Lecture 6 Assembler Directives

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Lecture 6

Assembler Directives

  • Code generation flow

  • Assembler directives—Introduction

  • Segment control

    • Generic segment (SEGMENT, RSEG)
    • Absolute segment (CSEG, DSEG and XSEG)
  • Address control

  • Symbol definition

  • Memory initialization/reservation

    • DB, DW, DD, DS
  • Example program template

Code Generation Flow

Assembler Directives—Introduction

  • Assembler Directives are not assembly language instructions as they do not generate any machine code

    • They are special codes placed in the assembly language program to instruct the assembler to perform a particular task or function
    • They can be used to define symbol values, reserve and initialize storage space for variables and control the placement of the program code
  • Assembler directives are typically specific to a particular assembler. We will be using the Keil A51 Assembler in this course.

  • The ASM directives are grouped into the following categories:

    • Segment control
    • Address control
    • Symbol definition
    • Memory initialization/reservation

Segment Control

  • In x51 CPU structure, a contiguous block of code or data memory is usually referred to as a segment

    • Examples:
      • A function definition (code memory)
      • An array (data memory)
  • There are two types of segments based on whether or not they are relocatable

    • Generic or relocatable
    • Absolute
  • Each of the two types of segments can be specified as one of five memory classes


Generic (Relocatable) Segment

  • Generic segments are created using the SEGMENT directive

  • The final memory location for a generic segment is assigned by the linker

  • The format is as follows:

  • SEGMENT <segment_memory_class>

  • Example:


  • The above directive defines a relocatable segment named as MYDATA, with a memory class of DATA


  • Whenever the above statement is encountered, the MYDATA segment will become the current active segment until the assembler comes across another RSEG directive, which will then define another segment area

Absolute Segment

  • Absolute segment means a fixed memory segment. Absolute segments are created by CSEG, DSEG and XSEG directives.

  • The final location of the segment is known at compile time

  • The format of this directive is as follows:

  • CSEG AT <address> ; defines an absolute code segment

  • DSEG AT <address> ; defines an absolute data segment

  • XSEG AT <address> ; defines an absolute external data segment

  • Example:

  • CSEG AT 0300H ;select code segment and set ;the starting address at 0300H

  • DSEG AT 0400H ;select data segment and set ;the starting address at 0400H

Address Control—ORG

  • The specified format for the ORG directive is:

  • ORG <expression>

  • The ORG directive is used to set the location counter in the current segment to an offset address specified by the expression

  • However, it does not alter the segment address. The segment address can only be changed by using the standard segment directives.

  • Example:

  • ORG 80H ;Set location counter to 80H

  • The ORG directive need not only be used in the code segment but can be used in other segments like the data segment as well.

  • For example, to reserve one byte memory space each at locations SECONDS and MINUTES in the data segment, we would write:

    • DSEG ;data segment
    • ORG 30H
    • SECONDS: DS 1
    • MINUTES: DS 1

Address Control—END

  • The specified format for the directive is:

  • END

  • The END directive indicates the end of the source file

  • It informs the assembler where to stop assembling the program

    • Hence any text that appears after the END directive will be ignored by the assembler
  • The END directive is required in every source file

    • If it is not written at the end of the program, the assembler will generate an error message

Symbol Definition

  • The symbol definition directive assigns a symbolic name to an expression or a register

  • Sometimes it is an advantage to use a symbol to represent a value or a register because it makes the program more meaningful to a user

  • Another advantage is, by equating the symbol to a value, the user only needs to change it once at the directive statement

    • The rest of the statements that make a reference to the symbol will be updated automatically

Symbol Definition—EQU, SET

  • The format of the EQU and SET directives are as follows:

      • Symbol EQU <expression>
      • Symbol EQU <register>
      • Symbol SET <expression>
      • Symbol SET <register>
  • This is similar to the “#define” macro definition in C

  • expression can include simple mathematical operators like ‘+’, ’-‘, ‘ * ‘, ‘/’, MOD

  • register includes A, R0, R1, R2, R3, R4, R5, R6 and R7

Symbol Definition—EQU, SET

  • Examples:

      • COUNT EQU R3 ;equate to a register
      • TOTAL EQU 200 ;equate to a constant
      • TABLE EQU 10

Symbol Definition—CODE, DATA, IDATA, XDATA

  • Each of these directives assigns an address value to a symbol. The format of the directive is as follows:

  • Symbol BIT <bit_address>

  • Symbol CODE <code_address>

  • Symbol DATA <data_address>

  • Symbol IDATA <idata_address>

  • Symbol XDATA <xdata_address>

  • bit_address The bit address which is available from bit-addressable location 00H through 7FH as an offset from byte location 20H

  • code_address The code address ranging from 0000H to 0FFFFH

  • data_address The address is from 00H to 7FH (internal data memory) and the special function register address from 80H to 0FFH

