TMS320F datasheet(72/389 Pages) TI

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Branches, Calls, and Returns

5-8

5.3

Branches, Calls, and Returns

Branches, calls, and returns break the sequential flow of instructions by trans-

ferring control to another location in program memory. A

branch only transfers

control to the new location. A

call also saves the return address (the address

of the instruction following the call) to the top of the hardware stack. Every

called subroutine or interrupt service routine is concluded with a

return instruc-

tion, which pops the return address off the stack and back into the program

counter (PC).

The ’C24x has two types of branches, calls, and returns:

- Unconditional. An unconditional branch, call, or return is always

executed. The unconditional branch, call, and return instructions are de-

scribed in sections 5.3.1, 5.3.2, and 5.3.3, respectively.

- Conditional. A conditional branch, call, or return is executed only if certain

specified conditions are met. The conditional branch, call, and return

instructions are described in detail in Section 5.4,

Conditional Branches,

Calls, and Returns, on page 5-10.

5.3.1

Unconditional Branches

When an unconditional branch is encountered, it is always executed. During

the execution, the PC is loaded with the specified program-memory address

and program execution begins at that address. The address loaded into the

PC may come from either the second word of the branch instruction or the low-

er sixteen bits of the accumulator.

By the time the branch instruction reaches the execute phase of the pipeline,

the next two instruction words have already been fetched. These two instruc-

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

execution continues at the branched-to address. The unconditional branch

instructions are B (branch) and BACC (branch to location specified by accu-

mulator).

5.3.2

Unconditional Calls

When an unconditional call is encountered, it is always executed. When the

call is executed, the PC is loaded with the specified program-memory address

and program execution begins at that address. The address loaded into the

PC may come from either the second word of the call instruction or the lower

16 bits of the accumulator. Before the PC is loaded, the return address is saved

in the stack. After the subroutine or function is executed, a return instruction

loads the PC with the return address from the stack, and execution resumes

at the instruction following the call.

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