7.2.1 TMS320C28x Register Use and Preservation

Table 7-2 summarizes how the compiler uses the TMS320C28x registers and shows which registers are defined to be preserved across function calls.

The FPU uses all the C28x registers as well as the registers described in Table 7-3.

Table 7-2 Register Use and Preservation Conventions

Register Usage Save on Entry Save on Call
AL Expressions, argument passing, and returns 16-bit results from functions No Yes
AH Expressions and argument passing No Yes
DP Data page pointer (used to access global variables) No No
PH Multiply expressions and Temp variables No Yes
PL Multiply expressions and Temp variables No Yes
SP Stack pointer (1) (1)
T Multiply and shift expressions No Yes
TL Multiply and shift expressions No Yes
XAR0 Pointers and expressions No Yes
XAR1 Pointers and expressions Yes No
XAR2 Pointers, expressions, and frame pointing (when needed) Yes No
XAR3 Pointers and expressions Yes No
XAR4 Pointers, expressions, argument passing, and returns 32-bit pointer values from functions No Yes
XAR5 Pointers, expressions, and arguments No Yes
XAR6 Pointers and expressions No Yes
XAR7 Pointers, expressions, indirect calls and branches (used to implement pointers to functions and switch statements) No Yes
(1) The SP is preserved by the convention that everything pushed on the stack is popped off before returning.

Table 7-3 FPU Register Use and Preservation Conventions

FPU32 Registers FPU64 Registers Usage Save on Entry Save on Call
R0H R0 = R0H:R0L Expressions, argument passing, and returns 32-bit float from functions No Yes
R1H R1 = R1H:R1L Expressions and argument passing No Yes
R2H R2 = R2H:R2L Expressions and argument passing No Yes
R3H R3 = R3H:R3L Expressions and argument passing No Yes
R4H R4 = R4H:R4L Expressions Yes No
R5H R5 = R5H:R5L Expressions Yes No
R6H R6 = R6H:R6L Expressions Yes No
R7H R7 = R7H:R7L Expressions Yes No