157 lines
6.8 KiB
Plaintext
157 lines
6.8 KiB
Plaintext
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BASELINE<4E>2091 Presents The First Guide To Coding The SNES
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Guide and Source Code by -Pan-
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Released on 4/20/93
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Welcome to the first installment of "How to code SNES"
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This first volume will show you how to make a simple text intro. It uses
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mode 0 graphics with no DMAs (Horizontal or General) and is the simplest
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type of intro you can make. We're starting small so you can easily understand
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what to do. Other future volumes will contain other graphic modes,
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Horizontal DMA (HDMA, the SNES version of an amiga copperlist), General DMA,
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Interrupts, and a brief section on Sound. Originally we were going to release
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the full Super Famicom Programmer's Manual but believe it or not, this manual
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a pile of garbage. Contrary to the popular belief that we always had the
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manual, this is NOT<4F>true. The White Knight happened to meet a very cool
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guy at the CEBIT in Germany and has gotten the manual 2 weeks ago!
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Yes! What you have seen from us before was true coding. We started out
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by hacking and working our way to the top. We didn't wait until we bought
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a manual. In fact, the manual was sent to us for FREE! This book isn't
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worth paying for!
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Let's clear up a few misunderstandings about that SNES<45>manual!
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1) This book does NOT teach you assembly language!!
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It would help if you knew a little before trying to code this machine!
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2) This book was not written such as other reference guides you can find in
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a store, like Mapping the Amiga, or even Mapping the C64.
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It tells you barely and confusingly what the registers do. Period.
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3) This book is about 148 pages long and that includes the Sound Section.
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Some have said it was the size of a phone book. Unless live in
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Mud Hole, Kentucky this is NOT the case!
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So much for the introduction. I personally feel that experience is better
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than a reference guide. Reading a text file does not give you the feel of the
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machine. In the included source file, you will notice that almost every line
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has a description of what it is doing. This is better than telling you the
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registers and letting you fiddle around. You know what it will do, and you
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can see it in action in the assembled output (also included). I suggest
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you examine the source code right after reading this brief introduction on
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how the SNES system operates.
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The SNES runs on a 65816 processor. This is similar to the 6502, but many
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new instructions are available. You will use the most popular commands
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like LDA, LDX, STA, PHA, PLA, RTS, JMP.. etc etc. There are some new
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ones but we will get into that subject in the next volume as it is not
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very important right now.
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The 65816 is a 16 bit processor that does 24 bit addressing.
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You can load and store 16 bit numbers, as well as 8 bit. The addressing is
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is different than the 6502 in that it includes a bank. If you have coded on
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the C64 you know that the addressing on the C64 was from $0000-$FFFF.
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That is 16 bit addressing. 24 bit includes 1 extra byte. This one byte
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is the BANK number. The SNES memory is broken down into fragments of
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32k blocks each. They are addressed from $8000-$FFFF and are stored into
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banks sequentially. If you wanted to access the first ROM byte in memory,
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the address would be $008000. The first $00 is the bank number, the first
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bank you can access. The $8000 is the 16 bit address. All banks (unless in
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high rom 64k bank memory) start at $8000!
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Remember that you can not write ROM. If you have coded on a C64 you have
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written a routine that looked like this:
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lda #$00
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sta $c000
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You cannot do this! $c000 is ROM and you can not write to ROM!
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To write to ram, simply write to any address between $0000-$1fff.
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If you need more memory you will find plenty at bank $7e and $7f
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These 2 banks contain memory from $0000-$ffff. These 2 banks each contain
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64k ofram totalling 128k for your own use!
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If you need to write to these directly, just use the LONG STA command
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example:
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sta $7ec000
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This will write to bank $7e at address $c000!
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Fair enough. This was only a brief lecture on how the memory works.
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The SNES hardware registers and how the work:
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You will notice when looking at the source code something very strange.
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Some registers are written to twice in a row! This because some registers
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need more than one 8 bit info, such as the scroll X registers. In these
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registers you can enter any number between $0-$07ff, but they are written
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as two 8 bit numbers, one right after another.
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example:
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lda #$07
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sta $210d
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lda #$00
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sta $210d
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This writes #$0007 to $210d, plane 0 scroll x register.
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Using 16 bit data storage will not work for this type of register!
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example:
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lda #$0007
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sta $210d
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This will not work because it will write #$07 to $210d, then a #$00 to $210e.
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Another strange register is the self-incrementing register such as the VRAM
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address registers $2116 and $2117.
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After writing to $2119 (or $2118 in another setting) the VRAM address in
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$2116 and $2117 will be increased. You do not have to do it yourself.
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This can be seen in the Character set (font set) transfer routine in the
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source code.
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Introduction to Video RAM (VRAM)
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The Super NES system has it own graphics processor. This requires its own
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ram to read/write graphics data. This ram can only be accessed through
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certain registers such as $2118 + $2119. To access Video Ram you MUST
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turn off the video or you must be in screen blank (horizontal or vertical).
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This is one of the downsides of the SNES.
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Video Ram allows the storage of map planes and tile graphics.
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VRAM is only 64k long and can not be used as regular ram. You can not
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program in it, it is a separate unit!
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In this volume we show you how to make a text screen in Mode 0. There are
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8 graphic modes numbered from Mode 0 to Mode 7. Mode 0 is the most
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simplistic. It allows only 4 colors per tile, but allows all four
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planes to be used.
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A normal video screen on the SNES is 32*32 tiles, which comes out to an
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even 1024 tiles. You can widen the screen but you still may only have
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1024 tiles. There are 2 parts to displaying a graphic on the screen.
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There is the tile graphic data which gives the tile its picture. Then there
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is the Map data. These are individually placed tiles placed on the screen
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to produce an image.
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example:
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BBBBB SSSSS LL
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BB BB SS LL
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BBBBB SSSS LL
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BB BB SS LL
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BBBBB SSSSS LLLLLLL
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Notice that all the small B's are the same. These woulds be drawn as
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Tile Graphics. They all form together to create the large B image.
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These B's together would be the Map data. The same would go for the S and L.
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This is enough info to understand the basics of this 2 color intro.
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This next installment on "How to code SNES" will feature more interesting
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subjects as:
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- How the color works
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- 16 color graphic mode
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- the entire 65816 instruction set with op-codes
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- more info on the joypad
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