Compaq ECQD2KCTE manual Addressing, Data Types, Byte, Word

Chapter 2

Basic Architecture

2.1 Addressing

The basic addressable unit in the Alpha architecture is the 8-bit byte. Virtual addresses are 64 bits long. An implementation may support a smaller virtual address space. The minimum vir- tual address size is 43 bits.

Virtual addresses as seen by the program are translated into physical memory addresses by the memory management mechanism.

Although the data types in Section 2.2 are described in terms of little-endian byte addressing, implementations may also include big-endian addressing support, as described in Section 2.3. All current implementations have some big-endian support.

2.2 Data Types

Following are descriptions of the Alpha architecture data types.

2.2.1 Byte

A byte is 8 contiguous bits starting on an addressable byte boundary. The bits are numbered from right to left, 0 through 7, as shown in Figure 2–1.

Figure 2–1: Byte Format

70

:A

A byte is specified by its address A. A byte is an 8-bit value. The byte is only supported in Alpha by the load, store, sign-extend, extract, mask, insert, and zap instructions.

2.2.2 Word

A word is 2 contiguous bytes starting on an arbitrary byte boundary. The bits are numbered from right to left, 0 through 15, as shown in Figure 2–2.

Basic Architecture 2–1