OS-9

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Home / Programming - OS-9

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Programming the 6809 using OS-9

This section needs help with the Assembler, Pascal and C.

Introduction

From The C Compiler's User Manual

C was originally developed at the Bel Telephone Laboratories as an implementation language for the UNIX operating system by Brian Kernighan and Dennis Ritchie. They also wrote a book titled The C Programming Language which is universally accepted as the standard for the language. It is an interesting reflection on the language that although no formal industry-wide "standard" was ever developed for C, programs written in C tend to be far more portable between radically different computer systems as compared to so-called "standardized" languages such as BASIC, COBOL, and PASCAL. The reason C is so portable is that the language is so inherently expandable that if some special function is required, the user can create a portable extension to the language, as opposed to the common practice of adding additional statements to the language. For example, the number of special-purpose BASIC dialects defies all reason. A lesser factor is the underlying UNIX operating system, which is also sufficiently versatile to discourage nonstandardization of the language. Indeed, standard C compilers and UNIX are intimately related. Fortunately, the 6809 microprocessor, the OS-9 operating system, and the C language form an outstanding combination. The 6809 was specifically designed to efficiently run high-level languages, and its stack-oriented instruction set and versatile repertoire of addressing modes handle the C language very well. As mentioned previously, UNIX and C are closely related, and because OS-9 is derived from UNIX, it also supports C to the degree that almost any application written in C can be transported from a UNIX system to an OS-9 system, recompiled, and corrected executed.

Differences From the K&R Specification

• Bit fields are not supported.
• Constant expressions for initializers may include arithmetic operators only if all the operands are of type int or char.
• The older forms of assignment operators, =+ or =*, which are recognized by some C compilers, are not supported. You must use the newer forms, +=, *=, etc.
• "#ifdef (#ifndef) ... [#else...] #endif" is supported but "#if <constant expression>" is not.
• It is not possible to extend macro definitions or strings over more than one line of source code.
• The escape sequence for newline '\n' refers to the ASCII carriage return character (used by OS-9 for end-of-line), not linefeed (hex 0A). Programs which use '\n' for end-of-line (which includes all programs in K&R) will still work properly.

Enhancements and Extensions

The "Direct" Storage Class

The 6809 microprocessor instruction for accessing memory via an index register or the stack pointer can be relatively short and fast when they are used in C programs to access "auto" (function local) variables or function arguments. The instructions for accessing global variables are normally not so nice and must be four-bytes long and correspondingly slow. However, the 6809 has a nice feature which helps considerably. Memory, anywhere in a single page (256 byte block), may be accessed with fast, two byte instructions. This is called the "direct page", and at any time its location is specified by the contents of the "direct page register" within the processor. The linkage editor sorts out where this should be, and it need not concern the program, who only needs to specify for the compiler which variables should be in the direct page to give the maximum benefit in code size and execution speed. To this end, a new storage class specifier is recognized by the compiler. In the manner of K&R page 192, the sc-specifier list is extended as follows:

Sc-specifier:

• auto
• static
• extern
• register
• typedef
• direct (extension)
• extern direct (extension)
• static direct (extension)

The new keyword may be used in place of one of the other sc-specifiers, and its effect is that the variable will be placed in the direct page. direct creates a global direct page variable. extern direct references an external-type direct page variable and static direct creates a local direct page variable. These new classes may not be used to declare function arguments. "Direct" variables can be initialized but will, as with other variables not explicitly initialized, have the value zero at the start of program execution. 255 bytes are available in the direct page (the linker requires one byte). If all the direct variables occupy less than the full 255 bytes, the remaining global variables will occupy the balance and memory above if necessary. If too many bytes of storage are requested in the direct page, the linkage editor will report an error, and the programmer will have to reduce the use of direct variables to fit the 256 bytes addressable by the 6809.

It should be kept in mind that direct is unique to this compiler, and it may not be possible to transport programs written using direct to other environments without modification.

Embedded Assembly Language

As versatile as C is, occasionally there are some things that can only be done (or done at maximum speed) in assembly language. The OS-9 C compiler permits user-supplied assembly-language statements to be directly embedded in C source programs.

Basic09

From the Basic09 manual: Basic09 is an enhanced structured BASIC language programming system specially created for the 6809 Advanced Microprocessor. In addition to the standard BASIC language statements and functions, Basic09 includes many of the most useful elements of the Pascal programming language so that programs can be modular, well-structured, and use sophisticated data structures. It also permits full access to almost all of the OS-9 Operating System commands and functions so it can be used as a systems programming language. These features make Basic09 an ideal language for many applications: scientific, business, industrial control, education and more.

Basic09 is unusual in that it is an Interactive Compiler that has the best of both kinds of language system: it gives the fast execution speed typical of compiler languages plus the ease of use and memory space efficiency typical of interpreter languages. Basic09 is truly a complete programming system that includes a powerful text editor, multi-pass compiler, run-time interpreter, high-level interactive debugger, and a system executive. Each of these components was carefully integrated so the user "sees" a friendly, highly interactive programming resource that provides all the tools and helpful "extra" facilities needed for fast, accurate creation and testing of structured programs.

• The Structure of I-Code