BASELINE 2091 Presents The First Guide To Coding The SNES
Guide and Source Code by -Pan-
Released on 4/20/93
Welcome to the first installment of "How to code SNES"
This first volume will show you how to make a simple text intro. It uses
mode 0 graphics with no DMAs (Horizontal or General) and is the simplest
type of intro you can make. We're starting small so you can easily understand
what to do. Other future volumes will contain other graphic modes,
Horizontal DMA (HDMA, the SNES version of an amiga copperlist), General DMA,
Interrupts, and a brief section on Sound. Originally we were going to release
the full Super Famicom Programmer's Manual but believe it or not, this manual
a pile of garbage. Contrary to the popular belief that we always had the
manual, this is NOT true. The White Knight happened to meet a very cool
guy at the CEBIT in Germany and has gotten the manual 2 weeks ago!
Yes! What you have seen from us before was true coding. We started out
by hacking and working our way to the top. We didn't wait until we bought
a manual. In fact, the manual was sent to us for FREE! This book isn't
worth paying for!
Let's clear up a few misunderstandings about that SNES manual!
1) This book does NOT teach you assembly language!!
It would help if you knew a little before trying to code this machine!
2) This book was not written such as other reference guides you can find in
a store, like Mapping the Amiga, or even Mapping the C64.
It tells you barely and confusingly what the registers do. Period.
3) This book is about 148 pages long and that includes the Sound Section.
Some have said it was the size of a phone book. Unless live in
Mud Hole, Kentucky this is NOT the case!
So much for the introduction. I personally feel that experience is better
than a reference guide. Reading a text file does not give you the feel of the
machine. In the included source file, you will notice that almost every line
has a description of what it is doing. This is better than telling you the
registers and letting you fiddle around. You know what it will do, and you
can see it in action in the assembled output (also included). I suggest
you examine the source code right after reading this brief introduction on
how the SNES system operates.
The SNES runs on a 65816 processor. This is similar to the 6502, but many
new instructions are available. You will use the most popular commands
like LDA, LDX, STA, PHA, PLA, RTS, JMP.. etc etc. There are some new
ones but we will get into that subject in the next volume as it is not
very important right now.
The 65816 is a 16 bit processor that does 24 bit addressing.
You can load and store 16 bit numbers, as well as 8 bit. The addressing is
is different than the 6502 in that it includes a bank. If you have coded on
the C64 you know that the addressing on the C64 was from $0000-$FFFF.
That is 16 bit addressing. 24 bit includes 1 extra byte. This one byte
is the BANK number. The SNES memory is broken down into fragments of
32k blocks each. They are addressed from $8000-$FFFF and are stored into
banks sequentially. If you wanted to access the first ROM byte in memory,
the address would be $008000. The first $00 is the bank number, the first
bank you can access. The $8000 is the 16 bit address. All banks (unless in
high rom 64k bank memory) start at $8000!
Remember that you can not write ROM. If you have coded on a C64 you have
written a routine that looked like this:
lda #$00
sta $c000
You cannot do this! $c000 is ROM and you can not write to ROM!
To write to ram, simply write to any address between $0000-$1fff.
If you need more memory you will find plenty at bank $7e and $7f
These 2 banks contain memory from $0000-$ffff. These 2 banks each contain
64k ofram totalling 128k for your own use!
If you need to write to these directly, just use the LONG STA command
example:
sta $7ec000
This will write to bank $7e at address $c000!
Fair enough. This was only a brief lecture on how the memory works.
The SNES hardware registers and how the work:
You will notice when looking at the source code something very strange.
Some registers are written to twice in a row! This because some registers
need more than one 8 bit info, such as the scroll X registers. In these
registers you can enter any number between $0-$07ff, but they are written
as two 8 bit numbers, one right after another.
example:
lda #$07
sta $210d
lda #$00
sta $210d
This writes #$0007 to $210d, plane 0 scroll x register.
Using 16 bit data storage will not work for this type of register!
example:
lda #$0007
sta $210d
This will not work because it will write #$07 to $210d, then a #$00 to $210e.
Another strange register is the self-incrementing register such as the VRAM
address registers $2116 and $2117.
After writing to $2119 (or $2118 in another setting) the VRAM address in
$2116 and $2117 will be increased. You do not have to do it yourself.
This can be seen in the Character set (font set) transfer routine in the
source code.
Introduction to Video RAM (VRAM)
The Super NES system has it own graphics processor. This requires its own
ram to read/write graphics data. This ram can only be accessed through
certain registers such as $2118 + $2119. To access Video Ram you MUST
turn off the video or you must be in screen blank (horizontal or vertical).
This is one of the downsides of the SNES.
Video Ram allows the storage of map planes and tile graphics.
VRAM is only 64k long and can not be used as regular ram. You can not
program in it, it is a separate unit!
In this volume we show you how to make a text screen in Mode 0. There are
8 graphic modes numbered from Mode 0 to Mode 7. Mode 0 is the most
simplistic. It allows only 4 colors per tile, but allows all four
planes to be used.
A normal video screen on the SNES is 32*32 tiles, which comes out to an
even 1024 tiles. You can widen the screen but you still may only have
1024 tiles. There are 2 parts to displaying a graphic on the screen.
There is the tile graphic data which gives the tile its picture. Then there
is the Map data. These are individually placed tiles placed on the screen
to produce an image.
example:
BBBBB SSSSS LL
BB BB SS LL
BBBBB SSSS LL
BB BB SS LL
BBBBB SSSSS LLLLLLL
Notice that all the small B's are the same. These woulds be drawn as
Tile Graphics. They all form together to create the large B image.
These B's together would be the Map data. The same would go for the S and L.
This is enough info to understand the basics of this 2 color intro.
This next installment on "How to code SNES" will feature more interesting
subjects as:
- How the color works
- 16 color graphic mode
- the entire 65816 instruction set with op-codes
- more info on the joypad