MASM Directives

.CODE [name]

Used with .MODEL - defines start of code segment.
name may be used with medium, large and huge models.

Default segment name for small and compact models: _TEXT

Used with .MODEL - defines start of NEAR data segment.
Segment name: _DATA

.MODEL model

Defines the memory model: SMALL, COMPACT, MEDIUM, LARGE, HUGE
.MODEL can only be used with the EXE format (not COM).

.MODEL SMALL

.STACK [size]

Used with .MODEL - defines start of stack segment.
Segment name: STACK
Default stack size: 256 (1024?) bytes - may be set with size.

.STACK 100h  ;256 bytes

Associates a segment-register (CS,DS,ES,SS) with a named segment.

      ASSUME CS:code, DS:code, ES:code, SS:code
code  SEGMENT

Define Byte : Allocates [and initializes] one ore more bytes of data
Char bytes (strings) may be delimited by either " or '.

msg   DB   "Message$"
      DB   "Hello World",13,10
      DB   'Hello World',0Dh,0Ah
      DB   ;uninitialized byte
  
      mov  dx,offset msg

Orders segments according to the Microsoft DOS segment conventions - overriding the ordering in the source text.
>.STACK, .DATA, .CODE

END [address]

Marks the end of a module [and sets entry point to address].

start: ;code starts here
       ;...
       END start

EQU : name EQU expression

Equate : Assigns expression to name.

msg    db "Hello",13,10
len    EQU $ - msg      ;len = location - OFFSET msg = 7

       mov dx,OFFSET msg 
       mov cx,len       ;mov cx,7

ORG

Sets the Location Counter.
In COM programs, ORG 100h leaves room for the DOS PSP.

code   SEGMENT
       ORG 100h
start: ;code at 100h

Defines a named program segment

code  SEGMENT
      ;code here
code  ENDS

Defines the program title - usually in first program line

TITLE hello