Encoding:

pre-Release 6

Release 6

Format:

JALR rd, rs

Purpose:

Jump and Link Register

To execute a procedure call to an instruction address in a register

Description:

 GPR[rd] = return_addr, PC = GPR[rs]

Place the return address link in GPR rd. The return link is the address of the second instruction following the branch, where execution continues after a procedure call.

For processors that do not implement the MIPS16e or microMIPS ISA:

• Jump to the effective target address in GPR rs. If the target address is not 4-byte aligned, an Address Error exception will occur when the target address is fetched.

For processors that do implement the MIPS16e or microMIPS ISA:

• Jump to the effective target address in GPR rs. Set the ISA Mode bit to the value in GPR rs bit 0. Set bit 0 of the target address to zero. If the target ISA Mode bit is 0 and the target address is not 4-byte aligned, an Address

Error exception will occur when the target instruction is fetched.

In both cases, execute the instruction that follows the jump, in the branch delay slot, before executing the jump itself.

In Release 1 of the architecture, the only defined hint field value is 0, which sets default handling of JALR. In

Release 2 of the architecture, bit 10 of the hint field is used to encode a hazard barrier. See the JALR.HB instruction description for additional information.

Restrictions:

Control Transfer Instructions (CTIs) should not be placed in branch delay slots or Release 6 forbidden slots. CTIs

include all branches and jumps, NAL, ERET, ERETNC, DERET, WAIT, and PAUSE.

Pre-Release 6: Processor operation is UNPREDICTABLE if a control transfer instruction (CTI) is placed in the delay slot of a branch or jump.

Release 6: If a control transfer instruction (CTI) is executed in the delay slot of a branch or jump, Release 6 implementations are required to signal a Reserved Instruction exception.

Jump-and-Link Restartability: Register specifiers rs and rd must not be equal, because such an instruction does not

have the same effect when re-executed. The result of executing such an instruction is UNPREDICTABLE. This restriction permits an exception handler to resume execution by re-executing the branch when an exception occurs in the delay slot.

Restrictions Related to Multiple Instruction Sets: This instruction can change the active instruction set, if more than

one instruction set is implemented.

If only one instruction set is implemented, then the effective target address must obey the alignment rules of the instruction set. If multiple instruction sets are implemented, the effective target address must obey the alignment rules of the intended instruction set of the target address as specified by the bit 0 or GPR rs.

For processors that do not implement the microMIPS32/64 ISA, the effective target address in GPR rs must be naturally-aligned. For processors that do not implement the MIPS16e ASE nor microMIPS32/64 ISA, if either of the two least-significant bits are not zero, an Address Error exception occurs when the branch target is subsequently fetched as an instruction.

For processors that do implement the MIPS16e ASE or microMIPS32/64 ISA, if target ISAMode bit is zero (GPR rs bit 0) and bit 1 is one, an Address Error exception occurs when the jump target is subsequently fetched as an instruction.

Availability and Compatibility:

Release 6 maps JR and JR.HB to JALR and JALR.HB with rd = 0:

Pre-Release 6, JR and JALR were distinct instructions, both with primary opcode SPECIAL, but with distinct function codes.

Release 6: JR is defined to be JALR with the destination register specifier rd set to 0. The primary opcode and function field are the same for JR and JALR. The pre-Release 6 instruction encoding for JR is removed in Release 6.

Release 6 assemblers should accept the JR and JR.HB mnemonics, mapping them to the Release 6 instruction encodings.

Operation:

I: temp = GPR[rs]
   GPR[rd] = PC + 8
I+1:if (Config3ISA = 0) and (Config1CA = 0) then 
      PC = temp
   else
      PC = tempGPRLEN-1..1 || 0
      ISAMode = temp0
   endif

Exceptions:

None

Programming Notes:

This jump-and-link register instruction can select a register for the return link; other link instructions use GPR 31.

The default register for GPR rd, if omitted in the assembly language instruction, is GPR 31.