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Re: cortex-m xml register descriptions for m-system
- From: Tristan Gingold <gingold at adacore dot com>
- To: Christopher Friedt <chrisfriedt at gmail dot com>
- Cc: Pedro Alves <palves at redhat dot com>, gdb-patches at sourceware dot org
- Date: Tue, 15 Dec 2015 12:44:20 +0100
- Subject: Re: cortex-m xml register descriptions for m-system
- Authentication-results: sourceware.org; auth=none
- References: <CAF4BF-RuPwFWfDa2Sp7MzYjF8bo1K3xb=jMThSpK4T7gTe+whQ at mail dot gmail dot com> <566F108D dot 1000401 at redhat dot com> <CAF4BF-TUH0V4=YY07u9n3q=dMecbjMr9cOrEm=2BDXeP3HrDQQ at mail dot gmail dot com>
> On 15 Dec 2015, at 00:11, Christopher Friedt <firstname.lastname@example.org> wrote:
> On Dec 14, 2015 1:55 PM, "Pedro Alves" <email@example.com> wrote:
>> On 12/14/2015 05:04 PM, Christopher Friedt wrote:
>>> Hi list,
>>> I've been using GDB and OpenOCD to debug ARM Cortex-M devices for
>>> quite a while. One thing that I always noticed when using OpenOCD is
>>> that the m-system registers are listed, which is *incredibly* useful
>>> for writing code on just about any Cortex-M microcontroller.
>>> Somewhat recently, Qemu has also begun to support Cortex-M based
>>> virtual devices, and it seems to be fairly usable.
>>> The down side, is that they do not expose the m-system registers,
>>> simply because binutils-gdb does not (at this time) have an XML file
>>> for them.
>>> Just to catch anyone up to speed who might be reading this, the
>>> m-system registers are
>>> MSP (main stack pointer)
>>> PSP (process stack pointer)
>>> PRIMASK (1-bit register that says if interrupts are enabled)
>>> BASEPRI (8-bit register that sets the NVIC base priority)
>>> FAULTMASK (1-bit register that says if fault interrupts are enabled)
>>> CONTROL (3-bit register that indicates presence of FP, whether PSP is
>>> selected, and whether running in unprivileged mode)
>>> Now, these are "system" registers, and on a full blown microprocessor,
>>> it might be unusual to expose them, but on a microcontroller, it's
>>> quite important. The other debuggers that I have seen (IAR,
>>> specifically) also list the m-system registers along with the general
>>> purpose ones for Cortex-M.
>>> The following XML is sufficient to describe the m-system registers so
>>> that they appear to the GDB client.
>>> <feature name="org.gnu.gdb.arm.m-system">
>>> <reg name="msp" bitsize="32" type="data_ptr"/>
>>> <reg name="psp" bitsize="32" type="data_ptr"/>
>>> <reg name="primask" bitsize="1" type="int8"/>
>>> <reg name="basepri" bitsize="8" type="int8"/>
>>> <reg name="faultmask" bitsize="1" type="int8"/>
>>> <reg name="control" bitsize="3" type="int8"/>
>> Does GDB need to be aware of these registers at all? That is, does gdb
>> need to be aware of org.gnu.gdb.arm.m-system? Usually GDB needs to
>> be aware of specific registers if for instance Dwarf can refer to them.
>> Otherwise, the design of xml descriptions is such that you're free
>> to send any additional registers you want without a specific feature.
>> GDB will show them.
> Hmm... It's hard for me to say. The MSP and PSP are banked stack
> pointers, control instructs the core which stack pointer to use, and
> they are also tightly coupled to exception entry, so I would lean
> towards yes?
I do think so too.
I have just written a patch so that gdb unwinds correctly on cortex-m
exceptions, and this of course requires that gdb knows about at least
I plan to submit it early January.