jump

eqc n

equals constant

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

Table 3.5 Primary instructions

mnemonic

name

opr n

operate

Table 3.6 Operate instruction

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