Rules Concerning Registers

9.1.2 Rules Concerning Registers

Registers having the same value before and after a function call is not guaranteed for some registers; some registers may change during a function call. Some registers are used for specific purposes according to the option settings. Table 3.27 shows the rules for using registers.

Table 9.1

Rules to Use Registers

Register	Register Value Does Not Change During Function Call	Function Entry	Function Exit	High-Speed Interrupt Register*1	Base Register*2	PID Register*3
R0	Guaranteed	Stack pointer	Stack pointer			
R1	Not guaranteed	Parameter 1	Return value 1			
R2	Not guaranteed	Parameter 2	Return value 2			
R3	Not guaranteed	Parameter 3	Return value 3			
R4	Not guaranteed	Parameter 4	Return value 4			
R5	Not guaranteed		(Undefined)			
R6	Guaranteed		(Value at function entry is held)			
R7	Guaranteed		(Value at function entry is held)			
R8	Guaranteed		(Value at function entry is held)		O	
R9	Guaranteed		(Value at function entry is held)		O	O
R10	Guaranteed		(Value at function entry is held)	O	O	O
R11	Guaranteed		(Value at function entry is held)	O	O	O
R12	Guaranteed		(Value at function entry is held)	O	O	O
R13	Guaranteed		(Value at function entry is held)	O	O	O
R14	Not guaranteed		(Undefined)			
R15	Not guaranteed	Pointer to return value of structure	(Undefined)			
ISP USP	Same as R0 when used as the stack pointer. In other cases, the values do not change. *4					
PC		Program counter*5				
PSW	Not guaranteed		(Undefined)			
FPSW	Not guaranteed		(Undefined)			
ACC	Not guaranteed*6		(Undefined) *6			
INTB BPC BPSW FINTV		No change*4				

Notes 1.

The high-speed interrupt function may use some or all four registers among R10 to R13, depending on the fint_register option. Registers assigned to the high-speed interrupt function cannot be used for other purposes. For details on the function, refer to the description on the option.

Notes 2.

The base register function may use some or all six registers among R8 to R13, depending on the base option. Registers assigned to the base register function cannot be used for other purposes. For details on the function, refer to the description on the option.

Notes 3.

The PID function may use one of R9 to R13, depending on the pid option. The register assigned to the PID function cannot be used for other purposes. For details on the function, refer to the description on the option.

Notes 4.

This does not apply in the case when the registers are set or modified through an intrinsic function or #pragma inline_asm.

Notes 5.

This depends on the specifications of the instruction used for function calls. To call a function, use BSR, JSR, BRA, or JMP.

Notes 6.

For the instructions that modify the ACC (accumulator), refer to the software manual for the target RX series product.