User`s guide

C for DSP56800E
Large Data Model Support
129Targeting MC56F83xx/DSP5685x Controllers
Large Data Model Support
The DSP56800E extends the DSP56800 data addressing range, by providing 24-bit
address capability to some instructions. 24-bit address modes allow user accesses
beyond the 64K-word boundary of 16-bit addressing. To control large data memory
model support, use the M56800E Processor panel (Figure 6.2
). See “M56800E
Processor” for explanations of this panel’s elements.
Figure 6.2 M56800E Processor Panel: Large Data Model
Extended data is data located beyond the 16-bit address boundary — as if it exists in
extended (upper) memory. Memory located below the 64K boundary is lower
memory.
The compiler default arrangement is using 16-bit addresses for all data accesses. This
means that absolute addresses (X:xxxx addressing mode) are limited to 16 bits. Direct
addressing or pointer registers load or store 16-bit addresses. Indexed addressing
indexes are 16-bit quantities. The compiler treats data pointers as 16-bit pointers that
you may store in single words of memory.
If the large data memory model is enabled, the compiler accesses all data by 24-bit
addressing modes. It treats data pointers as 24-bit quantities that you may store in two
words of memory. Absolute addressing occurs as 24-bit absolute addresses. Thus, you
may access the entire 24-bit data memory, locating data objects anywhere.