Bootstrapping An MSDOS Assembler With Batch Files

You have a clean MSDOS system, and you need to write some software for it. What do you do? You could use debug, of course. But there are no labels so while you can get machine code from mnemonics, you’ll still need to figure out the addresses on your own. That wasn’t good enough for [mniip], who created an assembler using mostly batch files. There are a few .COM files and it looks as if the first time you use debug to create those, but there’s also source you can assemble on subsequent builds with the assembler.

Why? We aren’t entirely sure. But it is definitely a hack. The technique sort of reminded us of our own universal cross assembler — sort of.

There are a few things that make this work. First, there are not many 8086 instructions to worry about. Second, you have to use a special format — essentially prefixing the op codes with CALL. This keeps the assembler from having to parse op codes. You actually call a batch file with the name of the instruction. For example:

CALL PUSH CS
CALL POP DS
CALL MOV DX WORD %String%

CALL LABEL String
REM H e l l o , w
CALL DB 72 101 108 108 111 44 32 119

That code snippet shows another nuance. You have to CALL LABEL to introduce a label. To use the label in an instruction, you have to surround it with percent signs.

Of course, as a practical matter, you could use gcc to build a proper assembler. But where’s the sport in that?