65c02 Assembly programming for the Atari Lynx

The Atari Lynx uses the 65c02... coupled with a 16 bit GPU, the Atari Lynx is was far more powerful than the Gameboy and Sega Master system

Although it's large size, and high power draw made it a commercial failure, it's an interesting system to develop for!

The official cartridges are encrypted, but we can make a binary that the emulator Handy can run quite easily!

Unlike most other systems, the Lynx does not have a tile array, a chunk of about 8k of internal memory is used for the screen, and we can write bytes directly to it (like on the CPC)... the hardware sprite function also writes to this buffer, so we have no effective sprite limit!

There were two models of the atari Lynx, but there's no real difference between them.
Cpu 4mhz 65C02
Ram 64k 
Vram Uses internal memory
Resolution 160x102
Sprites Unlimited... blits sprites to buffer in system memory
Tilemap None
Colors 16 onscreen from 4096
Sound chip 4 channel stereo

ChibiAkumas Tutorials

Lesson P4 - Bitmap Functions on the Atari Lynx

Lesson P13 - Joystick Reading on the Atari Lynx

Lesson P19 - Palette definitions on the Atari Lynx

Lesson P24 - Sound on the Atari Lynx

Binary file Header
While real cartridges are encrypted (causing copyright problems for the hobbyist), the Handy emulator can work with an unencrypted O file... this also means we do not need an official rom to run the emulator!

There is a 10 byte header...
FixedBytes:Many of the bytes are fixed, and should not be changed
StartPoint: Bytes 2 and 3 are the startpoint in Big Endian format.. . in our example the first program byte is $0300
Length: Bytes 4 and 5 are the length of the file.

Hardware Sprites
Unlike other systems, Lynx hardware sprites are not an extra layer! the 'Suzy' graphics chip draws the sprite into the Vram area of the 6502's addressable range

This may leave you wondering why not just do our sprites in software with the 6502... but the Suzy chip is VERY fast... it's a 16mhz 16 bit chip... and can even do dynamic scaling of sprites!

Sprites for the Suzy chip have to be held in RAM, and need a 'Sprite control block' to define the drawing of a sprite... this pointer is passed to the Suzy chip to get it to draw a sprite

Sprites can be 'Literal' (plain bmp) or 'RLE compressed' (defined by bit 7 of byte two of the SCB - SCBCTL1).... the colordepth is defined in SPRCTL0 (See later)

Each line of a sprite starts with a byte - this an offset to the next line... effectively the number of bytes in the line +1 .... effectively the pointer to the next line.

1 or 0 in this position have special meanings!... 0 means the end of the sprite... 1 means the end of the 'quardrent'... note this is optional! Akusprite does not use it!

Quadrent rendering is where the sprite is drawn in 4 sections from the middle... with a 1 byte marking each 1/4 of the sprite...  (followed by another 'offset to next line' byte)
the first quadrent is DownRight (default)... the second quadrent is UpRight
the third quadrent is UpLeft)... the fourth quadrent is DownLeft

Apparently there is a bug in the hardware - the last bit of each line must be 0! - we should always have a 0 at the end of our sprites to counter it - color 0 is transparent anyway!

You can see a Literal Sprite to the right... the Literal bitmap data is in green, and the header bytes are in cyan
Literal Sprite Example (BMP)

    db $8, $11, $11, $11, $11, $11, $10,0
    db $8, $10, $0, $0, $0, $0, $10,0
    db $8, $10, $04, $44, $44, $0, $10,0
    db $8, $10, $04, $3, $04, $0, $10,0
    db $8, $10, $04, $3, $04, $0, $10,0
    db $8, $10, $04, $44, $44, $0, $10,0
    db $8, $10, $0, $0, $0, $0, $10,0
    db $8, $11, $11, $11, $11, $11, $10,0
    db 0
RLE Sprite Data is a bit more tricky....
The first byte in a line is again an offset to the next line as before

The next BIT will be a 'block definition'... defining what the following data is...
1 marks that the next data will be LITERAL
0 marks that the next data will be RLE
    The next 4 bits will be the number of pixels to draw-1... so 0 means 1 pixel, and 15 means 16 pixels... we will call this N

If the block is RLE the next 1/2/3/4 bits (depending on bitdepth) will be used for the color to fill the next N pixels

If the block LITERAL the next N *(1/2/3/4) bits (depending on bitdepth) will be used for the color of the next N pixels

the next bit will be the next 'block definition'... this pattern repeats until the line is done.
4bpp RLE Sprite Example

db $8
(offset to next line)

db %01111000,%00000000
(RLE block...16 pixels... Color 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)

db %00001001,%10000000
(RLE block...2 pixles... Color 3,3)

db %10010000,%10010001,%10000000
(Literal block...3 pixels... Color 1,2,3)

(next line starts here)

The Sprite Control Block
The sprite control block defines the sprite onscreen, The first two bytes define the sprite type, how it's drawn, and the other data in the block... note if RR<2  we don't need the Scale or Tilt words... so these can be removed.

