ST20-C1 datasheet(178/205 Pages) STMICROELECTRONICS

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Compiling for the ST20-C1

C.1

Generating prefix sequences

Prefixing is intended to be performed by a compiler or assembler. Prefixing b y hand is

not advised.

Normally a value can be loaded into the instruction data value by a variety of different

prefix sequences . It is important to use the shortest possible sequence as this

enhances both code compaction and execution speed. The best method of optimizing

object code so as to minimize the number of prefix instructions needed is shown

below.

C.1.1 Prefixing a constant

The algorithm to generate a constant instruction data value

e for a function op is

described by the following recursive function.

prefix( op , e ) = if (e

< 16 AND e

op( e )

else if

16)

{prefix( pfix, e >> 4 ); op( e

∧ #F )}

else if

< 0)

{prefix( nfix, (~e) >> 4 ); op( e

∧ #F )}

where ( op, e ) is the instruction component with function code op and data

field e , ~ is a bitwise NOT, and >> is a logical right shift.

C.1.2 Evaluating minimal symbol offsets

Several primary instructions have an operand that is an offset between the current

value of the instruction pointer and some other part of the code. Generating the

optimal prefix sequence to create the instruction data value for one of these instruc-

tions is more complicated. This is because two, or more, instructions with offset

operands can interlock so that the minimal prefix sequences for each instruction is

dependent on the prefixing sequences used for the others.

For example consider the interlocking jumps below which can be prefixed in two

distinct ways. The instructions

j and cj are respectively jump and conditional jump.

These are explained in more detail later. The sequence:

cj +16; j -257

can be coded as

pfix 1; cj 0; pfix 1; nfix 0; j 15

but this can be optimized to be

cj 15; nfix 15; j 1

which is the encoding for the sequence

cj +15; j -255

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