Re: [Qemu-devel] [PATCH] target-mips: fix decoding of microMIPS POOL32Axf instructions

[Maciej W. Rozycki]

On Mon, 5 Aug 2013, Leon Alrae wrote:

> Just to make sure that we are refering to the same thing :)
> For MFHI:
> page no. 303 in MIPS Architecture for Programmers Volume II-B: The
> microMIPS32 Instruction Set (document MD00764)
> page no. 140 in MIPS Architecture for Programmers Volume IV-e: The MIPS
> DSP Module for the microMIPS32 Architecture (document MD00582)

 Where instruction encoding is concerned please remember to always refer 
to opcode tables rather than individual instruction description pages in 
case there is a documentation erratum.  If that happens, then opcode 
tables take precedence as they are generated automatically from actual 
architecture descriptions.  We had such a case already actually that was 
spotted late enough that it was architecture that had to be modified to 
match software expectations (QEMU included).

 So in this case it is:

Table 6.5 POOL32Axf Encoding of Minor Opcode Extension Field, p. 498, 
"MIPS Architecture for Programmers, Volume II-B: The microMIPS32 
Instruction Set", Document Number: MD00582, Revision 5.01, December 16, 
2012

and:

Table 5.5 POOL32Axf Encoding of Minor Opcode Extension Field, p. 66, "MIPS 
Architecture for Programmers, VolumeIV-e: The MIPSR DSP Module for the 
microMIPS32 Architecture", Document Number: MD00764, Revision 2.40, 
December 16, 2012

that you should be referring to (note that you've got the document numbers 
reversed too ;) ).

 I agree it looks weird and wasteful to have two separate encodings for 
instructions that operate on the HI[0] and LO[0] registers, but I suppose 
it makes the wiring of the DSP Disabled exception easier.

  Maciej