How can I write a far absolute JMP or CALL instruction using MASM? Specifically how do I get it to emit these instruction using the EA and CA opcodes, without manually emitting them using DB or other data directives?
For example consider the case of jumping to the BIOS reset entry point at FFFF:0000 in a boot sector. If I were using NASM I could code this in one instruction in the obvious way:
jmp 0xffff:0
With the GNU assembler the syntax is less obvious, but the following will do the job:
jmp 0xffff, 0
However when I try the obvious solution with MASM:
jmp 0ffffh:0
I get the following error:
t206b.asm(3) : error A2096:segment, group, or segment register expected
There are a number of possible workarounds I could use in MASM, like any of the following:
Hand assemble the instruction, emitting the machine code manually:
DB 0EAh, 0, 0, 0FFh, 0FFh
Use a far indirect jump:
bios_reset DD 0ffff0000h
...
jmp bios_reset ; FF 2E opcode: indirect far jump
Or push the address on the stack and use a far RET instruction to "return" to it:
push 0ffffh
push 0
retf
But is there anyway I can use an actual JMP instruction and have MASM generate the right opcode (EA)?
There's one way you can do it, but you need to use MASM's /omf switch so that it generates object files in the OMF format. This means the object files need to be linked with an OMF compatible linker, like Microsoft's old segmented linker (and not their current 32-bit linker.)
To do it you need to use a rarely used and not well understood feature of MASM, the SEGMENT directive's AT address attribute. The AT attribute tells the linker that the segment lives at a fixed paragraph address in memory, as given by address. It also tells the linker to discard the segment, meaning the contents of the segment aren't used, just its labels. This is also why the /omf switch has to be used. MASM's default object file format, PECOFF, doesn't support this.
The AT attribute gives you the segment part of the address we want to jump to. To get the offset part all you need to do is use the LABEL directive inside the segment. Combined with the ORG directive, this lets you create a label at the specific offset in the specific segment. All you then need to do is use this label in the JMP or CALL instruction.
For example if you want to jump to the BIOS reset routine you can do this:
bios_reset_seg SEGMENT USE16 AT 0ffffh
bios_reset LABEL FAR
bios_reset_seg ENDS
_TEXT SEGMENT USE16 'CODE'
jmp bios_reset
_TEXT ENDS
Or if you want to call the second stage part of your boot loader whose entry point is at 0000:7E00:
zero_seg SEGMENT USE16 AT 0
ORG 7e00h
second_stage LABEL FAR
zero_seg ENDS
_TEXT SEGMENT USE16 'CODE'
call second_stage
_TEXT ENDS