NULL mag Issue 04 13 Dos Assembler with batch files

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Published in 

null magazine

 · 3 years ago

  

https://hackaday.com/2018/12/10/ 
bootstrapping-an-msdos-assembler-with-batch-files/#more-336004 

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. 

https://github.com/mniip/BOOTSTRA 

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?