Strict conventions associate specific registers with specific operations in the C/C++ environment. If you plan to interface an assembly language routine to a C/C++ program, you must understand and follow these register conventions. The register conventions dictate how the compiler uses registers and how values are preserved across function calls. Section 2.3.1 shows the types of registers affected by these conventions.Section 2.3.1 summarizes how the compiler uses registers and whether their values are preserved across calls. For information about how values are preserved across calls, see Section 2.3.1. Table 6.7. How Register Types Are Affected by the Conventions Register Type Description Argument register Passes arguments during a function call Return register Holds the return value from a function call Expression register Holds a value Argument pointer Used as a base value from which a function's parameters (incoming arguments) are accessed Stack pointer Holds the address of the top of the software stack Link register Contains the return address of a function call Program counter Contains the current address of code being executed
Table 6.8. Register Usage Register Alias Usage Preserved by Function^[a] R0 A1 Argument register, return register, expression register Parent R1 A2 Argument register, return register, expression register Parent R2 A3 Argument register, expression register Parent R3 A4 Argument register, expression register Parent R4 V1 Expression register Child R5 V2 Expression register Child R6 V3 Expression register Child R7 V4, AP Expression register, argument pointer Child R8 V5 Expression register Child R9 V6 Expression register Child R10 V7 Expression register Child R11 V8 Expression register Child R12 V9, 1P Expression register, instruction pointer Parent R13 SP Stack pointer Child^[b] R14 LR Link register, expression register Child R15 PC Program counter N/A CPSR
Current program status register Child SPSR
Saved program status register Child ^[a] The parent function refers to the function making the function call. The child function refers to the function being called. ^[b] The SP is preserved by the convention that everything pushed on the stack is popped off before returning.
Table 6.9. VFP Register Usage 32-Bit Register 64-Bit Register Usage Preserved by Function^[a] FPSCR
Status register N/A S0 D0 Floating-point expression, return values, pass arguments N/A S1


S2 D1 Floating-point expression, return values, pass arguments N/A S3


S4 D2 Floating-point expression, return values, pass arguments N/A S5


S6 D3 Floating-point expression, return values, pass arguments N/A S7


S8 D4 Floating-point expression, pass arguments N/A S9


S10 D5 Floating-point expression, pass arguments N/A S11


S12 D6 Floating-point expression, pass arguments N/A S13


S14 D7 Floating-point expression, pass arguments N/A S15


S16 D8 Floating-point expression Child S17


S18 D9 Floating-point expression Child S19


S20 D10 Floating-point expression Child S21


S22 D11 Floating-point expression Child S23


S24 D12 Floating-point expression Child S25


S26 D13 Floating-point expression Child S27


S28 D14 Floating-point expression Child S29


S30 D15 Floating-point expression Child S31



D16-D31 Floating-point expression
^[a] The child function refers to the function being called.
Table 6.10. Neon Register Usage 64-Bit Register Quad Register Usage Preserved by Function^[a] D0 Q0 SIMD register N/A D1


D2 Q1 SIMD register N/A D3


D4 Q2 SIMD register N/A D5


D6 Q3 SIMD register N/A D7


D8 Q4 SIMD register Child D9


D10 Q5 SIMD register Child D11


D12 Q6 SIMD register Child D13


D14 Q7 SIMD register Child D15


D16 Q8 SIMD register N/A D17


D18 Q9 SIMD register N/A D19


D20 Q10 SIMD register N/A D21


D22 Q11 SIMD register N/A D23


D24 Q12 SIMD register N/A D25


D26 Q13 SIMD register N/A D27


D28 Q14 SIMD register N/A D29


D30 Q15 SIMD register N/A D31


FPSCR
Status register N/A ^[a] The child function refers to the function being called.