From source to program: assemble, link and run, the three toolchains side by side — 8086 assembly guide

An assembler turns each source file into an object module; a linker joins the modules and libraries into a program. The same steps with MASM, TASM and JWasm, side by side.

Two programs stand between a source file and something DOS can run. The assembler reads one .ASM file and writes an object module (.OBJ): the machine code, the names the module defines for others and the names it needs from them, and notes on every place that holds an address not yet known. The linker reads object modules and libraries, matches each needed name with its definition, lays the segments out one after another, fills in the addresses, and writes the program.

The program is a .EXE or a .COM. A .EXE starts with a header: where the code starts (CS:IP), where the stack is (SS:SP), and a table of the words DOS must adjust because the program is loaded at a segment nobody knew in advance. It can be as large as memory allows. A .COM is a plain memory image, loaded at offset 100h of one segment and started there; it has no header and no relocations, so it fits in 64 KB and uses no segment addresses of its own. Both assemblers and linkers can also write a listing (.LST: each source line with its offset and bytes) and a map (.MAP: where every segment and public name ended up).

The cheat sheet

To	MASM 6.11	JWasm	TASM
make a one-file .COM	ML /AT HELLO.ASM	JWASMR -bin -Fo=HELLO.COM HELLO.ASM	TASM HELLO, then TLINK /t HELLO
make a one-file .EXE	ML HELLOX.ASM	JWASMR -mz HELLOX.ASM	TASM HELLOX, then TLINK HELLOX
only assemble	ML /c MAIN.ASM PRINT.ASM	JWASMR MAIN.ASM PRINT.ASM	TASM MAIN, then TASM PRINT
link modules	LINK MAIN+PRINT,PROG;	JWLINK format dos file MAIN.OBJ,PRINT.OBJ name PROG.EXE	TLINK MAIN PRINT, PROG
build a library	LIB UTIL.LIB +PRINT.OBJ;	JWLIB -n UTIL.LIB +PRINT.OBJ	TLIB UTIL +PRINT
link with a library	LINK MAIN,PROG,,UTIL.LIB;	JWLINK format dos file MAIN.OBJ library UTIL.LIB name PROG.EXE	TLINK MAIN, PROG,, UTIL
write a listing	ML /Fl ...	JWASMR -Fl ...	TASM /l ...
write a map	ML /Fm ..., or LINK's third field	JWLINK ... option map	TLINK writes one (/x: none)
debug with symbols	ML /Zi, then CV PROG	DEBUG (no symbols)	TASM /zi, TLINK /v, then TD PROG
step in the console	DEBUG PROG.EXE	DEBUG PROG.EXE	DEBUG PROG.EXE

Every MASM and JWasm line above was run on a dosbench box, and so were TLINK and TLIB; TASM itself was not (see its article). The sources are the two below and the pair in the article on several modules.

The two kinds of program

HELLO.ASM: a .COM. The tiny model puts code and data in one segment; ORG 100h leaves room for the PSP.

        .MODEL  TINY
        .CODE
        ORG     100h
START:  MOV     DX, OFFSET MSG
        MOV     AH, 09h
        INT     21h
        MOV     AX, 4C00h
        INT     21h
MSG     DB      'Hello from a .COM', 13, 10, '$'
        END     START

HELLOX.ASM: a .EXE. The small model has a code segment and a data segment (with the stack); the program points DS at its data itself.

        .MODEL  SMALL
        .STACK  256
        .DATA
MSG     DB      'Hello from an .EXE', 13, 10, '$'
        .CODE
START:  MOV     AX, @DATA
        MOV     DS, AX
        MOV     DX, OFFSET MSG
        MOV     AH, 09h
        INT     21h
        MOV     AX, 4C00h
        INT     21h
        END     START

On a dosbench box

The workbench's Build button assembles the open file for you: with MASM 6.11 when the box has it in C:\MASM611, otherwise with JWasm from C:\JWASM, and keeps the listing so the debugger can show your source lines. Add JWasm, JWlink and DOS-debug to a box from the catalog; they are free. MASM and TASM are not, so bring your own copy in the box's files if you have one.