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AIX Version 4.3 Assembler Language Reference

.comm Pseudo-op


Defines an uninitialized block of storage called a common block, which can be common to more than one module.


.comm Qualname,Expression[,Number]

where QualName = Name[[StorageMappingClass]]

Note: Name is required. StorageMappingClass is optional and enclosed within brackets if specified. RW is the assumed default ifStorageMappingClass is omitted.


The .comm pseudo-op defines a block of storage specified by the Qualname parameter. The the block size is specified in bytes by the Expression parameter.

Note: By convention, use of the TD storage mapping class is restricted to common blocks no more than four (4) bytes long.

The valid values for StorageMappingClass are RW, TD, UC, and BS. These values are explained in the article on the .csect pseudo-op. If any other value is used for StorageMappingClass, the default value RW is used and a warning message is reported if the -w flag is in effect.

If TD is used for the storage mapping class, a block of zeroes, the length specified by the Expression parameter, will be written into the TOC area as an initialized csect in the .data section. If RW, UC, or BS is used as the storage mapping class, the block is not initialized in the current module and has symbol type CM (Common). At load time, the space for CM control sections with RW, UC, or BC storage mapping classes is created in the .bss section at the end of the .data section.

Several modules can share the same common block. If any of those modules have an external Control Section (csect) with the same name and the csect with the same name has a storage mapping class other than BS or UC, then the common block is initialized and becomes that other Control Section. If the common block has TD as its storage mapping class, the csect will be in the TOC area. This is accomplished at bind time.

If more than one uninitialized common block with the same Qualname is found at bind time, space is reserved for the largest one.

A common block can be aligned by using the Number parameter, which is specified as the log base 2 of the alignment desired.


Qualname Specifies the name and storage mapping class of the common block. If the StorageMappingClass part of the parameter is omitted, the default value RW is used. Valid StorageMappingClass values for a common block are RW, TD, UC and BS.
Expression Specifies the absolute expression that gives the length of the specified common block in bytes.
Number Specifies the optional alignment of the specified common block. This is specified as the log base 2 of the alignment desired. For example, an alignment of 8 (or doubleword) would be 3 and an alignment of 2048 would be 11. This is similar to the argument for the .align pseudo-op.


  1. The following example demonstrates the use of the .comm pseudo-op:
            .comm proc,5120
    # proc is an uninitialized common block of
    # storage 5120 bytes long which is
    # globally visible.
    # Assembler SourceFile A contains:
            .comm st,1024
    # Assembler SourceFile B contains:
            .globl st[RW]
            .csect st[RW]
            .long 1
            .long 2
    # Using st in the above two programs refers to
    # Control Section st in Assembler SourceFile B.
  2. This example shows how two different modules access the same data:
    1. Source code for C module td2.c :
      /* This C module named td2.c   */
      extern long   t_data;
      extern  void mod_s();
              t_data = 1234;
               printf("t_data is %d\n", t_data);
    2. Source for assembler module mod2.s :
              .file "mod2.s"
              .csect .mod_s[PR]
              .globl .mod_s[PR]
              .set    RTOC, 2
              l   5, t_data[TD](RTOC)  # Now GPR5 contains the
                                       # t_data value
              ai  5,5,14
              stu 5, t_data[TD](RTOC)
              .comm  t_data[TD],4  # t_data is a global symbol
    3. Instructions for making executable td2 from the C and assembler source:
      as -o mod2.o mod2.s
      cc -o td2 td2.c mod2.o
    4. Running td2 will cause the following to be printed:
      t_data is 1248

Related Information

Pseudo-ops Overview.

The .align pseudo-op, .csect pseudo-op, .globl pseudo-op, .lcomm pseudo-op, .long pseudo-op.

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