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Altera Corporation 15
GettingOverview
Overview
1
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.