PRB: SIZE Operator Value

• Microsoft Macro Assembler for MS-DOS, versions 4.0, 5.0, 5.1, and 6.0
• Microsoft Macro Assembler for OS/2, versions 5.1 and 6.0

SYMPTOMS

When trying to get the length of a text string, using the SIZE operator can produce undesired results. In the example below, MSG_LEN is set to a value of 1, but it seems it should be set to a value of 11:

   MSG DB 'Darth Vader'
   MSG_LEN EQU SIZE MSG

CAUSE

The value of MSG_LEN should be 1. The SIZE operator is defined to be the LENGTH * TYPE as stated in the documentation. The expression, LENGTH MSG, will be equal to 1 (when applied to a data label, the LENGTH operator returns the number of elements created by the DUP operator; otherwise it returns 1) and the expression, TYPE MSG, will be equal to 1. Therefore, the variable MSG_LEN will be equal to 1.

RESOLUTION

In Macro Assembler version 6.0, the SIZEOF operator can be used to calulate the total number of bytes allocated to a data item. In versions prior to 6.0, a simple way to calculate the value you want is as follows:

   MSG DB 'Darth Vader'
   MSG_LEN EQU $-MSG