ST20-C1 datasheet(16/205 Pages) STMICROELECTRONICS

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3.1 Values

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Architecture

This chapter describes the general architectural features of the ST20-C1 core which

are relevant to more than one instruction or group of instructions. Interrupts and traps

are described in Chapter 6 and support for multi-tasking is described in Chapter 7.

Other features which are related to specific tasks are descr ibed in Chapter 4. A full list

of constants and data structures is given in Appendix A.

The ST20-C1 instruction set covers:

• control flow

• arithmetic and logical operations

• bit field manipulations

• shifting and byte-swapping

• register manipulations

• memory access with various addressing modes and data sizes

• task scheduling

• direct input/output

3.1

Values

The ST20-C1 core supports data objects of different sizes, either signed or unsigned.

The sizes directly suppor ted are bytes (8-bit), half words (16-bit), words (32-bit) and

multiple words (64-bit, 96-bit etc.). Bytes, half-words and words may be loaded and

stored. Arithmetic operations are provided for signed words and multiple words. A half

word is called a

sixteen in the instruction names.

The most negative integer (0x80000000) is known as

MostNeg and the most positive

(0x7FFFFFFF) as

MostPos.

Boolean objects, taking one of the values

true or false, are also used by some instruc-

tions.

False is represented by the value 0 and true has the value 1. Section 4.7

describes how other values may be implemented for language compilation.

Several data structures are defined in this man ual. Each comprises a number of data

words (sometimes called

slots) that are referenced by name in the text and the

instruction descriptions and addressed as offsets from the base of the data structure.

A full list of these data structures and other constants is given in Appendix A.

3.1.1

Ordering of information

The ST20 is

little-endian - i.e. less significant data is always held in lower addresses.

This applies to bits in bytes, bytes in words and words in memory. Hence, in a word of

data representing an integer, one byte is more significant than another if its byte

selector is larger.

Figure 3.1 shows the ordering of bytes in words for the ST20.

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