Motorola DSP56305 User Manual download pdf (Page 82)

Pipeline Interlocks

Program Flow-Control Pipeline Interlocks

MOTOROLA Optimizing DSP56300/DSP56600 Applications 6-9

hardware stack so that it will not be full or empty and the execution

of instructions can continue. This activity of the stack extension

delays the execution phases by the number of cycles required to

move data to or from the stack, usually two cycles for each move.

6.3.1 Stack Extension Full/Empty Cases

The stack-full or stack-empty states are defined by the contents of

the SC (Stack Counter) register. When the stack counter equals 14, it

means that the on-chip hardware stack has fourteen words (a stack

word is a 48-bit long word combined from the low and the high

portions of the stack) inside. The stack is declared as stack-full, and

any additional push operation will activate the stack extension

mechanism. When the stack counter equals 2, it means that the

on-chip hardware stack has only two words inside. The stack is

declared as stack-empty, and any additional pop operation will

activate the stack extension mechanism.

6.3.2 Avoiding Stack Extension Delays

The best way to avoid stack extension delays is to make sure that

the number of stack levels used during execution of critical code

segments will not be larger than fourteen. If this is the case, and

upon entry to this piece of code, the stack was empty, absolutely no

stack extension delays will be added to the program flow. If

however, the stack was not empty, few stack extension delays will

be added until the code has reached its upper stack level.

6.4 PROGRAM FLOW-CONTROL PIPELINE

INTERLOCKS

During the execution of flow-control instructions, some boundary

non-frequent cases exist and introduce interlocks to the program

flow. These cases represent very unusual operations which

probably would never be used in a usual code. The generation of

interlock cycles in these cases is done in order to maintain object

code compatibility to the DSP56000 family of Digital Signal

Processors.

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Motorola DSP56305 User Manual Page 82