PRB: Jumping to an Absolute Addresss with MASM

ID: Q49900

The information in this article applies to:

Microsoft Macro Assembler 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

The Microsoft Macro Assembler (MASM) will not let you jump directly to a hexadecimal address. If you try to assemble:


   jmp 0FFFFh:0000h

You will receive the error message:

A2038: left operand must have segment

CAUSE

This is the expected behavior for MASM. To make a jump to an absolute address you must create a pointer to point to the offset inside of the segment.

RESOLUTION

To create a pointer to point to the offset inside of the segment in question you would create a segment at a specific location. For example:


   bios SEGMENT AT 0FFFFh
   ORG 0
   reset LABEL FAR
   bios ENDS

Then, any one of the following three jump instructions will perform the jump to offset 0000h within segment FFFFh:


   jmp FAR PTR reset
   jmp bios:reset
   jmp reset

Note that the opcode that is generated in the listing file is EA 0000 ----

MORE INFORMATION

This process is covered on Page 101 of the "Microsoft Macro Assembler 5.0: Programmer's Guide." The second paragraph states the following:

An AT segment typically contains no code or initialized data. Instead, it represents an address template that can be placed over code or data already in memory, such as a screen buffer or other absolute memory locations defined by hardware. The linker will not generate any code or data for AT segments, but existing code or data can be accessed by name if it is given a label in an AT segment.

Additional query words: 5.00 5.10 5.10a 6.00 6.00a 6.00b bios rom

Keywords :
Version : :5.0,5.1,5.1a,6.0,6.0a,6.0b
Platform :
Issue type :