This document discusses various directives used in assembly language programming for microcontrollers. It explains directives like EQU, SET, BIT, CODE, DATA, IDATA, XDATA, ORG, USING, END, and DB which are used to define symbols, assign memory locations, select register banks, and initialize data in memory. Directives provide commands to the assembler and help write assembly code, but do not directly influence the microcontroller's operation.

1. Designed by Manoj D. Harsule
Lecturer in Electronics &
Communication Dept.
G.H. Raisoni Polytechnic,
Amravati, Maharashtra, India
Microcontroller Basics Directives

2. System tools

3. Editor
Construct ALP
Type using Editor
Called as source program
Extension .asm

4. Assembler
ALP to machine code
Extension .obj or .lst

5. Linker
Combines more than one
Converts to single
executable

6. Simulator
Run 8051 program without
using 8051 kit
If not needed use Hex
Converter
Extension must be .hex

7. Data Types
 Sbit- special function reg.
 Address- 80 H to FF H
 Bit:- addressable RAM
 00 H to 7FH

8. Byte data type
Addressable byte by byte
No 16 bit word

9. Directives are commands of
assembly language itself and
have no influence on the
operation of the microcontroller
8051 DIRECTIVES

10. EQU directive
 The EQU directive is used to replace a number
by a symbol.
 Syntax: Name EQU Constant
 For example:
 MAXIMUM EQU 99 After using this directive,
every appearance of the label “MAXIMUM” in the
program will be interpreted by the assembler as
the number 99 (MAXIMUM = 99). Symbols may
be defined this way only once in the program. The
EQU directive is mostly used at the beginning of
the program therefore.

11. SET directive
 The SET directive is also used to
replace a number by a symbol. The
significant difference compared to the
EQU directive is that the SET directive
can be used an unlimited number of
times:
 SPEED SET 45SPEED SET 46SPEED
SET 57

12. BIT directive
 The BIT directive is used to replace a bit address
by a symbol. The bit address must be in the
range of 0 to 255 (00 H to FF H).
 Syntax: Name BIT 8051 bit
 For example:
 TRANSMIT BIT PSW.7; Transmit bit (the seventh
bit in PSW register); is assigned the name
“TRANSMIT”OUTPUT BIT 6 ;Bit at address 06 is
assigned the name “OUTPUT”RELAY BIT 81 ;Bit
at address 81 (Port 0)is assigned the name
;”RELAY”

13. CODE directive
 The CODE directive is used to assign a
symbol to a program memory address.
Since the maximum capacity of program
memory is 64K, the address must be in
the range of 0 to 65535(0000 H to FFFF
H).
 Syntax: Name CODE code address
 For example:
 RESET CODE 0 ;Memory location 00h
called “RESET”TABLE CODE 1024

14. DATA directive
 The DATA directive is used to assign a symbol to
an address within internal RAM and SFR. The
address must be in the range of 0 to 255 (00 H to
FF H). It is possible to change or assign a new
name to any register.
 Syntax: Name DATA data address
 For example:
 TEMP12 DATA 32 ;Register at address 32 is
named ;as “TEMP12”STATUS_R DATA D0h
;PSW register is assigned the name
;”STATUS_R”

15. IDATA directive
 The IDATA directive is used to change or
assign a new name to an indirectly addressed
register. It is an address of entire internal
RAM.
 Syntax: Name IDATA idata address
 For example:
 TEMP22 IDATA 32 ;Register whose address
is in register ;at address 32 is named as
“TEMP22”TEMP33 IDATA T_ADR ;Register
whose address is in ;register T_ADR is
named as “TEMP33”

16. XDATA directive
 The XDATA directive is used to assign a
name to registers within external
(additional) RAM memory. The
addresses of these registers cannot be
larger than 65535 (0000 h to FFFF H).
 Syntax: Name XDATA xdata address
 For example:
 TABLE_1 XDATA 2048 ;Register stored
in external; memory at address 2048 is
named; as “TABLE_1”

17. ORG directive : Origin
 The ORG directive is used to specify a
location in program memory where the
program following directive is to be placed.
 Syntax: ORG address
 Address can be given in either in hex or
decimal.
 For example:
 BEGINNING ORG 100 ... ...ORG
1000hTABLE ... ...This program starts
at location 100. The table containing data is to
be stored at location 1024 (1000h).

18. USING directive
 The USING directive is used to define which
register bank (registers R0-R7) is to be used in
the program.
 Syntax: USING Bank no.
 USING 0 ;Bank 0 is used (registers R0-R7 at
RAM-addresses 0-7)USING 1 ;Bank 1 is used
(registers R0-R7 at RAM-addresses 8-15)USING
2 ,Bank 2 is used (registers R0-R7 at RAM-
addresses 16-23)USING 3 ;Bank 3 is used
(registers R0-R7 at RAM-addresses 24-31)

19. END directive : End of
program
 The END directive is used at the end of
every program. The assembler will stop
compiling once the program encounters
this directive.
 Syntax: END
 For example:
 END ;End of program

20. DB directive
 The DB directive is used for writing specified
value into program memory. If several
values are specified, then they are
separated by a comma. If ASCII array is
specified, it should be enclosed within single
quotation marks.
 For example:CSEG DB 22,33,’Alarm’,44 If
this directive is preceeded by a lable, then
the label will point to the first element of the
array. It is the number 22 in this example.