Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

210 The Quintessential PIC Microcontroller

as used in Program 8.2. For instance, Table 8.4 shows the first part of Microchip’s file p16f84.inc which we will use for the rest of this text.

The process outlined up to here is known as absolute assembly. Here the source code is in a single file (plus maybe some included files) and the assembler places the resulting machine code in known (i.e. absolute) locations in the Program store. We stated earlier on page 201 that where many modules are involved, often written by di erent people or/and coming from outside sources and commercial libraries, some means must be found to link the appropriate modules together to give the final single absolute executable machine-code file. For example, you may have to call up one of the modules that Fred has written some time ago. You will not know exactly where in memory this module will reside until the project has been completed. What can you do? Well a module should have its entry point labelled; say, FRED in this case. Then you should be able to call FRED without knowing exactly what address this label represents.

The process used to facilitate this is shown in Fig. 8.3. Central to this modular tie-up is the linker program, which satisfies such external cross-references between the modules. Each module’s source-code file needs to have been translated into relocatable object code prior to the linkage. “Relocatable” means that its final location and various addresses of external labels have yet to be determined. This translation is done by a relocatable assembler. Unlike absolute assembly, it is the linker that determines where the machine code is to be located in memory, not the programmer.

Treating the linker as a type of task builder, its main functions are:

•To concatenate code and data from the various input module streams.

•To allocate values to symbolic labels which have not been explicitly given constant values by the programmer – eg. using equ directives.

•To generate the absolute machine-code executable file together with any symbol, listing and link-time error files.

In order to allow the linker to do its job, it must have knowledge of the memory architecture of the target processor; basically where the array of general-purpose file registers start and end, where the vectors reside in Program memory and where the code begins and ends. In the case of Microchip’s mplink.exe linker this information is supplied in the form of a linker command file.

A simple example of such a command file for a PIC16F84 is given in Table 8.5. Three directives are used in the file.8

codepage

The codepage directive is used for program code. These directives are here used to define two regions, one for the Reset and Interrupt vectors

8Microchip’s MPASM Users Guide with MPLINK and MPLIB manual gives a full list of linker directives.

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Table 8.5: The pic16f84.lkr linker command file.

//File: pic16f84.lkr

//Simple linker command file for 16F84

//Created by S.J. Katzen

//16/05/1999

in between 000h and 004h called vec and the other called program to be used for executable code from 005h through 3FFh. Notice the use of the prefix 0x to denote the hexadecimal base. This is the notation used in the C language.

databank

This is similar to codepage but is used for variable data in RAM. Here the file register array between File 0Ch and File 4Fh is called gprs.

section

This linker directive names two code streams. The first called VECTORS will be used by the programmer to store the two vector goto instructions while TEXT is used for the core program code. The source code assembler directive code with the appropriate label tells the linker which stream any following code is to be placed; for example, see Program 8.2. As many code sections from any codepage can be created as desired. For instance, all subroutines may be placed together in Program memory by modifying the linker file thus:

Sections can be made from DATABANK memory with RAM replacing the ROM attribute. In our case we have not defined a data section and the unlabelled assembler directive udata (Uninitialiized DATA) allows space to be reserved for labels in the general-purpose register array; see Program 8.4.

To illustrate the principle of linking we will implement the mathemat-

ical function NUM_12 + NUM_22, known as root mean square. There are three teams working on this problem.9 Tasks have been allocated by the project manager (a fourth person?) as follows:

1. The main function which sequences the steps:

9Obviously this is a ridiculously simple problem for team work but it illustrates the principle in a manageable space.