wrong opcode?

I curious as to why the code generated for these two move.w commands is completely different. In the first assembly I used two variables (one equal to 0 and the other equal to 8). I tried doing a move using the variables added together as my offset to register A1 and managed to get 33B9 0000 0000 0170 0000 0008. When I simply put an 8 in instead of the variables, I got the correct code. 3379 0000 0000 0008. Any ideas??????

I'm using gnu as on a pc-cygwin system with a m68k target.

3901 034a 0000 0008         .DC.L   C_WCP1_MSG_PARM
 3902 034e 0000 0000        .DC.L   HWPN_PARM_OFS

3903 0352 33B9 0000 MOVE.W SYS_SYS_VERS_W,C_WCP_MSG_PARM+HWPN_PARM_OFS(%A1)
3903 0000 0170
3903 0000 0008

 3901 034a 0000 0008        .DC.L   C_WCP1_MSG_PARM
 3902 034e 0000 0000        .DC.L   HWPN_PARM_OFS

3904
3905 0352 3379 0000 MOVE.W M_SYS_SYS_VERS_W,8(%A1)
3905 0000 0008
3906

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From what I see, in the case of 0x33b9, its using mode 6 for the destination which with the extension word of 0x0170 that decodes(look at page 2-2 of the MC68000 Family Programmer's Reference Manual) into a full extension word format where:

D/A = 0
REG = 0
W/L = 0
SCALE = 0
BS = 0
IS = 1
BDSIZE = 3
I/IS = 0

Which means that the index register is suppressed, the base register is not suppressed, the base displacement size is 32 bits, I/S indicates that no memory indirection is done(so its not pre-indexed or post-indexed), so this is equivilent to addressing mode 5 but with a 32 bit displacement instead of the 16 bits that mode 5 supports.

I think that this addressing mode is being picked since the assembler sees the expression C_WCP_MSG_PARM+HWPN_PARM_OFS and determines that the resulting expression contains 32 bits of significance(including all possible relocation) and hence can't stuff it into addressing mode 5.

Check how C_WCP_MSG_PARM and HWPN_PARM_OFS are being declared. is it with .equ, or is one of them a label(which requires 32 bits of significance to allow its relocation anywhere in the 4GB address space)?

Hope this helps...

Peter Barada                                   [email protected]
Wizard                                         781-852-2768 (direct)
WaveMark Solutions(wholly owned by Motorola)   781-270-0193 (fax)

<[email protected]> writes:

|> I curious as to why the code generated for these two move.w commands is
|> completely different. In the first assembly I used two variables (one equal to 0
|> and the other equal to 8). I tried doing a move using the variables added
|> together as my offset to register A1 and managed to get 33B9 0000 0000 0170 0000
|> 0008. When I simply put an 8 in instead of the variables, I got the correct
|> code. 3379 0000 0000 0008. Any ideas??????
|> 
|> I'm using gnu as on a pc-cygwin system with a m68k target. 
|> 
|> 
|> 3901 034a 0000 0008         .DC.L   C_WCP1_MSG_PARM
|>  3902 034e 0000 0000        .DC.L   HWPN_PARM_OFS
|>  3903 0352 33B9 0000 MOVE.W SYS_SYS_VERS_W,C_WCP_MSG_PARM+HWPN_PARM_OFS(%A1)
|>  3903      0000 0170 
|>  3903      0000 0008 

The other opcode is not wrong, it just uses the 32bit offset form. How are C_WCP_MSG_PARM and HWPN_PARM_OFS defined? Please provide a complete, self-contained test case.

Andreas.

Andreas Schwab, SuSE Labs, [email protected]
SuSE Linux AG, Deutschherrnstr. 15-19, D-90429 Nürnberg
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