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Compiler-specific Features
The function exits by setting the program counter to lr-4 and the CPSR to the value in
SPSR. No arguments or return values can be used with __irq functions.
Note
When compiling for a Thumb-only processor, the code is compiled to Thumb code
because interrupt handlers are entered in Thumb state. Otherwise, even when compiling
for Thumb using the --thumb option or #pragma thumb, any functions specified as __irq
are compiled for ARM.
See also
• --thumb on page 2-122
• #pragma thumb on page 4-73
• Chapter 6 Handling Processor Exceptions in the Developer Guide.
4.1.11 __packed
The __packed qualifier sets the alignment of any valid type to 1. This means that:
• there is no padding inserted to align the packed object
• objects of packed type are read or written using unaligned accesses.
The __packed qualifier applies to all members of a structure or union when it is declared
using __packed. There is no padding between members, or at the end of the structure.
All substructures of a packed structure must be declared using __packed. Integral
subfields of an unpacked structure can be packed individually.
Usage
The __packed qualifier is useful to map a structure to an external data structure, or for
accessing unaligned data, but it is generally not useful to save data size because of the
relatively high cost of access. Only packing fields in a structure that requires packing
can reduce the number of unaligned accesses.
Note
On ARM processors that do not support unaligned access in hardware, for example,
pre-ARMv6, access to unaligned data can be costly in terms of code size and execution
speed. Data accesses through packed structures must be minimized to avoid increase in
code size and performance loss.
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