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 one memory space, and the variables and data structures are loaded into the other. Each process includes separate execution and data directories and std in, std out and std err. In addition, NitrOS-9 keeps an separate per-process MMU table (DAT Image). So it is important to use the OS-9 calls F$MapBlk and F$ClrBlk calls to map in memory blocks with the MMU.

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.

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.

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.