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4–2 Alpha Architecture Handbook
• Qualifiers specific to the instructions in the group
• A description of the instruction operation
• Optional programming examples and optional notes on the instruction
4.1.1 Subsetting Rules
An instruction that is omitted in a subset implementation of the Alpha architecture is not per-
formed in either hardware or PALcode. System software may provide emulation routines for
subsetted instructions.
4.1.2 Floating-Point Subsets
Floating-point support is optional on an Alpha processor. An implementation that supports
floating-point must implement the following:
• The 32 floating-point registers
• The Floating-point Control Register (FPCR) and the instructions to access it
• The floating-point branch instructions
• The floating-point copy sign (CPYSx) instructions
• The floating-point convert instructions
• The floating-point conditional move instruction (FCMOV)
• The S_floating and T_floating memory operations
Software Note:
A system that will not support floating-point operations is still required to provide the 32
floating-point registers, the Floating-point Control Register (FPCR) and the instructions to
access it, and the T_floating memory operations if the system intends to support the
OpenVMS Alpha operating system. This requirement facilitates the implementation of a
floating-point emulator and simplifies context-switching.
In addition, floating-point support requires at least one of the following subset groups:
1. VAX Floating-point Operate and Memory instructions (F_ and G_floating).
2. IEEE Floating-point Operate instructions (S_ and T_floating). Within this group, an
implementation can choose to include or omit separately the ability to perform IEEE
rounding to plus infinity and minus infinity.
Note:
If one instruction in a group is provided, all other instructions in that group must be
provided. An implementation with full floating-point support includes both groups; a
subset floating-point implementation supports only one of these groups. The individual
instruction descriptions indicate whether an instruction can be subsetted.