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Question on PL/I source example from PL/I Lang Ref Manual


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Duncan Sharppe

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Joined: 13 Dec 2013
Posts: 20
Location: USA

PostPosted: Wed Sep 03, 2014 11:40 pm
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Hello people of the forum!

The following text is from the PL/I Language Reference (SC14-7285-02)
page 450.

AVAILABLEAREA returns a FIXED BINARY(31,0) value.
The value returned by AVAILABLEAREA is the size of the largest
single allocation that can be obtained from the area x.

The example cited in the book on page 450 is:

{Note that I've added the line numbers for ease of reference.)

1 ... dcl Uarea area(1000);
2 ... dcl Pz ptr;
3 ... dcl C99z char(99) varyingz based(Pz);
4 ... dcl (SizeBefore, SizeAfter) fixed bin(31);
5 ... SizeBefore = availablearea(Uarea); /* returns 1000 */
6 ... Alloc C99z in(Uarea);
7 ... SizeAfter = availablearea(Uarea); /* returns 896 */
8 ... dcl C9 char(896) based(Pz);
9 ... Alloc C9 in(Uarea);

From my understanding the allocate in statement number (6) allocated 100
bytes in area (Uarea) qualified by the based variable (C99z).

The returned value from availablearea in statement (7) is said to be
896; I believe this is incorrect it should be 900 as the varible (C99z)
has been the only thing allocate in area (Uarea).

It appears that the writer has assumed the pointer(Pz) been allocated
in the area as well. Or has mistakenly added the additional 4-bytes
of the pointer to the allocation and thus cites the number 896 in
the comments of line 8 as the remaining free space.

Is my understanding correct or I am missing something obvious.

TIA for any clirafications.

friarDuncan
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prino
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Joined: 07 Feb 2009
Posts: 1246
Location: Vilnius, Lithuania

PostPosted: Thu Sep 04, 2014 12:57 am
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The example is correct, the size of variables allocated inside an area is rounded up to the next multiple of 8 bytes.
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Duncan Sharppe

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PostPosted: Thu Sep 04, 2014 1:44 am
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Thank you!

I think I'll write ASM some code to examine the area and the "allocated data" within it or if I feel bolder I will TEST the proc and ASM code in TSO.

Whew!

friarDuncan
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prino
Warnings : 1

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Joined: 07 Feb 2009
Posts: 1246
Location: Vilnius, Lithuania

PostPosted: Thu Sep 04, 2014 2:55 am
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This should get you started, no assembler required:

Code:
dcl 1 * union,
      2 area(1000),
      2 ar_ea,                         /* Overlay on area            */
        3 use_list  bit        (1),    /* Free list in use           */
        3 filler    char       (3),
        3 nab       fixed bin (31),    /* Next available byte        */
        3 ffree     fixed bin (31),    /* Offset of first free byte  */
        3 eol       fixed bin (31),    /* Always 0, end of free list */
        3 contents  char    (1000);    /* Contents                   */
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Duncan Sharppe

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Joined: 13 Dec 2013
Posts: 20
Location: USA

PostPosted: Thu Sep 11, 2014 2:54 am
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Hehehe...

Sorry for not getting back to you sooner...

That is some code!!!

I really like your knowledge set and I love learning about internals.

Much Thanks.

friarDuncan
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