Syntax

              .fieldvalue[, size in bits]

Description

The .field directive initializes a multiple-bit field within a single word (16 bits) of memory. This directive has two operands:

• The value is a required parameter; it is an expression that is evaluated and placed in the field. The value must be absolute.
• The size in bits is an optional value from 1 to 32, which is the number of bits in the field. The default size is 16 bits. If you specify a size of 16 or more bits, the field starts on a word boundary. If you specify a value that cannot fit in size in bits, the assembler truncates the value and issues a warning message. For example, .field 3,1 causes the assembler to truncate the value 3 to 1 and print this message:

     *** WARNING! line 21: W0001: Field value truncated to 1
                     .field  3, 1

Successive .field directives pack values into the specified number of bits starting at the current word. Fields are packed starting at the least significant part of the word, moving toward the most significant part as more fields are added. If the assembler encounters a field size that does not fit into the current word, it writes out the word, increments the SPC, and begins packing fields into the next word. You can use the .align directive with an operand of 1 to force the next .field directive to begin packing into a new word.

The .field directive is similar to the .bits directive (see the .bits topic). However, the .bits directive does not force alignment to a field boundary and does not automatically increment the SPC when a word boundary is reached.

Use the .align directive to force the next .field directive to begin packing a new word.

If you use a label, it points to the word that contains the specified field.

When you use .field in a .struct/.endstruct sequence, .field defines a member's size; it does not initialize memory. For more information, see the .struct/.endstruct/.tag topic.

Example

This example shows how fields are packed into a word. The SPC does not change until a word is filled and the next word is begun.

       1              ************************************
       2              **    Initialize a 14-bit field.  **
       3              ************************************
       4 000000 0ABC          .field  0ABCh, 14
       5              
       6              ************************************************
       7              ** Initialize a 5-bit field in a new word.    **
       8              ************************************************
       9 000001 000A  L_F:    .field  0Ah, 5
      10              
      11              ************************************************
      12              ** Initialize a 4-bit field in the same word. **
      13              ************************************************
      14 000001 018A  X:      .field  0Ch, 4
      15              ************************************************
      16              ** Relocatable field in the next 2 words.     **
      17              ************************************************
      18 000002 0001'         .field  X
      19              ************************************
      20              **    Initialize a 32-bit field   **
      21              ************************************
      22 000003 4321          .field  04321h, 32
         000004 0000

Figure 6-5 shows how the directives in this example affect memory.

Figure 5-5 The .field Directive