Freescale-semiconductor StarCore SC140 Manual de usuario descargar pdf (Pagina 252)

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7-2 SC140 DSP Core Reference Manual

VLES Grouping Semantics

• All instructions in a VLES execute in parallel. This means:

— The assembly source order of instructions and labels within an unconditional VLES does not

change the results, with one exception being the assembly source order determines which

instruction (if any) updates the carry bit. Because source order within an unconditional VLES is

not required for correct code execution, the assembler sometimes reorders instructions in the

VLES during the encoding process.

— All instruction source registers and memory locations are read in parallel when the VLES starts

execution. That is, the order that multiple reads occur does not change the results.

— After all sources are read, all grouped instructions operate on them in parallel.

— All instruction results are written to registers and memory locations in parallel when the VLES

finishes execution. Parallel memory writes must write to different locations so the order that

multiple writes occur does not change the results.

• Instructions in a VLES can execute conditionally. This means:

— A VLES without conditional instructions executes all instructions unconditionally.

— A VLES can have conditional instructions (such as Bc, Jc, MOVEc, TFRc, where c is the

execution condition). The conditional instruction executes if its condition is true. If its condition

is false, the instruction does not execute (becomes a NOP).

— The IFc instruction conditionally executes all instructions that follow it (in assembly source

order) until the next IFc instruction or the end of the VLES. Multiple IFc instructions in a VLES

form su/jointfilesconvert/1233470/bgroups of instructions that are conditionally executed based on their associated IFc

condition. Multiple IFc su/jointfilesconvert/1233470/bgroups having the same condition are in the same su/jointfilesconvert/1233470/bgroup.

— Each unconditional instruction in an IFc group or su/jointfilesconvert/1233470/bgroup executes if its IFc condition is true.

If its IFc condition is false, the instruction does not execute (becomes a NOP).

— Each conditional instruction in an IFc group or su/jointfilesconvert/1233470/bgroup executes if the instruction’s condition

and its IFc condition are both true. If either condition is false, the instruction does not execute

(becomes a NOP).

• Only one carry-affecting instruction in a VLES may update the carry bit. This means:

— A VLES can have multiple carry-affecting instructions that execute in parallel.

— The carry-affecting instructions can execute unconditionally, or conditionally in an IFc group or

su/jointfilesconvert/1233470/bgroup.

— The last (in assembly source order) carry-affecting instruction whose execution condition is true

updates the carry bit, while the other instructions in the VLES do not affect the carry bit.

— If no carry-affecting instructions execute, the carry bit is not affected.

• Even though the SC140 may execute instructions grouped in a VLES during different pipeline

stages, the assembly source grouping for parallel instruction execution is enforced.

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