ST20-GP6 datasheet(80/123 Pages) STMICROELECTRONICS

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ST20-GP6

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Figure 14.2 Message formats

14.5 Peeking and poking the host from the target

The target CPU can

peek and poke the host via the diagnostic controller. This is done by reading or

writing a single word to a block of addresses within the DCU register block. The DCU will then send

peek or poke message to the host. After a host peek, the target CPU will wait until the host

responds with a

peeked message, which the DCU returns to the CPU as memory read data.

Peeking and poking the host from the target can be enabled or disabled. After reset, these bits are

cleared, so peek and poke from the target are disabled.

14.6 Abortable instructions

14.6.1 Properties of the hardware implementation

In the ST20-C2 core, some instructions are abortable, i.e. they may be “started” more than once. In

the instruction set chapter, abortable instructions are marked with an ‘A’ in the notes column, indi-

cating that the instruction can be aborted and later restarted.

The breakpoint mechanism in the DCU, follows the CPU behavior, and takes a trap in place of

starting an instruction with an

Iptr which matches. Care is taken in the hardware to ensure that any

interrupts which might have occurred following the preceding instruction are allowed in, and the

trap is taken only if the CPU was about to start the instruction with an

Iptr which matches.

If the DCU is programmed to break on an instruction, then normally, following the trap return

instruction, that instruction is executed. In this scenario, all instructions should be considered as

abortable. If an interrupt occurs between the end of the trap handler and the start of the instruction,

then when the interrupt completes the DCU will again trap on that instruction (if the breakpoint is

repeatable).

The user needs to be aware that setting a breakpoint on a given instruction may break more than

once on the same instruction in the same thread.

Poke

Command messages

Response messages

Address

First data word

Second data word

Peek

Address

Peeked

First data word

Second data word

Third data word

Triggered

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