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This is a follow up question to this. I'm writing code for an __AVR_HAVE_LPMX__ processor (avr25) using

  • GNU C (WinAVR 20100110) version 4.3.3 (avr) / compiled by GNU C version 3.4.5 (mingw-vista special r3), GMP version 4.2.3, MPFR version 2.4.1.

when I use lpm rd, Z in inline assembly, the compiler always translates this to lpm rd,Z+ (excerpt from lss-file):

asm volatile("lpm r24,Z");
248:    84 91           lpm r24, Z+

which is bad, if it's used in successive access to a look-up-table. The look-up value might be 0xff, so this unnecessarily increments ZH (r31) thus corrupting this solution.

Any suggestions to circumvent this behavior?

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Thomas
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1 Answers1

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There is nothing wrong in the solution; your disassembler (avr-objdump -d, part of binutils package) is buggy.

See page 97 of the Atmel AVR instruction set manual (PDF). The lpm instruction variants are encoded as

1001 0101 1100 1000 = 0x95C8   lpm r0,Z
1001 000? ???? 0100 = 0x9??4   lpm r?,Z
1001 000? ???? 0101 = 0x9??5   lpm r?,Z+

Assuming we trust the Atmel documentation better than your disassembler, then

84 91    lpm r24,Z

whereas

85 91    lpm r24,Z+

Indeed, avr-gcc (GCC) 4.8.2 compiles the inline assembly to the same two bytes (84 91) using avr-gcc-4.8.2 -O2 -fomit-frame-pointer -mmcu=avr25, and lists it as lpm r24,Z in the assembly source file (using -S option); when compiled to an object file, and disassembling with avr-objdump (GNU binutils) 2.23.1 using avr-objdump -d, the instruction is still 84 91 lpm r24,Z.

This makes me believe it is a bug in avr-objdump (part of GNU binutils). Ah yes, reported here, and apparently fixed in binutils-2.23.1 in October 2013.

In short, only the disassembly is affected; the disassembly incorrectly shows Z+ when it should show Z. It does not affect the generated code, only the human-readable output is incorrect. To fix, upgrade to binutils version 2.23.1 or later. If you can't, don't worry: you can safely ignore this error, as it only affects the human-readable disassembly.

Questions?

Nominal Animal
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  • That's right. In the simulator disassembler it shows: `+0000011B: 9184 LPM R24,Z`. I gotta learn which tool to trusts. Thanks! – Thomas Sep 09 '14 at 13:07