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24 Porting and Optimizing DSP56800 Applications to DSP56800E 

6.1 Extending Data Memory Size From 64K to 16M

There are two assembler switches that instruct the DSP56800E application to use more than 16 bits for

addresses: -od21 and -od24. Following these instructions, all addresses will become 24 bits long instead of

16 bits. Source code changes must be made to support this.

Instructions that are forced by the ‘>’ operator to use 16-bit data addresses must be forced with the new

‘>>’ operator to use 24-bit addresses. Code Example 35 displays the use of the force operator.

Code Example 35. Using the 24-Bit Force Operator Instead of 16-Bit Force Operator

move a1,x:>buffer ; forced to use 16-bit address

; DSP56800 original code

move.w a1,x:>>buffer ; forced to use 24-bit address

; DSP56800E ported code

Instructions that load addresses wider than 16 bits into address registers must also be modified, as shown in

Code Example 36.

Code Example 36. Loading 24-bit Immediates into Address Registers

move #buffer,r1

; DSP56800 original code

move.l #buffer,r1

; DSP56800E ported code

The memory storage size for pointers to data memory must also be extended from 16 bits to 32 bits as

shown in Code Example 37. Care must be taken to ensure that storage location is 2 word aligned.

Code Example 37. Extending Memory Storage Size for Pointers to Data Memory

pointer ds 1

;DSP56800 original code

pointer dsm 2

;DSP56800E ported code

Instructions that store the values of address registers in memory must also be modified to store not only

16 bits, but also 24 bits. These modifications are shown in Code Example 38.

Code Example 38. Saving 24-bit Values from Address Registers

move r1,x:pointer ; save LSP 16 bits of r1

; DSP56800 original code

move.l r1,x:pointer ; save all 24 bits of r1

; DSP56800E ported code

All the instructions that access these memory locations must be changed as shown in Code Example 39.

Code Example 39. Modifying Instructions to Access Memory on 32 Bits

inc x:pointer ; increment the word at x:pointer

; DSP56800 original code

inc.l x:pointer ; increment the 2-word value at x:pointer

; DSP56800E ported code

Note that on more complex applications which are ported to use data memory space above 64K require

more complex modifications. For example, 16-bit arithmetic on pointers must be replaced with 32-bit

arithmetic.

A summary of the extended data memory size for this project is presented in the Table 9.

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