ST20CX datasheet(14/212 Pages) STMICROELECTRONICS

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Error conditions

Any errors that can occur in instructions which are defined in ter ms of the modulo

operators are indicated explicitly in the instruction description. For example the

div

(

divide) instruction indicates the cases that can cause overflow, independently of the

actual division:

if (Areg = 0) or ((Breg = MostNeg) and (Areg = -1))

{

Areg

′← undefined

IntegerOverflow

}

else

Areg

′← Breg / Areg

Breg

′← Creg

Creg

′← undefined

Checked operators

To simplify the description of

checked arithmetic, the operators ‘

checked’, ‘−checked’,

etc. are used to indicate operations that perform checked arithmetic on signed

integers. These operators signal an

IntegerOverflow if an overflow, divide by zero, or

other arithmetic error occurs. If no trap is taken, the operators also deliver the modulo

result.

A number of comparison operators are also used and there are versions of some of

these that treat the operands as unsigned integers. A full list of the operators used in

the instruction definitions is giv en in Table 1.7.

1.11 Functions

Type conversions

The following function is used to indicate a type conversion:

unsign(x)

causes the bit-pattern in x to be interpreted as an unsigned integer.

1.12 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

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