ST20-C1 datasheet(26/205 Pages) STMICROELECTRONICS

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3.4 Instruction encoding

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One or more prefixing components may be needed to create the full instruction data

value. The prefixes are shown in Table 3.4 and explained below.

All instruction components initially load the four data bits into the least significant four

bits of the instruction data value.

pfix loads its four data bits into the instruction data value, and then shifts this value up

four places. Consequently, a sequence of one or more prefixes can be used to extend

the data value of the following instruction to any positive value. Instruction data values

in the range 16 to 255 can be represented using one

pfix.

nfix is similar, except that it complements all 32 bits of the instruction data value before

shifting it up, thus changing the sign of the instruction data value. Consequently, a

sequence of one or more

pfixes with one nfix can be used to extend the data value of

a following instruction to any negative value. Instruction data values in the range -256

to -1 can be represented using one

nfix.

When the processor encounters an instruction component other than

pfix or nfix,it

loads the 4-bit data field into the instruction data value. The instruction encoding is

now complete and the instruction can be executed. The instruction data value is then

cleared so that the processor is ready to fetch the next instruction component, by

building a new instruction data value.

For example, to load the constant 0x11, the instruction

ldc 0x11 is encoded with the

sequence:

pfix 1; ldc 1

The instruction

ldc 0x2A68 is encoded with the sequence:

pfix 2; pfix A; pfix 6; ldc 8

The instruction

ldc -1 is encoded with the sequence:

nfix 0; ldc F

3.4.3

Primary Instructions

Research has shown that computers spend most time executing a small number of

instructions such as:

• instructions to load and store from a small number of ‘local’ variables;

• instructions to add and compare with small constants; and

• instructions to jump to or call other parts of the program.

For efficiency, in the ST20 these are encoded directly as primary instructions using

the function field of an instruction component.

Thirteen of the instruction components are used to encode the most important opera-

tions performed by any computer executing a high level language. These are used (in

conjunction with zero or more prefixes) to implement the primary instructions. Primary

instructions interpret the instruction data value as an operand for the instruction. The

mnemonic for a primary instruction always includes this operand, shown in this

manual as

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