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The TI DSP evaluation boards you use have a large amount of memory; in addition to the 32K words internal to the DSP,there are another 256K words of memory installed on the EVM board. For many exercises, the data sets are small, and youworked with only the on-chip memory of the DSP and were not expected to consider how the use of memory impactedperformance. However, the large delays often required in audio processing, for example, require that many thousands ofsamples be stored in memory. There is not enough memory on the DSP microprocessor itself to store a second or more of samplesat a 44.1 kHz sample rate, so the off-chip memory must be used.
As you have seen, the TI TMS320C54x DSP has two separate
memory spaces, called Program and Data. Usually, Programcontains your assembled program, and Data contains data, but
sometimes it may be convenient or more efficient to violatethis convention. (For instance, the
firs
instruction requires filter coefficients in the Program
address space.) The Data space is 64K long and is accessedusing the 16-bit auxiliary registers
(
ARx
). Although the Program space is normally
accessed using 16-bit literals stored in your program code,the Program space is, in fact, significantly larger than 64K.
Using special "extended addressing" instructions, the TI DSPcan access up to 8192K-words of memory in the Program space.
The extended addressing instructions include far calls andjumps that reset the full 23-bit program counter, as well as
accumulator-addressed data-transfer instructions.
In many exercises, it is possible to store program instructions and data entirely in the DSP's on-chip("internal") memory. This internal memory has several advantages over off-chip ("external") memory: it is muchfaster (data stored can be accessed without delay), and multiple reads and writes can access the DSP's on-chipmemory simultaneously. However, many applications (including the audio delay effect of Using External Memory ) require a data buffer too large to fit into the on-chip memory. For these large buffers, wemust use the larger but slower external memory.
When writing programs that require large amounts of memory, use the internal memory to hold your code, filtercoefficients, and any small buffers you need. External memory should be used for large buffers that you only accessa few times per sample, like the delay buffer described in Audio Effects: Using External Memory .
As these memory maps show, the EVM's Data address space is
addressed fully by the 16-bit auxiliary registers(
ARx
) and address-extension words and the
mapping of Data memory is not affected by the
OVLY
bit. However, because the Program memory
space is much larger than can be addressed by the 16-bitaddressing register or the 16-bit literals stored in the
program, it is split up into 64K (16-bit) pages by thehardware. Normal instructions, such as
call
,
firs
, and
mvpd
accept only 16-bit
addresses, and can therefore only address the current "page"(usually address in the form
00xxxxh
, which
corresponds to the addresses the linker uses for yourprogram's code). To access the full 23-bit address space,
the DSP offers special accumulator-addressed load, store,and jump instructions.
Further complicating matters is the fact that the
OVLY
bit affects the mapping of the Program
memory space. If you remember, before we load our DSPprogram, we have to change the
PMST
to
FFE0h
. We do this to set the
OVLY
bit in the
PMST
, which maps the internal memory
into both the Program and Data spaces of the DSP. If
OVLY
is 1, the internal memory appears in both
the Data and Program memory address space at locations
0080h
to
07FFFh
. Therefore, with
OVLY
set, anything written into Data memory
below
07FFFh
will overwrite a program stored in
the same location.
6000h
-
7FFFh
- does not overlap
with any of the space allocated for the datasegments.0080h
-
7FFFh
of each
page. Therefore, with
OVLY
set, any addresses
to Program memory locations in the form of
xx0000h
-
xx7FFFh
reference internal
memory.
When
OVLY
is zero, internal memory is not
mapped into the Program space at all; in this case, theProgram space includes only external memory. In this mode,
all 192K words of external Program RAM are accessible,although several wait states will be required for accessing
each item of memory. In the overlay mode, only addresses inthe ranges of
08000h
-
0FF00h
,
1800h
-
1FFFFh
, and
28000h
-
2FFFFh
are available to
store your data buffers; the remaining addresses areunmapped or map to the on-chip RAM.
To escape this confusion and allow the full 192K-words of
external Program RAM to be used for your data buffers, thecore file provides mechanisms for manipulating the
PMST
indirectly. Instead of accessing the
external Program RAM directly, we can use the special macrosto access the RAM that is normally "hidden" by the internal
memory. This allows us to use the full range of externalmemory available: addresses
000000h
-
00FF00h
and
010000h
-
02FFFF
. However, since
addresses
00FF00h
-
00FFFFh
are
reserved by the core file, you must be careful not to writeto addresses in this range.
The core file provides two macros for accessing data stored in
the external Program RAM:
READPROG
and
WRITPROG
. These macros allow the processor to
copy data between data memory and external Programmemory. Both macros address external Program memory using the
value in accumulator
A
.
READPROG
reads data from the external Program memory location pointed
to by A and writes it to the data memory location pointed toby
AR1
.
WRITPROG
reads data from the
memory location pointed to by
AR1
and writes it
to the location in external Program RAM specified byaccumulator
A
. Both macros take one parameter, a
count; specifying 1 reads or writes one word from externalmemory, and specifying some other number
transfers
words starting at the locations pointed to by
A
and
AR1
.
AR1
is left
pointing at the word after the last word read or written; noother registers are modified.
For instance, the following code fragment loads the value
contained in memory location
023456h
into the
location
0064h
in data memory using the
READPROG
macro:
1 stm #64h,AR1 ; load 64h into AR1
2 ld #02h,16,A ; load 02h in high part of A
3 adds #3456h,A ; fill in low part of A
4 ; A contains 023456h
5 READPROG 1 ; read from 023456h in external Program RAM
6 ; into *AR1 in Data RAM
The
WRITPROG
macro can be used similarly to write
into extended Program RAM:
1 stm #64h,AR1 ; load 64h into AR1
2 ld #02h,16,A ; load 02h in high part of A
3 adds #3456h,A ; fill in low part of A
4 ; A contains 023456h
5 WRITPROG 1 ; write from *AR1 in Data RAM to
6 ; 023456h in external Program RAM
Note that Code Composer will not display or allow you to
change the contents of the external Program RAM on thememory-dump or disassembly screen, though you can view or
change it indirectly by watching the effects of the
READPROG
and
WRITPROG
macros on data
memory.
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