OS-9 Assembly Code Development

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Overview

Each process in NitrOS9 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. This includes separate execution and data directories and std in, std out and std err for each process. In addition, NitrOS9 keeps an separate per-process MMU table (DAT Image). So it is important to use the OS9 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 OS9 System Module Systm
$Dx OS9 File Manager Module FlMgr
$Ex OS9 Device Driver Module Drivr
$Fx OS9 Device Descriptor Module Devic

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

Program Modules

Data Modules

Here is a portion of the font module, which is a data module in NitrOS9.

               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

Accessing Variables and Data Structures

Accessing Table Data with PCR