ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 18

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

3.2 Memory
3.2
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
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
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
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bit position
bit position
Memory
Word address and byte selector
Alignment
Figure 3.2 Storing a signed integer in different length objects
31
31
1 1
these bit values not related to integer value
signed integer value (-10) stored as a 32-bit object (word)
signed integer value (-10) stored as an 8-bit object (byte)
...
1
8
8
7
7
1 1 1 1 0 1 1 0
1 1 1 1 0 1 1 0
0
0