ST20-C1 datasheet(13/205 Pages) STMICROELECTRONICS

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2 Notation

2.3.2

Conditions to instructions

In many cases, the action of an instruction depends on the current state of the

processor. In these cases the conditions are shown by an if clause; this can take one

of the following forms:

• if condition

statement

• if condition

statement

else

statement

• if condition

statement

else if

condition

statement

else

statement

These conditions can be nested. Braces, {}, are used to group statements which are

dependent on a condition. For example, the

cj (conditional jump) instruction contains

the following lines:

if (Areg = 0)

Iptr

′← next instruction + n

else

{

Iptr

′← next instruction

Areg

′← Breg

Breg

′← Creg

Creg

′← Areg

}

This says that if the value in Areg is zero, then the jump is taken (the instruction

operand, n, is added to the instruction pointer), otherwise the stack is popped and

execution continues with the next instruction.

2.4

Data structures and constants

A number of data structures have been defined in this man ual. Each comprises a

number of data slots that are referenced by name in the text and the instruction

descriptions.

These data structures are listed in the tables in Appendix A. Each table gives the

name of each slot in the structure and the word offsets from the base address of the

structure. A slot in a data structure is identified using the offset notation descr ibed in

section 2.2.5:

word[

base_address @ word_offset]

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