BUG: SUBSTR Index Past End of String Incorrectly Assembles

- Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1,

  6.1a, and 6.11

SYMPTOMS

MASM 6.x will incorrectly assemble the SUBSTR directive and not generate the expected error if the index value is 1 (one) greater than the length of the string. The expected error is:

   A2091: index value past end of string

STATUS

Microsoft has confirmed this to be a problem in the Microsoft Macro Assembler (MASM) versions 6.x. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The sample code below illustrates this behavior as well as the correct use of SUBSTR.

The SUBSTR directive selects a value from a string and assigns it to a symbol. The index value (start value) cannot be greater than the length of the string. The format of the directive is:

   <name> SUBSTR <string>, <index>

   -or-

   <name> SUBSTR <string>, <start>, <length>

Sample Code

; Assemble options needed: /c /Fl .model small .data

   op1 substr <12345678>,8  ;  4
   op2 sizestr op1          ;  5

   op3 substr <12345678>,9  ;  7 - should be A2091 here
   op4 sizestr op3          ;  8 - should be A2006 here

   op5 substr <12345678>,10 ; 10 - A2091 here
   op6 sizestr op5          ; 11 - A2006 here
END

               ; Assemble options needed: /c /Fl
               .model small
 0000          .data
 = 8              op1 substr <12345678>,8  ;  4
 = 0001           op2 sizestr op1          ;  5

 =                op3 substr <12345678>,9  ;  7 - should be A2091 here
 = 0000           op4 sizestr op3          ;  8 - should be A2006 here

                  op5 substr <12345678>,10 ; 10 - A2091 here

                  op6 sizestr op5          ; 11 - A2006 here

               END