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The object file, if it is absolute, contains only the binary bytes (00H-0FH) of the machine language program.

A relocatable object file will also contain a sysmbol table and other information required for linking and locating.

The listing file contains ASCII text codes (02H-7EH) for both the source program and the hexadecimal bytes in

the machine language program.

A good demonstration of the distinction between an object file and a listing file is to display each on the

host computer's CRT display (using, for example, the TYPE command on MS-DOS systems). The listing file

clearly displays, with each line of output containing an address, opcode, and perhaps data, followed by the

program statement from the source file. The listing file displays properly because it contains only ASCII text

codes. Displaying the object file is a problem, however. The output will appear as "garbage", since the object file

contains binary codes of an 8051 machine language program, rather than ASCII text codes.

ASSEMBLY LANGUAGE PROGRAM FORMAT

Assembly language programs contain the following:

Machine instructions

Assembler directives

Assembler controls

Comments

•

Machine instructions are the familiar mnemonics of executable instructions (e.g., ANL). Assembler directives

are instructions to the assembler program that define program structure, symbols, data, constants, and so on (e.g.,

ORG). Assembler controls set assembler modes and direct assembly flow (e.g., $TITLE). Comments enhance the

readability of programs by explaining the purpose and operation of instruction sequences.

Those lines containing machine instructions or assembler directives must be written following specific rules

understood by the assembler. Each line is divided into "fields" separated by space or tab characters. The general

format for each line is as follows:

[label:]

mnemonic [operand]

[, operand]

[…] [;commernt]

Only the mnemonic field is mandatory. Many assemblers require the label field, if present, to begin on the left in

column 1, and subsequent fields to be separated by space or tab charecters. With ASM51, the label field needn't

begin in column 1 and the mnemonic field needn't be on the same line as the label field. The operand field must,

however, begin on the same line as the mnemonic field. The fields are described below.

Label Field

A label represents the address of the instruction (or data) that follows. When branching to this instruction, this

label is usded in the operand field of the branch or jump instruction (e.g., SJMP SKIP).

Whereas the term "label" always represents an address, the term "symbol" is more general. Labels are

one type of symbol and are identified by the requirement that they must terminate with a colon(:). Symbols

are assigned values or attributes, using directives such as EQU, SEGMENT, BIT, DATA, etc. Symbols may be

addresses, data constants, names of segments, or other constructs conceived by the programmer. Symbols do not

terminate with a colon. In the example below, PAR is a symbol and START is a label (which is a type of symbol).

PAR

EQU

500

;"PAR" IS A SYMBOL WHICH

;REPRESENTS THE VALUE 500

START: MOV

A,#0FFH

;"START" IS A LABEL WHICH

;REPRESENTS THE ADDRESS OF

;THE MOV INSTRUCTION

A symbol (or label) must begin with a letter, question mark, or underscore (_); must be followed by letters,

digit, "?", or "_"; and can contain up to 31 characters. Symbols may use upper- or lowercase characters, but they

are treated the same. Reserved words (mnemonics, operators, predefined symbols, and directives) may not be

used.

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