BUG: SUBSTR Index Past End of String Incorrectly Assembles

ID: Q111502

The information in this article applies to:

Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, 6.11
Microsoft Macro Assembler (MASM) for Windows NT, version 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>

The directive is documented on pages 246-247 of the MASM 6.1x "Programmer[ASCII 146]s Guide."

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

Assembling this will correctly generate two errors (A2091 and A2006) on
lines 10 and 11 as shown in the following (partial) listing. These errors
should also be generated on lines 7 and 8 of the source, but aren't, and
the value 0 (zero) is incorrectly assigned to symbol op4.

               ; 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
test.asm(10): error A2091: index value past end of string
                  op6 sizestr op5          ; 11 - A2006 here
test.asm(11): error A2006: undefined symbol : op5
               END

Additional query words: 6.00 6.00a 6.00b 6.10 6.10a buglist6.10 buglist6.10b buglist6.11 buglist6.00 buglist6.00b buglist6.00a

Keywords :
Version : :6.0,6.0a,6.0b,6.1,6.11,6.1a
Platform :
Issue type :