Developing Software in Assembly Language
By Jonathan W. Valvano

     This article, which discusses assembly language programming, accompanies the book Embedded Microcomputer Systems: Real Time Interfacing published by Brooks-Cole 1999.

     For an introduction describing how to run the application TExaS, see the help files included with the application. To develop assembly language software, we first use an editor to create our source code. Source code contains specific commands in human-readable-form. Next, we use an assembler to translate our source code into object code. Object code contains the specific commands in machine-readable-form. When developing software for a real microcomputer, a loader is used to place the object code into our computer's memory. We test our program with the aid of a debugger. For more information on the allocation into specific type of memory within the embedded microcomputer see the section on Memory Allocation in Chapter 2 of Embedded Microcomputer Systems: Real Time Interfacing by Jonathan W. Valvano. In the final product, the power on reset is used to start software execution after power is supplied to the microcomputer. If the object code is to be stored in EPROM, we can use an EPROM programmer. Many microcomputers contain built in features that assist in programming its EPROM, while other microcomputers require a separate apparatus to program. The following figure outlines the software development process.

This document has four overall parts:

    The overview section includes a simple case study illustrating the software development process. The basic terminology used in assembly language development is defined and examples are given. The development environments of evaluation boards and simulation are introduced.

    The syntax section defines the various components of an assembly program. Some of specific topics include labels, op codes, pseudo-op codes, numbers, strings, operands, comments and assembly errors. Descriptions of the assembly opcodes and their addressing modes for the 6805, 6808, 6811 and 6812 can be found in the help file of the application TExaS. This application can be found on the CD accompanying the book.

Local variables
    Local variables are an important topic in assembly language programming. This document defines methodologies for implementing local variables on the 6808 6811 and 6812. There are examples of how C compilers generate local variables. The particular environments illustrated include the Symantec Think C version 7 for the 68K Macintosh, the ImageCraft ICC11 version 4.0 for the Motorola 6811, the ImageCraft ICC12 version 4.0 for the Motorola 6812, and the Borland C version 3 for the Intel 386 IBM-PC.

    Observing working examples is an effective method for learning assembly language programming. Included in subsection are many 6812 assembly language programs on if-then, for-loop, while-loop, addition, subtraction, multiplication, digital filters, table interpolation, shifting, stack initialization, interrupt vectors, first in first out queues, and command interpreters. The example subsection of this html document only includes 6812 code, but there are many more examples for the 6805, 6808, 6811 and 6812 installed with the application TExaS. Some examples are RTF files, which can be found in the MC6805, MC6808, MC6811 and MC6812 subdirectories. These examples can be loaded directly into the simulator and executed. Other examples can be found as part of the help file of TExaS.