Floating Point Contexts (changes)

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FPEmulator

» Floating point contexts

Floating point contexts

Multiple-context versions of the floating point system (version 4.12 or later of FPEmulator, indicated by an “M” suffix in the help string) allow rapid switching between processes using floating point, preserving the complete state of the system, including any pending asynchronous exceptions. A floating point context is an opaque block of memory used to hold the state of the system – an active context must be in an unpaged area of memory, such as the RMA. SWIs are provided to switch between contexts, and the floating point system provides a single default context which is activated when the module is loaded.

In all calls that take or return a context pointer, -1 is used to represent the default context. 0 indicates no context – if no context is active, then floating point instructions will generate undefined instruction exceptions. Where specified, some calls also accept -2 to denote the current context.

The contents of a floating point context should be regarded as a “black box”: software outside the floating point system itself should neither assume that it knows how to interpret them, nor alter them directly. However, the system does provide some facilities to allow inspection and modification of a floating point context.

Floating point context switching may be of use for performing floating point during interrupts or callbacks, or as part of threading schemes. But note that use of floating point instructions from SVC mode in versions of RISC OS prior to RISC OS 4 is only possible under the following constraints:

• The instruction must not use R13 or R14 in any way;
• R14 must be treated as a scratch register (ie. it will not necessarily be preserved by a floating point instruction);
• R13 must point correctly to a Full Descending stack with a sufficient amount of free space.

This rules out the use of floating point via the C compiler.

It should also be noted that on all versions of RISC OS floating point instructions will enable interrupts if they are handled by the emulator – this is possible even on systems fitted with hardware floating point.

All the new SWIs corrupt the N Z and C flags.

See also

• FPEmulator