SPRCTL0 bits 7,6 defines the Colordepth 1/2/3/4 bits per pixel for 2/4/8/16 colors
SPRCTL1 bit 7 defines the sprite type 1=Literal... 0=RLE

Note... if you have the BPP wrong on a RLE sprite it will be a total mess... a Literal sprite would just be stretched and a bit weird! (colors would be sort of 'dithered')

The Xpos and Ypos are relative to the defined screen boundaries... Wid and Hei are scales $100=100% $200=200%

The 'Palette' maps each 'color' in the sprite to a palette setting... this is most useful for 1/2bpp sprites - where we will need to select the colors each combination of bits will use. the example shown is a 4bpp 16 color sprite
       ;BBHV-TTT - SPRCTL0... B=bits per pixel H=hflip V=vflip T=type (7=normal)
    db %11000101       
         ;LSRRPSUl - SPRCTL1... L=Literal S=Sizing choice (0 only!) RR=Reloadable depth  (1=Use Size 3=Use Size,ScaleTilt)
    db %10010000         ;P=Palette reload (0=yes) s=skipsprite u=draw up l=draw left
    db 0            ;- SPRCOL - 0= OFF
    dw 0            ;Next SCB (0=none)
    dw LynxSprite    ;Sprite pointer
    dw 10            ;Xpos
    dw 10            ;Yos
    dw $200            ;Wid ($100 = 100%)
    dw $200            ;Hei ($100 = 100%)
;    dw 0            ;Scale - not needed if B4,B5 of SPRCTL<3
;    dw 0            ;Tilt - not needed if B4,B5 of SPRCTL<2
;Palette - maps nibbles to colors (useful for <4 bpp)
    db $01,$23,$45,$67,$89,$AB,$CD,$EF   

Sprite Drawing
Lets Draw a sprite... if our visible screen is at &C000 - the following will work!

Note when setting 16-bit Suzy values, we must set the LSB before the MSB...
A write to the LSB will ZERO the MSB automatically... so if we set FC09 first in this example it WILL NOT WORK!

    lda #$0        ;MUST SET LSB FIRST!
                       ;WRITE TO LSB will ZERO MSB
    sta $FC08    ;For sprites
    lda #$C0       ;Set screen ram pointer to $C000
    sta $FC09    ;For sprites
    lda #<(SCB)
    sta $fc10
    ldy #>(SCB)
    sty $fc11               
        lda #$5          ; 1 SprStart + 4 Everon
        sta $FC91    ;SPRGO
        sta $FD90    ;SDONEACK
        stz $FD91    ;CPUSLEEP
        lda $fc92      ;DISPCTL
        bcs WaitSuzy
        stz $FD90    ;SDONEACK
If we want our sprites to clip at the top left, we can set a screen offset - for example to set an offset of 8:

    LDA #8
    STA $FC04    ;SCR OFFSET
    STA $FC06    ;SCR OFFSET

Hardware Ports Memory Map
The Lynx memory map is pretty simple, the Zeropage is at $00xx , the Stack is at $01xx....
addresses from $0200-$FC00 are free for ram... we need to allocate $2000 for a screen buffer, which can be anywhere in ram we want (defined in address $FD94-$FD95), but the rest is ours to use!

Note: Cartridge Rom is not memory mapped, it acts more like a 'disk drive' which we have to access like an external system... which is annoying!

We need to use the hardware registers to control, and read from the devices attached to the system, they are listed below:

