PRB: Error "constant expected" with ORG Directive

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

SYMPTOMS

The statement 'ORG $+1' assembles without error, but the statements 'ORG $*2' and 'ORG $/2' produce an error. Under MASM 5.0, 5.1, and 5.1a the error message is:

   error A2042: Constant expected

   error A2026: constant expected

CAUSE

The location counter ($) has the same attributes as a near label. Thus, MASM emits an offset relative to the start of the current segment.

The linker must add the base address of the segment to do the fixup. When you use $+1, MASM emits an offset equal to one more than the current value of $. When you use $-1, MASM emits an offset equal to one less than the current value of $.

When the linker adds the segment base address, the desired value is obtained. Because the linker only adds the segment base address, there is no way to do multiplicatiion or division; only addition and subtraction can be performed. Since the Linker cannot multiply and divide, these operations cannot be applied to symbols that are relocatable.

RESOLUTION

This is the expected behavior of MASM.

Sample Code

; Assemble options needed: none

.MODEL small

.CODE start:

    mov ax,4C00h
    int 21h