Does Metal-C inline assembler support ternary address?



Help for C/C++ for MVS, OS/390 C/C++, z/OS C/C++ and C/C++ Productivity Tools for OS/390

Does Metal-C inline assembler support ternary address?

Postby chong_zhou » Sat Mar 09, 2019 12:23 pm

The MVC instruction requires 'ternary address' for the destination operand: D(L,B). If you want to use default L, you must write it like this: D(,B). If the comma is omitted, say D(L), the number in brackets is treated as L rather than B.

However, in Metal-C, if I write it like the following:


int val1, val2 = 0;
__asm(" MVC    %0,%1"
      : "=m"(val1)
      : "m"(val2)
      : );
 


The compiler will generates code like the following:

 MVC    124(13),128(13)
 


This is definitely wrong. The programme will probably ABEND because the instruction will copy 13 bytes of data rather than 4, and causes 'buffer overrun'. Actually I really tried this, and got 0C4 / 4 which is not a surprise.

So the question is how I can make the Metal-C compiler generate operand in such form: num(num, num), or at least num(,num)?

Thank you guys!!
chong_zhou
 
Posts: 24
Joined: Wed Nov 01, 2017 8:59 pm
Has thanked: 10 times
Been thanked: 0 time

Re: Does Metal-C inline assembler support ternary address?

 

Re: Does Metal-C inline assembler support ternary address?

Postby steve-myers » Sat Mar 09, 2019 10:08 pm

  • What do you mean by "ternary address"? AMODE 64? Then the problem is persuading the compiler to use 64-bit addresses and LG instructions to load base registers
  • MVC X(13),Y(13) assembles as MVC X(13,0),Y(13). If you think about it for a few seconds you can readily see the problem. Most Assembler programmers have done this several times in their careers. I certainly have!
Why not just write memcpy(x,y) where x and y are defined in the usual C fashion? I'd bet the compiler will write MVC X,Y with the appropriate base register assignment and the correct length. Much better than inline Assembler!

I admit I haven't done Metal C (or even much mainframe C), but memcpy strikes me as a much more usable approach! One of my early 16 bit C projects was a text window package. It used a lot of memcpy functions to build the elements of the window. No Assembler at all! Even on a single speed 8088 it was fast enough to please just about anyone even using the M$ Quick C compiler. Later on I got a copy of the M$ C Version 6 compiler and compiled my little package with it. I'm sure it was faster, but I couldn't see it!
steve-myers
Global moderator
 
Posts: 1998
Joined: Thu Jun 03, 2010 6:21 pm
Has thanked: 4 times
Been thanked: 216 times

Re: Does Metal-C inline assembler support ternary address?

Postby chong_zhou » Mon Mar 11, 2019 2:05 pm

Thank you for the quick reply, Steve. By 'ternary address', I meant the address expression consisted of 3 parts: D(X,B) or D(L,B). Sorry for the bad terminology.

If you look at the sample code that I posted at the beginning, you will notice that the metal C compiler generated 'MVC 124(13),...' which is wrong. Because from the whole listing I can see that it is using R13 as the base register, and the variable 'val1' is at offset 124, so it should be 'MVC 124(4,13),...'. I have no idea how I can make the metal C compiler do the right thing.

I don't think it needs a PMR for IBM, because if it were the metal C compiler's fault, it would have been already found out. I am not the first metal C programmer after all.

memcpy is a better choice for the the sample code I posted, but in my the case, I am actually extracting 'fields' from an area, and I only have a huge dsect macro.
chong_zhou
 
Posts: 24
Joined: Wed Nov 01, 2017 8:59 pm
Has thanked: 10 times
Been thanked: 0 time

Re: Does Metal-C inline assembler support ternary address?

Postby enrico-sorichetti » Mon Mar 11, 2019 7:54 pm

I remember doing an MVC using a pointer
something along the lines of
  __asm(" MVC 0(4,%0),%1"
            : "r"(_ptr1->var1)
            : "m"(var2)
            : );


but it was a Loong time ago and I cannot recover the source
cheers
enrico
When I tell somebody to RTFM or STFW I usually have the page open in another tab/window of my browser,
so that I am sure that the information requested can be reached with a very small effort
enrico-sorichetti
Global moderator
 
Posts: 2843
Joined: Fri Apr 18, 2008 11:25 pm
Has thanked: 0 time
Been thanked: 151 times


Return to C, C++

 


  • Related topics
    Replies
    Views
    Last post