User`s manual

Program Control

Using the Stack Extension

MOTOROLA Optimizing DSP56300/DSP56600 Applications 3-7

3.3 USING THE STACK EXTENSION

The hardware stack could be extended to the data memory (X or Y),

and it’s depth could be set by the user according to need. After

initialization, the stack extension works automatically without any

user overhead, giving the same functionality as the hardware stack.

The registers participating in stack extension operation are listed in

Table 3-2.

Stack extension initialization bits in the OMR include the XYS (X Y

Select) bit, by which the user selects the data space (X or Y) in which

the stack extension will reside, and the SEN (Stack Extension

Enable) bit, by which the user activates the stack extension after all

the relevant registers are initialized.

The SP register counts the number of entries in the stack. If the stack

extension is disabled, the values of SP are bounded to 0–15, and

selection of other values cause a stack error exception. When the

stack extension is enabled, SP may hold values from 0 up to the

value stored in SZ. A push increments SP by 1, a pop decrements it

by 1.

SZ stores the maximum stack depth. During stack extension

operation, if SP becomes greater than SZ, a stack overflow exception

occurs. SZ has no default value, and therefore, must be initialized

by the user before enabling the stack extension. Set the SZ value

according the amount of memory available to the user, using the

Table 3-2 Registers Involved in Stack Extension Operation

OMR operating

mode register

stack extension initialization (bits:SEN,XYS)

stack extension status (bits: WRP,EOV,EUN)

SZ stack extension

size

maximum stack depth, in word pairs.

SP stack pointer current total stack depth, in word pairs

SC stack counter current hardware stack depth.

EP stack extension

pointer

pointer to the last address written in the

memory extension.