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- 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
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