Texas Instruments TMS320 DSP User Manual
Page 21
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Scratch
Algorithm A
Scratch
Algorithm B
Write-Once C
Scratch
Algorithm C
Scratch
Physical
Memory
Persistent B
Persistent A
Persistent A
Persistent B
Write-Once C
0000
FFFF
0
1
Algorithm C
Scratch
Scratch
Write-Once C
Data Memory
Notice that both a() and b() freely use some of the same scratch registers and no saving and restoring of
these registers is necessary. This is possible because both functions, a() and b(), agree on the set of
scratch registers and that values in these registers are indeterminate at the beginning of each function.
By analogy, we partition all memory into two groups: scratch and persistent.
•
Scratch memory is memory that is freely used by an algorithm without regard to its prior contents, i.e.,
no assumptions about the content can be made by the algorithm and the algorithm is free to leave it in
any state.
•
Persistent memory is used to store state information while an algorithm instance is not executing.
Persistent memory is any area of memory that an algorithm can write to assume that the contents are
unchanged between successive invocations of the algorithm within an application. All physical memory
has this behavior, but applications that share memory among multiple algorithms may opt to overwrite
some regions of memory (e.g., on-chip DARAM).
A special variant of persistent memory is the write-once persistent memory. An algorithm's initialization
function ensures that its write-once buffers are initialized during instance creation and that all subsequent
accesses by the algorithm's processing to write-once buffers are strictly read-only. Additionally, the
algorithm can link its own statically allocated write-once buffers and provide the buffer addresses to the
client. The client is free to use provided buffers or allocate its own. Frameworks can optimize memory
allocation by arranging multiple instances of the same algorithm, created with identical creation
parameters, to share write-once buffers.
Note that a simpler alternative to declaring write-once buffers for sharing statically initialized read-only
data is to use global statically linked constant tables and publish their alignment and memory space
requirements in the required standard algorithm documentation. If data has to be computed or relocated at
run-time, the write-once buffers approach can be employed.
The importance of making a distinction between scratch memory and persistent memory is illustrated in
.
Figure 2-1. Scratch vs Persistent Memory Allocation
All algorithm scratch memory can be "overlaid" on the same physical memory. Without the distinction
between scratch and persistent memory, it would be necessary to strictly partition memory among
algorithms, making the total memory requirement the sum of all algorithms' memory requirements. On the
other hand, by making the distinction, the total memory requirement for a collection of algorithms is the
sum of each algorithm's distinct persistent memory, plus any shared write-once persistent memory, plus
the maximum scratch memory requirement of any of these algorithms.
SPRU352G – June 2005 – Revised February 2007
General Programming Guidelines
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