PRB: SEGMENT AT Does Not Initialize Data at Absolute Address

• Microsoft Macro Assembler (MASM) for MS-DOS, versions 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b
• Microsoft Macro Assembler for OS/2, versions 5.1, 5.1a, 6.0, 6.0a, 6.0b

SYMPTOMS

When the SEGMENT directive is used with the AT combine type to place code or initialized data at an absolute address, the generated executable file does not initialize the segment at the specified absolute address. MASM versions 5.0, 5.1, and 5.1a do not generate any error or warning messages. MASM versions 6.0, 6.0a, and 6.0b generate the following warning message:

   A4014: instructions and initialized data not supported in AT
          segments

CAUSE

This warning is expected and caused by the design of the Microsoft Linker.

The Microsoft Linker (LINK), distributed with Microsoft programming languages, is designed to generate relocatable programs to run with MS-DOS, Microsoft Windows, or OS/2. LINK does not generate code or data at absolute addresses. MASM versions 6.0, 6.0a, and 6.0b generate a warning message because LINK does not support SEGMENT AT in this manner.

RESOLUTION

Link and locate tools that generate absolute addresses are available from third-party vendors.

MORE INFORMATION

The SEGMENT AT directive is used to access a specified memory address, for example, a code routine or data in the ROM BIOS. While SEGMENT AT allows an application to create a template for memory at a fixed address, it does not support initializing code or data at that address.

The code fragment below demonstrates the problem.

Sample Code

; Assembler options needed: none

interrupt_table SEGMENT AT 0 ORG 8*4 irq0 DD 12345678h irq1 DD 9ABCDEF0h interrupt_table ENDS

_text SEGMENT WORD PUBLIC 'CODE' ASSUME cs:_text start:

    mov ax, 4C00h
    int 21h

END start