  • idata_address The address is ranging from 00H to 0FFH

  • xdata_address The external data space ranging from 0000H to 0FFFFH

Symbol Definition—CODE, DATA, IDATA, XDATA

  • Example:

  • Act_bit BIT 2EH ;Use bit location 2EH ;as Act_bit

  • Port2 DATA A0H ;A special function ;register, P2

Memory Initialization/Reservation

  • The directives for memory initialization and reservation are DB, DW and DD

  • These directives will initialize and reserve memory storage in the form of a byte, a word or a double word in code space

  • The directive to reserve memory without initialization is DS

  • This directive will reserve specified number of bytes in the current segment

DB (Define Byte)

  • The DB directive initializes code memory with a byte value

  • The directive has the following format:

  • <label>: DB <expression>, ,

  • label

  • is the starting address where the byte values are stored

  • expression

  • is the byte value, it can be a character string, a symbol, or an 8-bit constant

DB (Define Byte)

  • Example:

  • CSEG AT 200H

  • MSG: DB ‘Please enter your password’, 0

  • ARRAY: DB 10H,20H,30H,40H,50H

  • The above string of characters will be stored as ASCII bytes starting from location 200H, which means location [200H]=50H, [201H]=6CH and so on

  • Notice that the DB directive can only be declared in a code segment

    • If it is defined in a different segment, the assembler will generate an error

DW (Define Word)

  • The DW directive initializes the code memory with a double byte or a 16-bit word

  • The directive has the following format:

  • <label>: DW <expression>, ,

  • Example:

  • ;2 words allocated

  • CNTVAL: DW 1025H, 2340H

  • ;10 values of 1234H starting from location XLOC

  • XLOC: DW 10 DUP (1234H)

  • The DUP operator can be used to duplicate a sequence of memory contents

  • The DW directive can only be used in the code segment

    • If it is defined in other segments, the assembler will give an error message

DD (Define Double Word)

  • The DD directive initializes the code memory with double word or 32-bit data value

  • The directive has the following format:

  • <label>: DD <expression>, ,

  • Example:

  • ADDR: DD 820056EFH, 10203040H

  • EMPT: DD 3 DUP ( 0 )

  • Same as the DB and DW directives, DD can only be specified in the code segment

    • If it is declared in other segment it risks having error message generated by the assembler

DS (Define Storage)

  • The DS directive reserves a specified number of bytes in the current segment

  • It can only be used in the currently active segment like CSEG, ISEG, DSEG or XSEG

  • The DS directive has the following format:

  • DS <expression>

  • The expression can not contain forward references, relocatable symbols or external symbols

DS (Define Storage)

  • Example:

  • XSEG AT 1000H ;select memory block from ;external memory, starting ;address from 1000H

  • Input: DS 16 ; reserve 16 bytes

  • Wavetyp: DS 1 ; reserve 1 byte

  • The location counter of the segment is incremented by one byte every time the DS statement is encountered in the program

  • The programmer should be aware that no more than 16 byte values should be entered starting from the address ‘Input’ as shown in the above example

  • Notice that the bytes are not initialized, just reserved

Example Program Template

  • ;-----------------------------------------------------------

  • $include (c8051f020.inc) ;Include register definition file

  • ;-----------------------------------------------------------


  • ;-----------------------------------------------------------

  • CR EQU 0DH ;Set CR (carriage return) to 0DH

  • ;-----------------------------------------------------------


  • ;-----------------------------------------------------------

  • ; Reset Vector

  • CSEG AT 0 ; Jump to the start of code at

  • LJMP Main ; the reset vector

  • ; Timer 4 Overflow Vector

  • ORG 83h ; Jump to the start of code at

  • LJMP TIMER4INT ; the Timer4 Interrupt vector

  • ;-----------------------------------------------------------


  • ;-----------------------------------------------------------


  • RSEG MYDATA ; Switch to this data segment.

  • ORG 30h

  • Input: DS 16

  • temp: DS 1

Example Program Template

  • ;-----------------------------------------------------------


  • ;-----------------------------------------------------------


  • RSEG MYCODE ; Switch to this code segment

  • USING 0 ; Specify register bank

  • ; for main code.

  • Main: ; Insert Main Routine of program here

  • ; … …

  • ; … …

  • ;-----------------------------------------------------------

  • ; Timer 4 Interrupt Service Routine

  • ;-----------------------------------------------------------

  • TIMER4INT: ; Insert Timer 4 ISR here

  • ; … …

  • ; … …

  • RETI

  • ;-----------------------------------------------------------

  • ; Global Constant

  • ;-----------------------------------------------------------

  • Rdm_Num_Table:

  • DB 05eh, 0f0h, 051h, 0c9h, 0aeh, 020h, 087h, 080h

  • DB 092h, 01ch, 079h, 075h, 025h, 07ch, 02bh, 047h

  • ;-----------------------------------------------------------

  • ; End of file.

  • END


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