ST20-C1 datasheet(17/205 Pages) STMICROELECTRONICS

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3 Architecture

Figure 3.1 Bytes and bits in words

For example, the most significant bit of a word is bit 31, and the most significant byte is

byte 3, consisting of bits 24 to 31. This ordering is compatible with Intel processors,

but not Motorola or SPARC.

For compatibility with other devices, a

swap32 instruction is provided to reverse the

order of bytes within a word.

3.1.2

Signed integers and sign extension

A signed object is stored in twos-complement format. A signed value may be repre-

sented by an object of any size. Most commonly a signed integer is represented by a

single word, but as explained, it may be stored, for example, in a 64-bit object, a 16-bit

object, or an 8-bit object. In each of these formats, all the bits within the object contain

useful information.

The length of the object that stores a signed value can be increased, so that the object

size is increased without changing the value that is represented. This operation is

known as

sign extension. All the extra bits that are allocated for the larger object, are

meaningful to the value of the signed integer; they must therefore be set to the appro-

priate value. The value for all these extra bits is the same as the value of the most

significant bit - i.e. the sign bit - of the smaller object. The ST20-C1 provides instruc-

tions that sign extend byte and half-word objects to words.

The example shown in Figure 3.2 shows how the value -10 is stored in a 32-bit

meaningful for the 32-bit object but not for the 8-bit object. These bits are set to 1 in

the 32-bit object.

Bytes in a word

Most

significant

Least

significant

Bits in a word

Most

significant

Least

significant

ST20-C1 Datasheet(PDF) 17 Page - STMicroelectronics