Linux86-64 Programming Model
RFLAGS
The flags register contains the system flags, such as the direction flag and the carry flag. The direction flag must be set to the "forward" (i.e., zero) direction before entry and upon exit from a function. Other user flags have no specified role in the standard calling sequence and are not preserved.
Floating Point Control Word
The control word contains the floating-point flags, such as the rounding mode and exception masking.
This register is initialized at process initialization time and its value must be preserved.
Signals can interrupt processes. Functions called during signal handling have no unusual restriction on their use of registers. Moreover, if a signal handling function returns, the process resumes its original execution path with registers restored to their original values. Thus, programs and compilers may freely use all registers without danger of signal handlers changing their values.
Function Return Values
Functions Returning Scalars or No Value
• A function that returns an integral or pointer value places its result in the next available register of the sequence %rax, %rdx.
• A function that returns a floating point value that fits in the XMM registers returns this value in the next available XMM register of the sequence %xmm0, %xmm1.
• An X87 floating-point return value appears on the top of the floating point stack in %st(0) as an 80-bit X87 number. If this X87 return value is a complex number, the real part of the value is returned in %st(0) and the imaginary part in %st(1).
• A function that returns a value in memory also returns the address of this memory in %rax.
• Functions that return no value (also called procedures or void functions) put no particular value in any register.
Functions Returning Structures or Unions
A function can use either registers or memory to return a structure or union. The size and type of the structure or union determine how it is returned. If a structure or union is larger than 16 bytes, it is returned in memory allocated by the caller.
To determine whether a 16-byte or smaller structure or union can be returned in one or more return registers, examine the first eight bytes of the structure or union. The type or types of the structure or union’s fields making up these eight bytes determine how these eight bytes will be returned. If the eight bytes contain at least one integral type, the eight bytes will be returned in %rax even if non-integral types are also present in the eight bytes. If the eight bytes only contain floating point types, these eight bytes will be returned in %xmm0.
If the structure or union is larger than eight bytes but smaller than 17 bytes, examine the type or types of the fields making up the second eight bytes of the structure or union. If these eight bytes contain at least one integral type, these eight bytes will be returned in %rdx even if non-integral types are also present in the eight bytes. If the eight bytes only contain floating point types, these eight bytes will be returned in %xmm1.
If a structure or union is returned in memory, the caller provides the space for the return value and passes its address to the function as a "hidden" first argument in %rdi. This address will also be returned in %rax.
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