User`s manual

Program Control

The Hardware Stack

MOTOROLA Optimizing DSP56300/DSP56600 Applications 3-5

The next example shows loop and subroutine nesting. Figure 3-1

shows the state of the stack at the time the fast interrupt is executing

(label I_IRQA, that enters execution when PC = $000529). The first

DO instruction pushes the existing data on LA and LC (0 and

$FFFFFF, respectively, in this example).

Note: A fast interrupt does not effect the stack. Only long

interrupts (that have a subroutine call) push data into the

stack. Had IRQA

been a long interrupt, another push would

have been done, the saved values being SSH:$529 (PC) and

SSL: $C18300 (SR). The different values of the LF and FV bits

in SR are saved as the nesting proceeds (no loop, finite loop,

infinite loop).

exit DO loop

immediately

BRKcc (condition true)

PC: = LA + 1;

SR: = SSL

SP: = SP – 1

LA: = SSH, LC: = SSL

SP: = SP – 1

Figure 3-1 State of the Stack When IRQA Is Serviced

Table 3-1 Implicit Stack Activity (Continued)

Activity

Triggered by Instruction or

Condition

Implicit Stack

Actions Taken

$109 (PC)

$FFFFFF (LC)

$520 (PC)

$525 (PC)

$530 (LA)

$C00300 (SR)

$C08300 (SR)

$6 (LC)

$0 (LA)

LowHigh

AA0833