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 much faster (data stored can be accessed without delay), and multiple reads and writes can access the DSP's on-chip memory 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, we must use the larger but slower external memory.

When writing programs that require large amounts of memory, use the internal memory to hold your code, filter coefficients, and any small buffers you need. External memory should be used for large buffers that you only access a few times per sample, like the delay buffer described in Audio Effects: Using External Memory.

Figure 1: DSP EVM memory maps

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-bit addressing register or the 16-bit literals stored in the program, it is split up into 64K (16-bit) pages by the hardware. 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 your program'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 DSP program, 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. 1 In addition, copies of the internal memory also appear in the extended Program address space, occupying locations 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, the Program 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 in the ranges of 08000h-0FF00h, 1800h-1FFFFh, and 28000h-2FFFFh are available to store your data buffers; the remaining addresses are unmapped 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, the core file provides mechanisms for manipulating the PMST indirectly. Instead of accessing the external Program RAM directly, we can use the special macros to access the RAM that is normally "hidden" by the internal memory. This allows us to use the full range of external memory available: addresses 000000h-00FF00h and 010000h-02FFFF. However, since addresses 00FF00h-00FFFFh are reserved by the core file, you must be careful not to write to addresses in this range.