Re: [help] Issues with 'g' packet and MIPS - gdb interprets the packet reply wrong

[Christopher Bainbridge <cb0128@my.bristol.ac.uk>]

Many thanks!

It turns out that I mistook the M14000 to be the same as the M14k... 
which, annoyingly, they aren't.

So doing:

     set architecture mips:isa32r2

has solved my problem.

Thanks a lot!
Chris

On 21/10/14 21:34, Maciej W. Rozycki wrote:
> Hi Christopher,
>
>> I am trying to implement a remote stub for a MIPS cpu (using GDB version 7.8).
>> When GDB asks for the general registers using  the 'g' packet, I reply with:
>>
>> 00000000000000000d01000000000000fffdffff00000000000000000080c0bf30000000f0fec0bf000000002e0000000000000000000000000000008080808000000000000000000000000000000000000000000000000000000000000000000000000000000000000001800000000000000000e8fec0bf00000000702ec0bf0000000000000000000000000000000000000000702ec0bf
>>
>> As each register is 32 bits (represented by 8 hex characters), this should be
>> all the registers up to and including the PC.
>   What makes you assume the registers are 32 bits each?
>
>> However, GDB prints this out:
>>
>> info reg
>>            zero       at       v0       v1       a0       a1 a2       a3
>>   R0   00000000 0000010d fffffdff 00000000 00000030 00000000 00000000 00000000
>>              t0       t1       t2       t3       t4       t5 t6       t7
>>   R8   00000000 00000000 00000000 00000000 00000000 80010000 00000000 00000000
>>              s0       s1       s2       s3       s4       s5 s6       s7
>> Sending packet: $p13#d4...Ack
>> Packet received: 00000000
>> Sending packet: $p14#d5...Ack
>> Packet received: 00000000
>> Sending packet: $p15#d6...Ack
>> Packet received: 00000000
>> Sending packet: $p16#d7...Ack
>> Packet received: 00000000
>> Sending packet: $p17#d8...Ack
>> Packet received: 00000000
>>   R16  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>>              t8       t9       k0       k1       gp       sp s8       ra
>> Sending packet: $p18#d9...Ack
>> Packet received: 00000000
>> Sending packet: $p19#da...Ack
>> Packet received: 00000000
>> Sending packet: $p1a#02...Ack
>> Packet received: 00000180
>> Sending packet: $p1b#03...Ack
>>
>> etc
>>
>> This looks to be that it is determining the size of each register incorrectly,
>> and is thus asking for more registers using the 'p' packet.
>>
>> Is this a bug on my end or in GDB?
>> I use the command
>>
>> set processor mips:14000
>>
>> beforehand, as this is the processor we're using.
>   You mean:
>
> (gdb) set architecture mips:14000
>
> I presume, right?
>
>   The R14000 is a 64-bit processor so its registers are 64-bit and will be
> treated as such by default by GDB.  You may be able to limit the width of
> registers expected by selecting a 32-bit processor instead or by selecting
> a 32-bit ABI such as `o32'.  The latter can be done with:
>
> (gdb) set mips abi o32
>
> or by selecting a file to debug that has been built for that ABI.
>
>   Please note that this is a grey area though, with a bare-metal stub you
> should be really exchanging registers with GDB in their native sizes and
> letting GDB truncate and extend them as required depending on the ABI used
> by the program being debugged.  GDB is already capable of doing that,
> however in order to make use of that capability both the stub and GDB
> would have to support XML register descriptions which is something that
> owing to the vast number of CP0 register set variants in the MIPS
> architecture has never been implemented.  So in fact you may be hitting
> problems regardless of the ABI selection noted above.
>
>   You can always determine the widths of registers GDB expects with the:
>
> (gdb) maintenance print registers
>
> command -- see the `Type' column on the right for the internal type used
> and note that the registers exchanged with a remote stub are those in the
> low half of indices (`Nr' == `Rel'), the so called "raw registers".
>
>   HTH,
>
>    Maciej