The Architecture

direct access to low-level hardware functions. PALcode supports optimizations for mul- tiple operating systems, flexible memory-management implementations, and multi- instruction atomic sequences.

The Alpha architecture performs byte shifting and masking with normal 64-bit, regis- ter-to-register instructions. The 21264/EV67 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/EV67 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/EV67 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/EV67 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

Alpha 21264/EV67 Hardware Reference Manual

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Compaq EV67, 21264 specifications Architecture, Addressing, Integer Data Types, Floating-Point Data Types