STC89C53RC datasheet(149/165 Pages) STC

149 / 165 page

There are two categories of assembler controls: primary and general. Primary controls can be placed in the

invocation line or at the beginnig of the source program. Only other primary controls may precede a primary

control. General controls may be placed anywhere in the source program.

LINKER OPERATION

In developing large application programs, it is common to divide tasks into subprograms or modules containing

sections of code (usually subroutines) that can be written separately from the overall program. The term "modular

programming" refers to this programming strategy. Generally, modules are relocatable, meaning they are not

intended for a specific address in the code or data space. A linking and locating program is needed to combine the

modules into one absolute object module that can be executed.

Intel's RL51 is a typical linker/locator. It processes a series of relocatable object modules as input and creates

an executable machine language program (PROGRAM, perhaps) and a listing file containing a memory map and

symbol table (PROGRAM.M51). This is illustrated in following figure.

FILE3.OBJ

FILE2.OBJ

FILE1.OBJ

RL51

PROGRAM.ABS

PROGRAM.MAP

Linker operation

Legend

Utility program

User file

As relocatable modules are combined, all values for external symbols are resolved with values inserted into

the output file. The linker is invoked from the system prompt by

RL51

input_list

[T0 output_file]

[location_controls]

The input_list is a list of relocatable object modules (files) separated by commas. The output_list is the name

of the output absolute object module. If none is supplied, it defaults to the name of the first input file without any

suffix. The location_controls set start addresses for the named segments.

For example, suppose three modules or files (MAIN.OBJ, MESSAGES.OBJ, and SUBROUTINES.OBJ) are

to be combined into an executable program (EXAMPLE), and that these modules each contain two relocatable

segments, one called EPROM of type CODE, and the other called ONCHIP of type DATA. Suppose further that

the code segment is to be executable at address 4000H and the data segment is to reside starting at address 30H (in

internal RAM). The following linker invocation could be used:

RS51

MAIN.OBJ, MESSAGES.OBJ, SUBROUTINES.OBJ TO EXAMPLE & CODE

(EPROM (4000H) DATA (ONCHIP (30H))

Note that the ampersand character "&" is used as the line continuaton character.

If the program begins at the label START, and this is the first instruction in the MAIN module, then

execution begins at address 4000H. If the MAIN module was not linked first, or if the label START is not at the

beginning of MAIN, then the program's entry point can be determined by examining the symbol table in the

listing file EXAMPLE.M51 created by RL51. By default, EXAMPLE.M51 will contain only the link map. If

a symbol table is desired, then each source program must have used the SDEBUG control. The following table

shows the assembler controls supported by ASM51.

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