Overview
Absolute-Jump Instructions
There are two absolute (computed) jump instructions: JMP and CALL. The jump-target address is given by the contents of a general-purpose register. The register contents are left-shifted by one and transferred into the PC. CALL is identical to JMP except that the return-address is saved in %o7. Details of the return-address computation are provided in the description of the CALL instruction. Both JMP and CALL are unconditional. Conditional jumps are implemented by preceding JMP or CALL with a SKP-type instruction.
Both JMP and CALL instructions have branch delay slot behavior: The instruction immediately following a JMP or CALL is executed after JMP or CALL, but before the instruction at the jump-target. The LRET pseudo- instruction, which is an assembler alias for JMP %o7, is conventionally used to return from subroutines.
Trap Instructions
The Nios processor implements two instructions for software exception processing: TRAP and TRET. See “TRAP” on page 102 and “TRET” on page 103 for detailed descriptions of both these instructions. Unlike JMP and CALL, neither TRAP nor TRET has a branch delay-slot: The instruction immediately following TRAP is not executed until the exception-handler returns. The instruction immediately following TRET is not executed at all as part of TRET's operation.
Conditional Instructions
There are five conditional instructions (SKPs, SKP0, SKP1, SKPRz, and SKPRnz). Each of these instructions has a converse assembler-alias pseudo-instruction (IFs, IF0, IF1, IFRz, and IFRnz, respectively). Each of these instructions tests a CPU-internal condition and then executes the next instruction or not, depending on the outcome. The operation of all five SKP-type instructions (and their pseudo-instruction aliases), are identical except for the particular test performed. In each case, the subsequent (conditionalized) instruction is fetched from memory regardless of the test outcome. Depending on the outcome of the test, the subsequent instruction is either executed or cancelled.
While SKP and IF type conditional instructions are often used to conditionalize jump (JMP, CALL) and branch (BR, BSR) instructions, they can be used to conditionalize any instruction. Conditionalized PFX instructions (PFX immediately after a SKPx or IFx instruction) present a special case; the next two instructions are either both cancelled or both executed. PFX instruction pairs are conditionalized as an atomic unit.