From To Name Description Bits Meaning
FC00 FC01 TMPADRL Temporary Address LH

FC02 FC03 TILTACUM Accumulator for tilt value LH

FC04 FC05 HOFF Offert to H edge of screen

FC06 FC07 VOFF Offert to V edge of screen

FC08 FC09 VIDBAS Base address of video build buffer

FC0A FC0B COLLBAS Base address of Coll build buffer

FC0C FC0D VIDADRL Current Video Build Addres

FC0E FC0F COLLARL Current Collision Build Address

FC10 FC11 SCBNEXT Address of next SCB

FC12 FC13 SPRDLINE Start of Sprite Data Line Address

FC14 FC15 HPOSSTRT Starting Hpos

FC16 FC17 VPOSSTRT Starting Vpos




FC1E FC1F TILT H Position Adder

FC20 FC21 SPRDOFF Offset to next sprite data line

FC22 FC23 SPRVPOS Current Vpos

FC24 FC25 COLLOFF Offset to collision depository

FC26 FC27 VSIZACUM Vertical Size Accumulator

FC28 FC29 HSIZOFF Horizontal size offset

FC2A FC2B VSIZOFF Vertical Size Offeet

FC2C FC2D SCBADR Address of current SCB

FC2E FC2F PROCADR Current Spr data Proc Address  

   BBHV-TTT  B=bits per pixel
H=hflip V=vflip
T=type (7=normal)
LSRRPSUl L=Literal
S=Sizing choice (0 only!)
RR=Reloadable depth
P=Palette reload (0=yes)
s=skipsprite u=draw up l=draw left


FC88 FC88 SUZYHrev Suzy Hardware Revision R

FC89 FC89 SUZYHrev Suzy Hardware Revision W

FC90 FC90 SUZYBUSEN Suzy bus enable FF

FC91 FC91 SPRGO Sprite Process start bit ---E-S S=Sprites on E=Everon detector(?)
FC92 FC92 SPRSYS System Cotrlol Bits (RW)

FCB0 FCB0 JOYSTICK Read Joystick and Switches  UDLR12IO 
FCB1 FCB1 SWITCHES Read other switches -----CCP


FCC2 FCC2 PPT Paralell port Status RW

FCC3 FCC3 PPT DATA Paralell port Data RW

FCC4 FCC4 Howie Howie (RW)

FD00 FD03
Timer Channel 0 and hcount

FD04 FD07
Timer Channel 1 and mag0a

Timer Channel 2 and vcount

Timer Channel 3 and mag0b

FD10 FD13
Timer Channel 4 and serial rate

FD14 FD17
Timer Channel 5 and mag1a

Timer Channel 6

Timer Channel 7 and mag1b

FD20 FD20
Audio Channel 0 – 2’s compliment Volume control
FD21 FD21
Audio Channel 0 – Shift register feedback enable
eg %00010000
FD22 FD22
Audio Channel 0 – Audio Output Value (Raw Data)

Eg $80
FD23 FD23
Audio Channel 0 –Lower 8 bits of shift register
Eg 0
FD24 FD24
Audio Channel 0 – Audio Timer Backup Value
eg 0-63
FD25 FD25
Audio Channel 0 – Audio Control Bits
FTIRCKKK eg %00011110
FD26 FD26
Audio Channel 0 – Audio Counter

FD27 FD27
Audio Channel 0 –Other Audio Bits
Eg 0
Audio Channel 1 – Same as Channel 0

FD30 FD37
Audio Channel 2 – Same as Channel 0

Audio Channel 3 – Same as Channel 0

FD40 FD40 ATTENREG0 LLLLRRRR – Audio Attenuation

FD41 FD41 ATTENREG1 LLLLRRRR – Audio Attenuation

FD42 FD42 ATTENREG2 LLLLRRRR – Audio Attenuation

FD43 FD43 ATTENREG3 LLLLRRRR – Audio Attenuation

FD44 FD44 MPAN Stereo attenuation selection

FD50 FD50 MSTEREO Stereo disable LLLLRRRR 0=all on 255=all off
FD80 FD80 INTRST Interrupt poll 0

FD81 FD81 INTSET Interrupt poll 1

FD84 FD84 MAGRDY0 Mag tape Ready Channel 0

FD85 FD85 MAGRDY1 Mag tape Ready Channel 1

FD86 FD86 AUDI Audio In


FD88 FD88 MIKEYHREV Mikey Hardware Revision R

FD89 FD89 MikeySREV Mikey Software Revision W

FD8A FD8A IODIR Mikey Paralell IO Data direction

FD8B FD8B IODAT Mikey Paralell data

FD8C FD8C SERCTL Serial Control Register

FD8D FD8D SERDAT Serial Data

FD90 FD90 SDONEACK Suzy Done Acknowledge

FD91 FD91 CPUSLEEP Cpu Bus Request Disable

FD92 FD92 DISPCTL Video Bus Request Enable

FD93 FD93 PBKUP Magic P count

FD94 FD95 DISPADR Display Address LH LLLLLLLL HHHHHHHH Address of video screen



FDA0 FDAF Green – Colors (0-15)
FDB0 FDBF Blue/Red – Colors (0-15)

Memory Map Control
C---VRMS Turn on Rom/CPU Cycles
FFFC FFFD CPU Reset Vector
FFFE FFFF CPU Interrupt Vector


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