IBM Mainframe Forum

[bobguo]

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 000D68 D200 5260 C000 00260 00000  3067          MVC   PREFIX(1),0(R12)                                         08150000
 000D6E D201 5261 C000 00261 00000  3068          MVC   PREFIX+(1),0(R12)                                        08160000 

i think 'MVC PREFIX+(1),0(R12) ' can be rewirted to 'MVC PREFIX+1(1),0(R12)'.
but i have one question, why it was assembled to

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D201 5261 C000 00261

instead of

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D200 5261 C000 00261

[enrico-sorichetti]

because somewhere PREFIX was defined as a 2 bytes field !

[bobguo]

because somewhere PREFIX was defined as a 2 bytes field !

you are right! it was defined as a 2 bytes field.
but i don't know how you can ensure its length is 2. could you explain 'MVC PREFIX+(1),0(R12)' in detail. Thx!

[enrico-sorichetti]

thats because of the format of the SS instruction

8 bits OP CODE
8 bits LENGTH - 1
...
...

since no length was specified the implied length from the DS/DC was used

[bobguo]

ooo. i see.
Thank you very much
i mistaked '+(1)' as the length of move.
but as Enrico said, it should be offset.

[steve-myers]

There are two types of SS instructions: instructions with one length, from 1 to 256, and instructions with two lengths like PACK, UNPK and the packed decimal arithmetic instructions.

As others have said, it can be very easy to screw up the specification. One would certainly think that after 40+ years writing S/360 style Assembler I'd never mess this up. WRONG! Every few months I blow it, though somewhat differently, like this:

MVC offset(reg),SOURCE

rather than

MVC offset(length,reg),SOURCE

After the S0C4 I smack my forehead (again), throw out useful comments like Stupid, along with other words not to be repeated in mixed company.