Compaq EV68A Architecture, Addressing, Integer Data Types, Floating-Point Data Types

The Architecture

The Alpha architecture performs byte shifting and masking with normal 64-bit, regis- ter-to-register instructions. The 21264/EV68A performs single-byte and single-word load and store instructions.

1.1.1 Addressing

The basic addressable unit in the Alpha architecture is the 8-bit byte. The 21264/ EV68A supports a 48-bit or 43-bit virtual address (selectable under IPR control).

Virtual addresses as seen by the program are translated into physical memory addresses by the memory-management mechanism. The 21264/EV68A supports a 44-bit physical address.

1.1.2 Integer Data Types

Alpha architecture supports the four integer data types listed in Table 1–1.

Table 1–1 Integer Data Types

Data Type	Description
Byte	A byte is 8 contiguous bits that start at an addressable byte boundary.
	A byte is an 8-bit value.
Word	A word is 2 contiguous bytes that start at an arbitrary byte boundary.
	A word is a 16-bit value.
Longword	A longword is 4 contiguous bytes that start at an arbitrary byte boundary. A
	longword is a 32-bit value.
Quadword	A quadword is 8 contiguous bytes that start at an arbitrary byte boundary.
Note:	Alpha implementations may impose a significant performance penalty
	when accessing operands that are not naturally aligned. Refer to the Alpha
	Architecture Handbook, Version 4 for details.

1.1.3 Floating-Point Data Types

The 21264/EV68A supports the following floating-point data types:

•Longword integer format in floating-point unit

•Quadword integer format in floating-point unit

•IEEE floating-point formats

–S_floating

–T_floating

•VAX floating-point formats

–F_floating

–G_floating

–D_floating (limited support)

1–2 Introduction 21264/EV68A Hardware Reference Manual