Chapter7 programming rules, 1 vles sequencing semantics, 2 vles grouping semantics – Freescale Semiconductor StarCore SC140 User Manual
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SC140 DSP Core Reference Manual
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Chapter 7
Programming Rules
The SC140 has programming rules for correct construction and execution of assembly language code.
These rules define the ability to group or sequence instructions that activate various execution units,
because of their use of shared resources. This chapter describes the SC140 programming rules and
guidelines for correct code construction. These programming rules must be followed to ensure that
software applications produce expected results, and are compatible with future processor implementations.
The SC140 assembler and simulator assist the programmer in conforming to these programming rules.
7.1 VLES Sequencing Semantics
The SC140 VLES execute in sequence according to the following semantics
1
:
•
One VLES finishes execution before the next VLES begins execution, with one exception being
delayed change-of-flow instructions finish execution after the delay slot. This means:
— The results of one VLES are immediately available to the next sequential VLES.
— The latency of various SC140 instructions is not exposed in the assembly source code.
— The number of cycles for a VLES is the number of cycles taken by its longest instruction.
•
Even though the SC140 pipeline overlaps the execution of several VLES, the assembly source order
for sequential VLES execution is enforced.
7.2 VLES Grouping Semantics
The SC140 instructions grouped in a VLES are executed according to the following semantics:
•
Instructions to be executed together are explicitly grouped in a VLES by the programmer. This
means:
— The SC140 does not make any grouping decisions. However, it will take an illegal execution set
exception if it decodes a grouping error.
— Grouped instructions are placed on the same assembly source line, or on one or more lines
surrounded by brackets [...].
1. Semantics is the meaning of language. In this context, the function of instructions, the relationship between instructions in a VLES, and the
relationship between VLES in a program.