Assembler directives are special instructions that provide information to the
assembler. Assembler directives do not generate code.
Segment directives, equ, assume, and end mnemonics are all directives.
These are not valid 80x86 instructions.
Directives may be divided into the following classes:
Pseudo-Opcode directive or Processor Code Generation Directives
Memory Model Directives
Segment Definition Directives
Segment Ordering Directives
Data Allocation Directives
Logical and Bit Oriented Directives
Macro Definition Directives
Program Listing and Documentation Directives
Conditional Assembly Directives
User Message Directives
Other Operators and Directives
A pseudo-opcode is a message to the assembler just like an assembler directive.
However, a pseudo-opcode will emit object code bytes. Examples of pseudo-opcodes
include byte, word, dword, qword, and tbyte. These instructions emit the bytes
of data specified by their operands but they are not true 80X86 machine
At this level only the most commonly used directives are briefly presented.
Memory Model Directives:
These are already seen in the section related to models. The following table
summarizes the main points about segment directives.
|.MODEL model ||defines memory model to be one of the following:|
SMALL, COMPACT, MEDIUM, LARGE or HUGE;
must be used prior to any other segment directive
|.CODE [name] ||starts code segment; must follow .MODEL directive
|.DATA ||starts a near data segment for initialized data |
must follow .MODEL directive;
|.STACK [size] ||indicates start of stack segment named 'STACK'|
with size indicating number of bytes to reserve.
Table 6: Model and Segment directives
Segment Definition Directives:
|name PROC [NEAR|FAR] ||defines procedure; NEAR/FAR has .MODEL default
|name ENDP ||ends procedure 'name'
|PUBLIC name[,name...] ||makes symbol 'name', which could be a variable or|
|END [name] ||marks end of source module and sets program|
start address (CS:IP) if 'name' is present
a procedure available to other modules.
Table 7: Segment Definition directives
Data Allocation Directives:
|[name] DB init[,init...] ||define byte
|[name] DW init[,init...] ||define word (WORD, 2 bytes)
|[name] DD init[,init...] ||define double word (DWORD, 4 bytes)
|[name] DF init[,init...] ||define far word (FWORD, 386, 6 bytes)
|[name] DQ init[,init...] ||define quad word (QWORD, 8 bytes)
|[name] DT init[,init...] ||define temp word (TBYTE, 10 bytes)
|count DUP(init[,init...]) ||duplicate 'init' 'count' times; DUP can be
Table 8: Data Allocation directives
These are available only if simplified segments are used.
|@code ||contains the current code segment
|@codesize ||0 for small and compact; 1 for large medium and huge
|@datasize ||0 for small and medium; 1 for compact ;2 for large
|@data ||contains segment of define by .DATA
|@stack ||contains segment of define by .STACK
Table 9: Equates directives
|.RADIX expr ||sets radix [2..16] for numbers (dec. default)
|.RADIX B ||binary data specifier
|.RADIX Q ||octal data specifier
|.RADIX O ||octal data specifier
|.RADIX D ||decimal data specifier (default)
|.RADIX H ||hex
Table 10: Radix Specifiers directives
Macro Definition Directives:
|name MACRO [parm[,parm...]] ||defines a macro and it's parameters
|ENDM ||terminates a macro block
|LOCAL name[,name...] ||defines scope symbol as local to a macro
|EXITM ||exit macro expansion immediately
|REPT expr ||repeats all statements through ENDM statement|
for 'expr' times
Table 11: Macro Definition directives