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Program Control

The Hardware Stack

MOTOROLA Optimizing DSP56300/DSP56600 Applications 3-3

Note: The BRKcc instruction has the same functionality as the C

language “break”, (i.e., terminating the loop and resuming

execution after the end of the loop). A similar instruction is

the ENDDO instruction, which exits the loop after finishing

the current loop iteration. ENDDO is not a conditional

instruction, therefore normal use generally includes testing a

condition and skipping the ENDDO instruction accordingly.

The following example counts the number of bits in A1, terminating

if the register turns 0 before the full 24 iterations.

clr b #0,x0

do #24,_END_LOOP

tst a

jne _CONT

enddo

_CONT

lsr a

addc x0,b

_END_LOOP

Bit 0 of the result (B1) could be used as the parity of the original

operand (A1).

Note: Both ENDDO and BRKcc have sequence restrictions, as

shown in the

DSP56300

and

DSP56600 Family Manuals

Appendix B.

3.2 THE HARDWARE STACK

The DSP56300/DSP56600 hardware stack enables the user to nest

DO loops and subroutines (called by software or interrupts) with no

software overhead. With the Stack Extension enabled, the hardware

stack can accommodate an unlimited nesting level of DO loops,

JSRs, or a combination of them. The only overhead of a very deep

nesting level is some additional cycles required to copy data to or

from the stack extension memory. Examples of stack extension use

are given in Section 3.3 on page 3-7.

The hardware stack mechanism works in parallel to opcode

execution, thus saving execution time, as well as software overhead

compared to conventional software stacks. These advantages make

the DSP56300/DSP56600 especially suitable for multi-tasking, and

running real-time operating systems and program code generated

from high-level languages.

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