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 6-3 shows the types of registers affected by these conventions. Table 6-4 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 6.4.
| 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 |
| Program counter | Contains the current address of code being executed |
| Register | Alias | Usage | Preserved by Function(1) |
|---|---|---|---|
| R0 | PC | Program counter | N/A |
| R1 | SP | Stack pointer | N/A(2) |
| R2 | SR | Status register | N/A |
| R3 | Constant generator | N/A | |
| R4-R10 | Expression register | Child | |
| R11 | Expression register | Parent | |
| R12 | Expression register, argument pointer, return register | Parent | |
| R13 | Expression register, argument pointer, return register | Parent | |
| R14 | Expression register, argument pointer, return register | Parent | |
| R15 | Expression register, argument pointer, return register | Parent |