Encoding:

Format:

B offset 

Purpose:

Unconditional Branch

To do an unconditional branch.

Description: branch

B offset is the assembly idiom used to denote an unconditional branch. The actual instruction is interpreted by the hardware as BEQ r0, r0, offset.

An 18-bit signed offset (the 16-bit offset field shifted left 2 bits) is added to the address of the instruction following the branch (not the branch itself), in the branch delay slot, to form a PC-relative effective target address.

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.

Operation:

I:    target_offset = sign_extend(offset || 02)
I+1:  PC = PC + target_offset

Exceptions:

None

Programming Notes:

With the 18-bit signed instruction offset, the conditional branch range is ± 128 Kbytes. Use jump (J) or jump register

(JR) or the Release 6 branch compact (BC) instructions to branch to addresses outside this range.