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I am trying to figure out the encoding for unconditional JMPs on SPARC, i.e the JMP. After disassembling a few binaries.

In my IDA disassembly the encoding for JMP %g1 is:

81 c0 40 00 

Digging through the spark manuals, I can't seem to find a record of how this is encoded. I am also confused as to why IDA refers to a "JMP" as opposed to the "JMPL" in the docs.

The JMPL encoding recommendations given in the SPARC9 manual are a little arcane to me and I struggle with what they are getting at:

10-RD-OP3-RS1-i-[-]-rs2 

or

10-RD-OP3-RS1-i-siMM3

"If either of the low-order two bits of the jump address is nonzero, a mem_address_not_aligned exception occurs"

Well, I'm not sure how that squares with the instruction that IDA found. Can someone break down how this maps to JMP %g1? How would this change for JMP %g2?

  • I suspect it sees the 'L' as an implementation detail - often you'd just write JMP and leave it up to the assembler whether it generates a short or long jump (but my SPARC is rusty). %g2 - you should probably get hold of a SPARC assembler and experiment, e.g. GNU binutils is easy to build targeting any chip - if you can build it in a Linux VM or a Mac at least, or maybe Cygwin just works too nowadays. Or you could hex-edit your SPARC binary and run it through IDA again at a pinch to see if you can change it to g2 yourself. – Rup May 31 '17 at 6:59
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81 c0 40 00 can be broken down as follows

    
10 00000 111000 00001 0 0000000000000

^--op1
   ^--rd
         ^--op3
                ^--rs1
                      ^--i
                         ^--rs2/simmm13

To change the target from %g1 to %g2, just change the rs1 field from 00001 to 00010.

The 'L' is simply an artifact of at&t syntax vs intel syntax. In at&t syntax, the instruction name encodes information about the argument size. In intel syntax, that's done via the decorating of arguments.

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