ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

B.4 Support code requirements
Common VFP Subarchitecture Specification
When an instruction is bounced, control passes to the Undefined Instruction exception handler provided by
the operating system.
The operating system is expected to:
1. Perform a standard exception entry sequence, preserving process state and re-enabling interrupts.
2. Decode the bounced instruction sufficiently to determine whether it is a coprocessor instruction, and
if so, for which coprocessor.
3. Check whether the bounced instruction is conditional, and if it is conditional, check whether the
condition was passed. This ensures correct execution on implementations that perform the bounce
even for an instructions that would fail its condition code check.
4. Check whether the coprocessor is enabled in the access control register, and take appropriate action
if not. For example, in the lazy context switch case described in Context switching with the Advanced
SIMD and VFP extensions on page B1-69, the operating system context switches the VFP state.
5. Call an appropriate second-level handler for the coprocessor, passing in:
the instruction that bounced
the state of the associated process.
6. The second-level handler must indicate whether the bounced instruction is to be retried or skipped.
It can also signal an additional exception that must be passed on to the application.
7. Restore the original process, transferring control to an exception handler in the application context if
necessary.
If the bounced instruction is a VFP instruction, control is passed to a second-level handler for VFP
coprocessor instructions. For the Common VFP subarchitecture this:
1. Uses the FPEXC.EX and FPEXC.DEX bits to determine the bounced instruction and associated
handling. The three possible cases are:
FPEXC.EX == 0, FPEXC.DEX == 0
The bounce was synchronous. The exception-generating instruction is the instruction that
bounced:
If the exception-generating instruction is not a CDP instruction, or the version of the
subarchitecture is before version 3, the bounce was caused by an unallocated
instruction encoding or a VFP access permission fault. Branch to operating system
specific code that takes appropriate action.
If the exception-generating instruction is a CDP instruction, check whether the
bounce was caused by a VFP access permission fault:
— If it is a VFP access permission fault, branch to operating system specific
code that takes appropriate action.
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