ST20-C1 datasheet(18/205 Pages) STMICROELECTRONICS

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3.2 Memory

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Figure 3.2 Storing a signed integer in different length objects

3.2

Memory

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

address space. This section explains how data is arranged in that address space. The

address of an object is the address of the base, i.e. the byte with the lowest address.

3.2.1

Word address and byte selector

A machine address, or pointer, is a single word of data which identifies a byte 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 arithmetic and comparison functions to be used on pointer 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. A full list of names and values of constants used in this

manual is given in Appendix A.

In particular, the null process pointer (known as

NotProcess) has the value MostNeg,

since zero could be a valid process address.

3.2.2

Alignment

A data object is said to be

word-aligned if it is at an address with a byte selector of

zero, i.e. the full address of the object is divisible by 4. Similarly, a data object is said to

11 11 0110

these bit values not related to integer value

bit position

11 11 0110

bit position

...

signed integer value (-10) stored as an 8-bit object (byte)

signed integer value (-10) stored as a 32-bit object (word)

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