Microcontroller directives
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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.
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Microcontroller directives
- 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.