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4 Using ST20-C1 instructions

Accessing a word addressed array

Let

Wa_ptr be a pointer to an array Wa that starts at a word boundary, and in which all

component types are measured in words. Let

e be a subscript expression. The

address of component

e of Wa is

e; Wa_ptr; wsub;

or if

e is a constant expression this can be optimized to:

Wa_ptr; ldnlp e;

Accessing a byte addressed array

Similarly, let

Ba_ptr be a pointer to an array (Ba) which may start at any byte location,

and in which each component type is measured in bytes. Let

e be a subscript expres-

sion.

The address of component

e of Ba is:

e; Ba_ptr; add;

4.6

Comparisons and jumps

This section describes the arithmetical comparison instructions and their use in condi-

tional program behavior. Unconditional jumps are also described. Functions and

procedures are described in section 4.10, and evaluation of boolean expressions is

described in section 4.7.

Comparisons, conditional behavior and jumps are provided by the instructions listed in

Table 4.9.

4.6.1

Representation of true and false

The ST20 uses 0 as

false and 1 as true. These values are generated by predicate

operations (for example comparisons). They can be loaded with single byte load

constant instructions.

Mnemonic

Name

eqc n

equal to constant

greater than

gtu

greater than unsigned

order

orderu

order unsigned

cj n

conditional jump

jump

jab

jump absolute

Table 4.9 Comparison and jump instructions

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