ST20-C1 datasheet(46/205 Pages) STMICROELECTRONICS

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4.7 Evaluation of boolean expressions

46/205

The following shows the correspondence between C logical expressions and ST20-C1

instructions.

X and Y represent expressions,and K represents a constant. The symbol

‘

¬’ is a logical NOT (see section 4.7.1).

true

ldc 1

false

ldc 0

¬(X)

X== Y

X; Y; sub; eqc 0

X!= Y

¬(X; Y; sub; eqc 0)

X== K

X; eqc K

X!= K

¬(X; eqc K)

X> Y

X; Y; gt

X< Y

Y; X; gt

X>= Y

¬(Y; X; gt)

X<= Y

¬(X; Y; gt)

Further optimizations can be made to the ‘not equals’ comparison when followed by a

conditional jump.

X != Y; cj L

X; Y; sub; cj L

X != 0; cj L

X; cj L

4.7.1

Evaluation of NOT

If zero represents false and 1 represents true, then logical NOT can be performed by

eqc 0.

4.7.2

Evaluation of AND and OR

For evaluation of logical AND and OR operations, the instruction sequence depends

on whether strict or non-strict evaluation is used, i.e. whether both operands are

always evaluated. This is important if side-effects may occur, such as a trap, or if the

second operand is not always defined, as in:

if ((ptr != NULL) && (ptr->tag == TAG_VAL)) ...

In this example, ptr->tag is not defined if ptr is NULL. For languages such as ANSI

C, non-strict evaluation is required, so the following short-cuts must be used:

X OR Y

¬(¬(X); cj L; ¬(Y); L:)

X AND Y

X; cj L; Y; L:

For non-strict evaluation, the following laws should be applied to the compilation of

conditional expressions before code is generated to ensure that the jump is taken as

early as possible:

¬(X AND Y)

(

¬X) OR (¬Y)

¬(X; cj L; Y; L:)]

¬(X OR Y)

(

¬X) AND (¬Y)

¬(X); cj L; ¬(Y); L:]

(X OR Y); cj L

(

¬X); cj M; Y; cj L; M:

(X AND Y); cj L = X; cj L; Y; cj L

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