3.3. Register Conventions

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. Table 1 below shows the types of registers affected by these conventions. Table 2 summarizes how the compiler uses registers and whether their values are preserved across calls. Table 3 and 4 provide register usage information for VFP and Neon registers, respectively. For information about how values are preserved across calls, see Function Structure and Calling Conventions.

Table 1: 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 2: Register Usage

Register	Alias	Usage	Preserved by Function (Note 1 below)
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 (Note 2 below)
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

Note 1) The parent function refers to the function making the function call. The child function refers to the function being called.

Note 2) The SP is preserved by the convention that everything pushed on the stack is popped off before returning.

Table 3: VFP Register Usage

32-Bit Register	64-Bit Register	Usage	Preserved by Function (Note 1 below)
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

Note 1) The child function refers to the function being called.

Table 4: Neon Register Usage

64-Bit Register	Quad Register	Usage	Preserved by Function (Note 1 below)
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

Note 1) The child function refers to the function being called.