HCS08 datasheet(248/290 Pages) MOTOROLA

248 / 290 page

Development Support

MC9S08GB/GT

248

MOTOROLA

15.4.2 Bus Capture Information and FIFO Operation

The usual way to use the FIFO is to setup the trigger mode and other control options, then arm the

debugger. When the FIFO has filled or the debugger has stopped storing data into the FIFO, you would

read the information out of it in the order it was stored into the FIFO. Status bits indicate the number of

words of valid information that are in the FIFO as data is stored into it. If a trace run is manually halted by

writing 0 to ARM before the FIFO is full (CNT = 1:0:0:0), the information is shifted by one position and

the host must perform ((8 – CNT) – 1) dummy reads of the FIFO to advance it to the first significant entry

in the FIFO.

In most trigger modes, the information stored in the FIFO consists of 16-bit change-of-flow addresses. In

these cases, read DBGFH then DBGFL to get one coherent word of information out of the FIFO. Reading

DBGFL (the low-order byte of the FIFO data port) causes the FIFO to shift so the next word of information

is available at the FIFO data port. In the event-only trigger modes (see 15.4.5 Trigger Modes), 8-bit data

information is stored into the FIFO. In these cases, the high-order half of the FIFO (DBGFH) is not used

and data is read out of the FIFO by simply reading DBGFL. Each time DBGFL is read, the FIFO is shifted

so the next data value is available through the FIFO data port at DBGFL.

In trigger modes where the FIFO is storing change-of-flow addresses, there is a delay between CPU

addresses and the input side of the FIFO. Because of this delay, if the trigger event itself is a

change-of-flow address or a change-of-flow address appears during the next two bus cycles after a trigger

event starts the FIFO, it will not be saved into the FIFO. In the case of an end-trace, if the trigger event is

a change-of-flow, it will be saved as the last change-of-flow entry for that debug run.

The FIFO can also be used to generate a profile of executed instruction addresses when the debugger is

not armed. When ARM = 0, reading DBGFL causes the address of the most-recently fetched opcode to be

saved in the FIFO. To use the profiling feature, a host debugger would read addresses out of the FIFO by

reading DBGFH then DBGFL at regular periodic intervals. The first eight values would be discarded

because they correspond to the eight DBGFL reads needed to initially fill the FIFO. Additional periodic

reads of DBGFH and DBGFL return delayed information about executed instructions so the host debugger

can develop a profile of executed instruction addresses.

15.4.3 Change-of-Flow Information

To minimize the amount of information stored in the FIFO, only information related to instructions that

cause a change to the normal sequential execution of instructions is stored. With knowledge of the source

and object code program stored in the target system, an external debugger system can reconstruct the path

of execution through many instructions from the change-of-flow information stored in the FIFO.

For conditional branch instructions where the branch is taken (branch condition was true), the source

address is stored (the address of the conditional branch opcode). Because BRA and BRN instructions are

not conditional, these events do not cause change-of-flow information to be stored in the FIFO.

Indirect JMP and JSR instructions use the current contents of the H:X index register pair to determine the

destination address, so the debug system stores the run-time destination address for any indirect JMP or

JSR. For interrupts, RTI, or RTS, the destination address is stored in the FIFO as change-of-flow

information.

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