HCS12 datasheet(86/452 Pages) FREESCALE

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S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

5.28 Background Mode and Null Operations

Background debug mode (BDM) is a special CPU12 operating mode that is

used for system development and debugging. Executing enter background

debug mode (BGND) when BDM is enabled puts the CPU12 in this mode.

For complete information, refer to Section 8. Instruction Queue.

Null operations are often used to replace other instructions during software

debugging. Replacing conditional branch instructions with branch never

(BRN), for instance, permits testing a decision-making routine by disabling

the conditional branch without disturbing the offset value.

Null operations can also be used in software delay programs to consume

execution time without disturbing the contents of other CPU registers or

memory.

Table 5-28 shows the BGND and null operation (NOP) instructions.

Table 5-28. Background Mode and Null Operation Instructions

Mnemonic

Function

Operation

BGND

Enter background debug mode

If BDM enabled, enter BDM;

else resume normal processing

BRN

Branch never

Does not branch

LBRN

Long branch never

Does not branch

NOP

Null operation

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