STC89LE53RC datasheet(134/165 Pages) STC

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ASSEMBLER OPERATION

There are many assembler programs and other support programs available to facilitate the development of

applications for the 8051 microcontroller. Intel's original MCS-51 family assembler, ASM51, is no longer

available commercially. However, it set the standard to which the others are compared.

ASM51 is a powerful assembler with all the bells and whistles. It is available on Intel development systems

and on the IBM PC family of microcomputers. Since these "host" computers contain a CPU chip other than the

8051, ASM51 is called a cross assembler. An 8051 source program may be written on the host computer (using

any text editor) and may be assembled to an object file and listing file (using ASM51), but the program may not

be executed. Since the host system's CPU chip is not an 8051, it does not understand the binary instruction in the

object file. Execution on the host computer requires either hardware emulation or software simulation of the target

CPU. A third possibility is to download the object program to an 8051-based target system for execution.

ASM51 is invoked from the system prompt by

ASM51 source_file [assembler_controls]

The source file is assembled and any assembler controls specified take effect. The assembler receives a source

file as input (e.g., PROGRAM.SRC) and generates an object file (PROGRAM.OBJ) and listing file (PROGRAM.

LST) as output. This is illustrated in Figure 1.

Since most assemblers scan the source program twice in performing the translation to machine language,

they are described as two-pass assemblers. The assembler uses a location counter as the address of instructions

and the values for labels. The action of each pass is described below.

PROGRAM.SRC

ASM51

PROGRAM.OBJ

PROGRAM.LST

Figure 1 Assembling a source program

Legend

Utility program

User file

Pass one

During the first pass, the source file is scanned line-by-line and a symbol table is built. The location counter

defaults to 0 or is set by the ORG (set origin) directive. As the file is scanned, the location counter is incremented

by the length of each instruction. Define data directives (DBs or DWs) increment the location counter by the

number of bytes defined. Reserve memory directives (DSs) increment the location counter by the number of bytes

reserved.

Each time a label is found at the beginning of a line, it is placed in the symbol table along with the current

value of the location counter. Symbols that are defined using equate directives (EQUs) are placed in the symbol

table along with the "equated" value. The symbol table is saved and then used during pass two.

Pass two

During pass two, the object and listing files are created. Mnemonics are converted to opcodes and placed in

the output files. Operands are evaluated and placed after the instruction opcodes. Where symbols appear in the

operand field, their values are retrieved from the symbol table (created during pass one) and used in calculating

the correct data or addresses for the instructions.

Since two passes are performed, the source program may use "forward references", that is, use a symbol

before it is defined. This would occur, for example, in branching ahead in a program.

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