Programs in several modules: PUBLIC, EXTRN and libraries — 8086 assembly guide

A module offers names with PUBLIC and asks for others with EXTRN; the linker joins them. A library is a file of modules the linker takes only when they are needed.

A name in a module is private to it until the module declares it PUBLIC; then the object file carries it for the linker. A module that uses a name defined elsewhere declares it EXTRN with its type, so the assembler knows how to encode each use: NEAR and FAR for procedures (a near or far CALL), BYTE, WORD and DWORD for variables, ABS for a constant. The assembler leaves the address blank and the linker fills it in.

MAIN.ASM

; MAIN.ASM: the program; PRINT comes from another module
        .MODEL  SMALL
        .STACK  256
        .DATA
MSG     DB      'Two modules, one program', 13, 10, '$'
        .CODE
        EXTRN   PRINT:NEAR
START:  MOV     AX, @DATA
        MOV     DS, AX
        MOV     DX, OFFSET MSG
        CALL    PRINT
        MOV     AX, 4C00h
        INT     21h
        END     START

PRINT.ASM

; PRINT.ASM: a module of routines for others to call
        .MODEL  SMALL
        .CODE
        PUBLIC  PRINT
; PRINT: write the $-terminated string at DS:DX
PRINT   PROC    NEAR
        MOV     AH, 09h
        INT     21h
        RET
PRINT   ENDP
        END

Both modules use the small model, so their .CODE sections are the same segment (_TEXT) and their .DATA sections the same _DATA: the linker puts the pieces of a segment next to each other, and a near CALL from MAIN reaches PRINT. Only one module ends with END start-label; that label is the program's entry point. Only one needs .STACK.

Assemble each, link once

Each prints "Two modules, one program".

ML /c MAIN.ASM PRINT.ASM           ; MASM: two .OBJ files
LINK MAIN+PRINT,PROG,PROG.MAP      ;    ; objects, program, map
 
ML /FePROG.EXE MAIN.ASM PRINT.ASM  ; MASM: the same in one command
 
JWASMR MAIN.ASM PRINT.ASM          ; JWasm: two .OBJ files
JWLINK format dos file MAIN.OBJ,PRINT.OBJ name PROG.EXE option map

When one module changes, only it is assembled again before the link. That is the point of modules in a big program, and what NMAKE (MASM) and MAKE (Borland) automate.

Libraries

A library (.LIB) is a file of object modules with an index of their public names. The linker searches it only for names still missing after the object files, and takes just the modules that define them, so a library of a hundred routines adds only the ones the program calls.

LIB UTIL.LIB +PRINT.OBJ       ;          ; MASM: create UTIL.LIB, add PRINT
LINK MAIN,PROG,,UTIL.LIB      ;         ; libraries are the fourth field
 
JWLIB -n UTIL.LIB +PRINT.OBJ  ; JWlib: -n makes a new library
JWLINK format dos file MAIN.OBJ library UTIL.LIB name PROG.EXE
 
TLIB UTIL +PRINT              ; Borland
TLINK MAIN, PROG,, UTIL

Pitfalls

A name declared EXTRN that no module makes PUBLIC stops the link: LINK reports error L2029 "unresolved external", TLINK "Undefined symbol", JWlink "undefined reference". The type in EXTRN must match the definition: a near CALL to a FAR procedure, which ends in a far return, does not come back where it should. A .EXE without a stack segment gets LINK's warning L4021 "no stack segment" and runs on whatever stack DOS left it.