I wrote this in 1991, when I was writing Amiga and Atari ST games for Ocean Software in Manchester, UK. I think at the time I was working on Parasol Stars. It’s an interesting look at a simpler time in games programming.

An explanation of my 68000 development system - by Mick West

The purpose of this document is twofold:

	1 - To make the meaning of my code a touch more accesible
            to those that may come after me.

	2 - To remind me of the meaning of my code.


	A 68000 development system, at present only encompassing
the Atari ST and the Commodore Amiga. My present System is PC
based, using the SNASM assembler, though this could change.

	I try to be modular and to this end I have lots of small
INCLUDE files that contain short modules of code to do things
like random numbers, heap management, sprite printing, etc.
	These modules are (hopefully) written to use the minimum
of extrenal references and ideally to incorporate the functions
of an Abstract Data Type - ie a 'Black Box' of code that can
only be accessed by calling a number of specified routines. The
ADT module can do whatever function it is required to do in
whatever way it wants so long as it fullfills the specifications.
	A good example is the scroll routines I wrote at the
start of the 'Darkman' project. I actually wrote three diffent
scroll routines and they all did the same thing, but in
different ways, some quicker, some using less memory. All three
had the same external interface with the same named routines
(SETUP_MAP_BUFFER, MOVE_LEFT_4, etc) to facilitated the screen
scrolling. I could simply include one instead of the other and
the program would assemble exactly the same.

	There are a set of fundamental routines that practically
all games require, You need to read the keyboard, you need a
Vertical Blank Interrupt, you need a double buffered screen, you
need some way of reserving areas of memory. These basic routines
are provide by the file KERNAL.S.
	For all my projects, the first part of the source code
will INCLUDE \DEV\KERNAL.S. I should point out at this point
that all my modules that are not entirely specific to a
particular project are kept in a folder called \dev, which
should also include this file you are reading now.
	Let's have a look at the very simplest program you can
write using this method:

; SIMPLE.S - A very simple program

	org	$1000
	regs	pc=$1000,sr=2700
	include	c:\dev\kernal.s	
	bra	main
	trap	#0

; data areas
		rsset	free_memory
something	rs.b	10000

; end of SIMPLE.S

	Ignoring the comments and blank lines, the first two
lines are to tell SNASM where to assemble the program to and to
turn the interrupts off before running.
	The next line includes the kernal routines, as it is at
the start of the program then the first thing that will be
executed will be a small routine at the start of KERNAL.S that
kills the interrupt vectors, sets up the double buffered screen,
installs a keyboard interrupt and sets up a VBL. 
	The kernal needs two external labels: MAIN and MY_VBL.
The first, MAIN (memories of C) is just the start of your
program. The second, MY_VBL is a routine that is called once per
Vertical Blank Interrupt.
	In the example above, all MAIN does is go into a loop.
All MY_VBL does is a TRAP #0 to give SNASM access to the
machine. This is not done in the kernal because you might want
some control over when you let SNASM get it's claws in, maybe
one per game cycle or whatever.

	The last couple of lines are rather fundamental in that
they illustrate how I do my memory organisation. The Kernal
allows a fixed amount (configurable, the default is 100K) for
your raw code (ie - the assembled code, including all modules).
All memory above that is assumed to be free and is reserved
using the RS directive (which I use extensivly, so you have to
understand it to understand me).
	The kernal will reserve about 70k itself for the two
screens and the stack. Then the lable FREE_MEMORY will be SET to
the end of this reserved memory. Any modules that you include
may also reserve memory, if so they will set FREE_MEMORY to the
end of the area they have just reserved. So in your main
program, if you want to reserve areas of memory then you will
simply use 'RSSET free_memory' to set the __RS variable
correctly and the just use RS to Reserve Space as desired.
	If you are writing a module that needs memory then in
addition to the above you will also need to SET the FREE_MEMORY
variable so other modules and your main code recognises what you
have done.
	This may sound complicated (it does to me!) but if you
just look back at the example program you can see that in
practice it is really very simple.

	More about the Kernal - since this is common to every
program I write, I shall describe it's functions here. Modules
should (if I have the time/inclination/energy) have a full
specification at the start of the source. So if you want to know
what it does, read the comments not the code!

Kernal Configuartion variables (compile time)
if these are undefined then they will be given a default
	ST		true if target is an ST (true) 
	THE_RPF		rasters per frame (1)
	MAX_CODE	max code size (100*1024)
	RASTER_COUNT	if true then will display a raster count (false)

Kernal Variables for external usage.
	TIME.L		starts as zero, increments every VBL
	SCREEN.L	address of the virtual screen
	REAL.L		address of the real screen

Major kernal routines
	FLIP_SCREENS	flips screen and real (waits for RPF)
Major macros
	PUSHEM		save registers (always long)
	PUSH.s		save a register (sized)
	PAUSE jiffies	hangs for a certain time
	CLS screen	fills 32000 bytes with 0

This is not everything in the kernal, you should look at the
source comments for that.

Mick's Code Style Conventions
	I am trying to develop a consistent style of writing
code, this will cut down on having to write unique routines
every time I write a new program. The style should not just be
about code, but about data structures and how the code accesses
them. Eventually I hope to have macros for the most common code
structures as soon as I have them fully formalised.

Mick's Maxims (not in order!!) (!)

1 - Use Meaningful Labels!
2 - Use Local Labels!
3 - Save Registers! 
4 - Don't repeat code, put it in a routine or a macro!
5 - Don't use numbers, use equates!
6 - Use lower case!
7 - Use data structures!
8 - Use consistent program structures!
9 - Use Comments! 
10- Make Backups! 

