Qualifiers specific to the instructions in the group

A description of the instruction operation

Optional programming examples and optional notes on the instruction

4.1.1Subsetting 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.

4–2Alpha Architecture Handbook

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Compaq ECQD2KCTE manual Subsetting Rules, Floating-Point Subsets