ST20-C1 datasheet(44/205 Pages) STMICROELECTRONICS

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4.6 Comparisons and jumps

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Implementation of languages with different representations of

true and false

It is easy to implement programming languages that use a different representation of

true and false. For example, using

eqc X; not; adc 1

in place of

eqc X and

gt; not; adc 1

in place of

gt, does not affect the representation of a false result, but changes the

representation of

true to -1, which is used in some programming languages.

4.6.2

Comparison

The primary instruction

eqc n loads Areg with a truth value — true if Areg is initially

equal to the instruction operand (

n), false otherwise. Breg and Creg are unaffected.

gt and gtu take integer operands in Areg and Breg and produce a boolean result

which is loaded into Areg. They also load the value in Creg into Breg, saving a copy

of the initial Areg in Creg.

The

gt instruction loads Areg with true if Breg > Areg, false otherwise, treating Areg

and Breg as signed values. Similarly

gtu loads Areg with true if the unsigned value of

Breg is greater than the

unsigned value of Areg; false otherwise.

4.6.3

Jump and conditional jump

There are two relative jump instructions; both are primary instructions.

The unconditional jump instruction,

jn, adds its operand (n) to the address of the

instruction immediately following it and puts the result into Iptr, thus transferring

execution to another part of the program.

The conditional jump instruction,

cj n, performs a jump if the value in Areg is 0 and

does not affect the evaluation stack. If the value in Areg is not 0

cj rotates the value in

Areg to the bottom of the evaluation stack and continues with the next instruction.

Consequently

cj n serves as ‘jump if false’ provided that the language being imple-

mented interprets 0 as

false (see section 4.6.1).

4.6.4

Conditional transfer of control

The conditional expressions used in a conditional branch of an if construct are

compiled using the conditional jump. The statement:

if (

E){

}

This compiles to:

E; cj L;

P; j ENDIF;

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