1 - Use Meaningful Labels! By meaningful I mean use full words,
not abbriviations, use underscores to separate words, use the
proper tense for the word and the correct plurality. EG:
2 - Use local labels! Only use global labels for routines that
can be accessed externally. ALL labels within a routine
(including any local variables it uses) MUST be local. EG:

	clr.w	d0
	add.w	#1,.counter
	bsr	another_routine
	bne	.loop
.counter	dc.w	0

3 - Save Registers! Unless you formally specify otherwise, all
registers not used to return values should be UNCHANGED when the
routine returns to caller. If you want to write faster versions
then do so, but SPECIFY that they may corrupt registers.

4 - Don't repeat code! Put it in a routine or a macro!

5 - Don't use numbers! Use equates! EG:

	add.w	#4,man_x		; is WRONG
	add.w	#walking_speed,man_x	; is RIGHT

6 - Use lower case! Upper case programs are a useless reminder
of the ancient days of computing. They make programs difficult
to read, and your comments either have to be in upper case, or
you have to mess around with the CAPS LOCK key. It's just a
waste of time.

7 - Use data structures! This is a big subject, addressed later.

8 - Use consistent program structures! Like mine for example.

9 - Use Comments! At the very minimum each routine should have
one comment, on  the line before the label. This should give a
brief description of what the routine does and it's input and
output requirements. A full specification of a routine should
include the following.
	Exact input/output requirements
	Register corruption
	The external references it needs
	Brief explanation (one line is ok) of the algorithm
	Possible areas for improvement
	Known bugs still to be fixed
As well as the comments at the start of a routine it is usually
a good idea to have comments in the routine itself. For
espescially complex routines it is a great help to development
to comment EVERY line to provide information on: Flow of
control, meaning of decisions, contents of registers and just
general elucidation of exactly what you think you are doing.

10 - Make Backups! Not strictly a coding requirement, but still
a vital part of development. I use an Archive program and have a
batch file to archive everything in the \DEV folder and
everything in the folder containing the project that I am
currently working on. They are archived to hard disk and then
copied to floppy, which is then taken home and copied to my hard
disk there.
	Besides the obvious frustration of having to rewrite
code that you have already slaved hours over, there is the fact
that there is a lot of money at stake here. Think money, make backup!

	Right, that's enough maxims for one day (ten was good
enough for Moses so it'll do for me) Now, let's expand on the
dual topics of Data Structures and Program structures. 
	At university they taught me somthing called 'Data
Driven Design', the idea was that you built your program around
the structure of what you were doing things too, the data.
	They also waffled a lot about Abstract Data Types which
I described briefly earlier and involve keeping the data
structure seperate from the program code.
  	Data driven design is really a very simple concept. Put
at it's most basic level all you do is list everything in the
program and then write routines to handle each of them.

	This document is not supposed to be a beginners guide to
structured programming, so I'll just describe what I do, and
what I think I might do if I get around to it.

	What came first, the program structure or the data
structure? I usually start with a loose outline of the program
structure and then write some specific data structures and then
the program and the data structures evolve together as they
respond to changing needs and new idea. This is not the way it
should be, but it has happend like that because of the usual
lack of time and energy. 

	Anyway, enough waffling, what is a data structure? Well,
it's like a RECORD in pascal or a STRUCT in C, which probably
leaves you none the wiser. Essentially it is an array or a list
of groups of bytes. It is best illustrated by an example.
	Say you are writing galaxians or something similar,
obviously you need to store the positions of all the aliens. To
do this you will just have a list of say 20 bytes for each
alien, in those 20 bytes you will store information such as the
x and y position, the speed, the sprite number, the energy, what
it will score, how long before it next does something and things
like that, depending on your game.
	To get information on a specifc alien you point an
address register to the base of the alien and then use offsets
to access the various bits fo data in that alien. EG:

	lea	alien,a0		; address of alien
	move.w	(a0),d0			; get x
	move.w	2(a0),d1		; get y
	move.w	4(a0),d2		; get sprite number
	bsr	draw_alien		; draw it

	In line with my maxim "don't use numbers", I dont use
numbers as the offsets. Instead I define my structures using the
RS command. This let's you set up a data structure as if you
were defining variables using DS, but instead of the labels
being absolute addresses they will be offsets relative to the
start of the code. For example:

		rsreset			; new data structure
alien_flag	rs.w	1
alien_x		rs.w	1
alien_y		rs.w	1
alien_xv	rs.w	1
alien_yv	rs.w	1
alien_sprite	rs.w	1
alien_energy	rs.w	1
alien_len	rs.w	0

	Having set up a data structure I will define an area of
memory at the end of the program (using RS and explained a page
or so ago) like:

alien_table	rs.w	max_alien*alien_len+2

	The table will be initialised by:

		fill	#alien_table,#max_alien*alien_len,#0
		move.w	#-2,alien_table+max_alien*alien_len

	So the extra word at the end of the table will be in
effect a dummy entry in the list with a flag value of -2. I
usually end my lists with a negative value in the first field.

	There will be a routine called NEW_ALIEN that will
return a0 as the first free baddy in the list. My usual way of
going through all the objects in a list is as follows:

	lea	alien_table-alien_len,a0
	lea	alien_len(a0),a0
	tst.w	alien_flag(a0)
	beq	.next_alien
	bmi	.done

; just an example of the sort of thing I might do with it.
	move.w	alien_x(A0),d0
	move.w	alien_y(a0),d1
	move.w	alien_sprite(a0),d2
	bsr	draw_sprite
	bra	.next_alien

	You can kill a object using

		clr.w	alien_flag(A0)

	This sort of structure is the fundamental basis of my
programs at the moment.

	I have written this in a brief attempt to slightly
formalise what I am doing, mostly to get a few things clear in
my own mind. So if you are not me and this does not seem
entirely sensible to you then hard luck...