ST20C2 datasheet(26/212 Pages) STMICROELECTRONICS

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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. However two instruction components are used to extend the range of the

instruction data value. Hence one or more prefixing components may be needed to

create the correct instruction data value. These are shown in Table 4.1 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.

nfix is similar, except that it complements the instruction data value†

before shifting it up. Consequently, a sequence of one or more prefixes can be

included to extend the value. Instruction data values in the range -256 to 255 can be

represented using one prefix instr uction.

When the processor encounters an instruction component other than

pfix or nfix,it

loads the data field into the instr uction data value. The instruction encoding is now

complete and the instruction can be executed. When the processor is ready to fetch

the next instruction component, it starts to create a new instruction data value.

4.1.3

Primary Instructions

Research has shown that computers spend most of the time executing 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 therefore, these are encoded directly as

primary instructions using the function field of an instr uction component.

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

operations 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 will therefore normally include a

this operand - n - when referenced.

The mnemonics and names for the primary instructions are listed in Table 4.2.

mnemonic

name

pfix n

prefix

nfix n

negative prefix

Table 4.1 Prefixing instr uction components

mnemonic

name

adc n

add constant

Table 4.2 Primary instructions

† Note that it inverts all 32 bits of the instruction data value.

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