TMS320F datasheet(76/389 Pages) TI

76 / 389 page

Conditional Branches, Calls, and Returns

5-12

The conditional branch instructions are BCND (branch conditionally) and

BANZ (branch if currently selected auxiliary register is not equal to 0). The

BANZ instruction is useful for implementing loops.

5.4.4

Conditional Calls

The conditional call (CC) instruction is executed only when the specified condi-

tion or conditions are met (see Table 5–3 on page 5-10). This allows your pro-

gram to choose among multiple subroutines based on the data being pro-

cessed. If all the conditions are met, the PC is loaded with the second word

of the call instruction, which contains the starting address of the subroutine.

Before branching to the subroutine, the processor stores the address of the

instruction following the call instruction (the return address) to the stack. The

function must end with a return instruction, which takes the return address off

the stack and forces the processor to resume execution of the calling program.

By the time the conditions of the conditional call instruction have been tested,

the two instruction words following the call instruction have already been

fetched in the pipeline. If all the conditions are met, these two instruction words

are flushed from the pipeline so that they are not executed, and then execution

continues at the beginning of the called function. If the conditions are

not met,

the two instructions are executed instead of the call. Because there is a wait

cycle for conditions to become stable, the conditional call takes one more cycle

than the unconditional one.

5.4.5

Conditional Returns

Returns are used in conjunction with calls and interrupts. A call or interrupt

stores a return address to the stack and then transfers program control to a

new location in program memory. The called subroutine or the interrupt service

routine concludes with a return instruction, which pops the return address off

the top of the stack and into the program counter (PC).

The conditional return instruction (RETC) is executed only when one or more

conditions are met (see Table 5–3 on page 5-10). By using the RETC instruc-

tion, you can give a subroutine or interrupt service routine more than one pos-

sible return path. The path chosen then depends on the data being processed.

In addition, you can use a conditional return to avoid conditionally branching

to/around the return instruction at the end of the subroutine or interrupt service

routine.

TMS320F Datasheet(PDF) 76 Page - Texas Instruments