OS-9 Assembly Code Development

Overview

Each process in NitrOS-9 is allocated its own 64K address space. When run, the program object code is loaded into the end of the memory space, and the variables and data structures are loaded into the beginning. In addition, NitrOS-9 keeps an separate per-process MMU table (DAT Image), so you must use F$MapBlk to map and F$ClrBlk to unmap 8K blocks in your process' address space. The operating system has its own 64K address space as well.

When a new process is created, registers U, Y, DP and SP are set with U = start of data area, Y = end of data area, DP = page # of beginning page, SP = end of data area + 1. Each process includes its own execution and data directories, along with std in, std out and std err.

Memory and DAT Image

mmap and pmap output the current status of the memory in the system. mmap shows which memory blocks are currently in use. pmap shows the blocks that are mapped in the DAT Image for a particular process.

Program and Data Module Structure

There are 9 different types of modules in 4 possible languages:

Module Types				Language
Code	Module Type	Name		Code	Language
$1x	Program Module	Prgm		$x0	Data (non-executable)
$2x	Subroutine Module	Sbrtn		$x1	6809 Object Code
$3x	Multi-module	Multi		$x2	BASIC09 I-Code
$4x	Data Module	Data		$x3	PASCAL I-Code
$5x-$Bx	User-definable module			$x4-$xF	Reserved for Future Use
$Cx	OS-9 System Module	Systm
$Dx	OS-9 File Manager Module	FlMgr
$Ex	OS-9 Device Driver Module	Drivr
$Fx	OS-9 Device Descriptor Module	Devic

The two most used will be Program Modules and Data Modules.

Program Modules

The format of the program module varies depending on whether you have a single source file for your program or multiple source files.

Note: multiple source files format cannot use direct page addressing due to the way they are compiled.

Single Source File Format

The following is a skeleton for a single source file memory module (program).

********************************************************************
* [Program Name]
* [Description]
*
* by [Author Name]
*
* Edt/Rev  YYYY/MM/DD  Modified by
* Comment
* ------------------------------------------------------------------
               nam       [executablename]
               ttl       [Full Program Name]

               ifp1
               use       defsfile
               endc

tylg           set       Prgrm+Objct
atrv           set       ReEnt+rev
rev            set       $00
edition        set       1

               mod       eom,name,tylg,atrv,start,size

        [Data Section - Variables and Data Structures Here]

size           equ       .
name           fcs       /[executablename]/
               fcb       edition

start

        [Program Section - Program Code Here]
        
               os9 F$Exit

               emod
eom            equ *
               end

OS9 can load programs (modules) only if they are in module header format. A module (program) is defined between the mod and emod pseudo instructions.

mod: Creates the module header and starts CRC check. mod has 6 attributes:

1. Module size (computed from the eom label at the end of the code - 2 bytes)
2. Module name offset (offset where the string of the name is contained, here that is defined at the name label with the fcs pseudo instruction - 2 bytes
3. Module type & language (in this case a Program in 6809 object code - 1 byte)
4. Module attribute-revision (1 byte). Attributes are reentrant (ReEnt) or not reentrant (0). Revision is a number 0-15. If two or more modules with the same name are in the module directory, then OS-9 only keeps the highest revision number module.
5. Module execution offset (here that is defined with the start label - 2 bytes)
6. Module variable storage size (defined by the size label immediately after the variable area, includes stack space - 2 bytes)

emod: Outputs the correct 3 byte CRC. This is generated by the assembler over the entire module.

For more information about memory modules, see OS-9 Fundamentals:

OS-9 Fundamentals on YouTube by Boisy Pitre

Data Modules

Here is a portion of the font module, which is a data module in NitrOS-9.

               nam       font
               ttl       F256 font

               use       defsfile

tylg           set       Data
atrv           set       ReEnt+rev
rev            set       $01

               mod       eom,name,tylg,atrv,start,0
name           fcs       /font/

start
L0000          fcb   $00,$00,$00,$00,$00,$00,$00,$00  
L0008          fcb   $7C,$82,$AA,$82,$BA,$92,$82,$7C 

*More data here

               emod
eom            equ       *
               end

Writing Position Independent Code

All code in NitrOS-9 must be position independent. Addresses are not known until a program module is loaded and executed. All addressing must be "program counter relative addressing". All branch and long branch instructions are program counter relative.

Rules:

Use BRA and LBRA instead of JMP

Use BSR and LBSR instead of JSR

Use program counter relative indexed addressing for all load, store, arithmetic and logical instructions.