Compaq 21264 - Manual (Page 26)

1–2 Introduction Alpha 21264/EV67 Hardware Reference Manual
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.
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)
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.