ST20C2 datasheet(17/212 Pages) STMICROELECTRONICS

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Addressing and data representation

The ST20 processor is a 32-bit word machine, with byte addressing and a 4 Gbyte

address space. This chapter explains how data is loaded from and stored into that

address space, explains how signed arithmetic is represented, and defines the

arithmetic significance of order ing of data items.

2.1

Word address and byte selector

A machine address is a single word of data which identifies a b yte in memory - i.e. a

byte address. It comprises two parts, a word address and a byte selector. The byte

selector occupies the two least significant bits of the word; the word address the thirty

most significant bits . An address is treated as a signed value, the range of which

starts at the most negative integer and continues, through zero, to the most positive

integer. This enables the standard comparison functions to be used on pointer

(address) values in the same way that they are used on numerical values.

Certain values can never be used as pointers because they represent reserved

addresses at the bottom of memory space. They are reserved for use by the

processor and initialization. In this text, names are used to represent these and other

values (e.g.

NotProcess.p, Disabling.p). A full list of names and values of constants

used in this book is given in section 1.9.

2.2

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 the larger of the two. Figure 2.1 shows the ordering of bytes in

words and memory for the ST20. Note that this ordering is compatible with Intel

processors, but not Motorola or SPARC.

Figure 2.1 Bytes in memory and words

words (wordlength is 32 bits)

X is a word-aligned byte address

X+n is the byte n bytes past X

X+7

X+6

X+5

X+4

X+3

X+2

X+1

X+0

Memory

(bytes)

X+3

X+2

X+1

X+0

MSB

LSB

24 23

16 15

8 7

X+7

X+6

X+5

X+4

MSB

LSB

24 23

16 15

8 7

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