The register usage convention for MicroBlaze is given in the following table.
| Register | Type | Enforcement | Purpose |
|---|---|---|---|
| R0 | Dedicated | HW | Value 0 |
| R1 | Dedicated | SW | Stack Pointer |
| R2 | Dedicated | SW | Read-only small data area anchor |
| R3-R4 | Volatile | SW | Return Values/Temporaries |
| R5-R10 | Volatile | SW | Passing parameters/Temporaries |
| R11-R12 | Volatile | SW | Temporaries |
| R13 | Dedicated | SW | Read-write small data area anchor |
| R14 | Dedicated | HW | Return address for Interrupt |
| R15 | Dedicated | SW | Return address for Sub-routine |
| R16 | Dedicated | HW | Return address for Trap (Debugger) |
| R17 | Dedicated | HW/SW | Return address for Exceptions HW, if configured to support hardware exceptions, else SW |
| R18 | Dedicated | SW | Reserved for Assembler/Compiler Temporaries |
| R19 | Non-volatile | SW | Must be saved across function calls. Callee-save |
| R20 |
Dedicated or Non-volatile |
SW | Reserved for storing a pointer to the global offset table (GOT) in position independent code (PIC). Non-volatile in non-PIC code. Must be saved across function calls. Callee-save. |
| R21-R31 | Non-volatile | SW | Must be saved across function calls. Callee-save. |
| RPC | Special | HW | Program counter |
| RMSR | Special | HW | Machine Status Register |
| REAR | Special | HW | Exception Address Register |
| RESR | Special | HW | Exception Status Register |
| RFSR | Special | HW | Floating-Point Status Register |
| RBTR | Special | HW | Branch Target Register |
| REDR | Special | HW | Exception Data Register |
| RPID | Special | HW | Process Identifier Register |
| RZPR | Special | HW | Zone Protection Register |
| RTLBLO | Special | HW | Translation Look-Aside Buffer Low Register |
| RTLBHI | Special | HW | Translation Look-Aside Buffer High Register |
| RTLBX | Special | HW | Translation Look-Aside Buffer Index Register |
| RTLBSX | Special | HW | Translation Look-Aside Buffer Search Index |
| RPVR0-12 | Special | HW | Processor Version Register 0 through 12 |
The architecture for MicroBlaze defines 32 general purpose registers (GPRs). These registers are classified as volatile, non-volatile, and dedicated.
- The volatile registers (also known as caller-save) are used as temporaries and do not retain values across the function calls. Registers R3 through R12 are volatile, of which R3 and R4 are used for returning values to the caller function, if any. Registers R5 through R10 are used for passing parameters between subroutines.
- Registers R19 through R31 retain their contents across function calls and are hence termed as non-volatile registers (a.k.a callee-save). The callee function is expected to save those non-volatile registers, which are being used. These are typically saved to the stack during the prologue and then reloaded during the epilogue.
- Certain registers are used as dedicated registers and programmers
are not expected to use them for any other purpose.
- Registers R14 through R17 are used for storing the return address from interrupts, sub-routines, traps, and exceptions in that order. Subroutines are called using the branch and link instruction, which saves the current Program Counter (PC) onto register R15.
- Small data area pointers are used for accessing certain memory locations with 16- bit immediate value. These areas are discussed in the memory model section of this document. The read only small data area (SDA) anchor R2 (Read-Only) is used to access the constants such as literals. The other SDA anchor R13 (Read-Write) is used for accessing the values in the small data read-write section.
- Register R1 stores the value of the stack pointer and is updated on entry and exit from functions.
- Register R18 is used as a temporary register for assembler operations.
-
MicroBlaze includes special
purpose registers such as:
- program counter (rpc)
- machine status register (rmsr)
- exception status register (resr)
- exception address register (rear)
- floating-point status register (rfsr), branch target register (rbtr)
- exception data register (redr)
- memory management registers (rpid, rzpr, rtlblo, rtlbhi, rtlbx, rtlbsx)
- processor version registers (0-12)
These registers are not mapped directly to the register file; and hence, the usage of these registers is different from the general purpose registers. The value of a special purpose registers can be transferred to or from a general purpose register by using mts and mfs instructions respectively.