RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
• 0-255 (an 8-bit value) in a 16-bit Thumb instruction.
real_num
Is the value passed in r7 to the handler to determine the function to
perform.
Usage
Thumb applications on ARM Linux use __svc_indirect_r7 to make kernel syscalls.
You can also use this feature to implement indirect SVCs.
Example
long __svc_indirect_r7(0) \
SVC_write(unsigned, int fd, const char * buf, size_t count);
#define write(fd, buf, count) SVC_write(4, (fd), (buf), (count))
Calling:
write(fd, buf, count);
compiles to SVC #0 with r0 = fd, r1 = buf, r2 = count, and r7 = 4.
Errors
When an ARM architecture variant or ARM architecture-based processor that does not
support an SVC instruction is specified on the command line using the --cpu option, the
compiler generates an error.
See also
• __value_in_regs
• --cpu=name on page 2-15
• SVC on page 4-130 in the Assembler Guide.
4.1.18 __value_in_regs
The __value_in_regs qualifier instructs the compiler to return a structure of up to four
integer words in integer registers or up to four floats or doubles in floating-point
registers rather than using memory.
__value_in_regs is a function qualifier. It affects the type of a function.
Syntax
__value_in_regs return-type function-name([argument-list]);
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