ST20-C1 datasheet(47/205 Pages) STMICROELECTRONICS

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4 Using ST20-C1 instructions

In other languages, evaluation of boolean expressions may be strict (for example, ADA

gives the programmer the choice) and so both expressions in dyadic logical opera-

tions may need to be evaluated.

Where

false is represented by 0, and true is represented by any fixed bit pattern other

than 0 (e.g.

true is always 1, or true is always -1), then the following transformations

apply:

X OR Y

X BITOR Y

X AND Y

X BITAND Y

and the bitwise instructions given in section 4.8 can be used:

Note that even for some non-strict evaluations, the above sequence may be prefer-

able. Where

Y is a simple boolean expression such as a local variable, its evaluation

does not cause any side-effects, and so it does no harm to implement a non-strict

evaluation using a bitwise operation.

4.8

Bitwise logic and bit operations

Bitwise logic and bit operations are provided by the instructions listed in Table 4.10.

The

not operation has only one operand that is taken from Areg. The result of this,

which is a bitwise inversion of all bits in the operand, is loaded into Areg, leaving Breg

and Creg unaffected.

and, or and xor are bitwise logical operations on two operands that are taken from

Areg and Breg. For each, the result is loaded into Areg. The data previously held in

Creg is popped into Breg and the initial Areg is left in Creg. These operations are

commutative.

bitld, bitst and bitmask are used for setting, clearing and testing bits of a word. bitld

returns the value of a single bit from a value in Breg,

bitst sets or clears a single bit

and

bitmask creates a mask with a single bit set. In each case the bit number is

initially in Areg and the result is put in Areg. For

bitld and bitst, the value containing

the bit to be tested, set or cleared is initially in Breg.

Mnemonic

Name

and

xor

exclusive or

not

bitwise not

bitld

bit load

bitst

bit store

bitmask

bit mask

rmw

memory read modify write

Table 4.10 Bitwise logic and bit instructions

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