Inc. Typewriter User Manual

66 www.xilinx.com MicroBlaze Processor Reference Guide

1-800-255-7778 UG081 (v6.0) June 1, 2006

Chapter 3: MicroBlaze Application Binary Interface

The register usage convention for MicroBlaze is given in Table 3-2.

The architecture for MicroBlaze deﬁnes 32 general purpose registers (GPRs). These

registers are classiﬁed as volatile, non-volatile, and dedicated.

• The volatile registers (a.k.a 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 sub-routines.

• 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.

Table 3-2: Register usage conventions

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,ifconﬁgured

to support HW

exceptions, else

Return Address for Exceptions

R18 Dedicated SW Reserved for Assembler

R19-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

RPVR0-

RPVR11

Special HW Processor Version Register 0 thru 11