Atari 2600 VCS Programming

game program™
ATARI 2600
PROGRAMMING
Use with Keyboard Controllers
PROGRAM CONTENTS 2011-2013 CHESTER, INC.
©

What
An overview of the Atari 2600
architecture, covering everything
needed to create a “Hello,World!”
program that can run on emulators
or even on a real Atari
http://slideshare.net/chesterbr

Why
● Pure nostalgia
● Homebrew games
● Demoscene
● Appreciate masterworks such as
Enduro, Pitfall! or River Raid
● Feel better about today's tools
and hardware limitations :-)

@chesterbr
http://chester.me
Who

Atari 2600
(Video Computer System)

Over 600 titles...imagem: mitchelaneous.com

but why were they so...“Atari-ish”?

Let's look inside and find out!
(Atari 2600 Jr. printed circuit board)

Look ma, no O.S.!
Atari programs talk directly to the
hardware – there is no middle man!
Its 6502 CPU only “understands”
memory reads and writes, so all the
other chips are hard-wired to act as
memory (even those who aren't)

Memory Map
(very, very, very simplified*)
0000-002C – TIA (write)
0030-003D – TIA (read)
0080-00FF – RIOT (RAM)
0280-0297 – RIOT (I/O,Timer)
F000-FFFF – Cartridge (ROM)
*http://nocash.emubase.de/2k6specs.htm

Memory Map
(this is not your biggest problem...)
4 KBytes!

Memory Map
(and still not your biggest problem)
128 BYTES!!!!!
(1/8 of a KB)

VRAM
(frame buffer)
Typical video chips translate
bit patterns stored onVideo RAM
(VRAM) into pixels and colors

VRAMVRAM
VRAM
38
44
44
7C
44
44
EE
00

VRAM
Screen resolution and color depth
are subject toVRAM size limits
Memory was expensive on the
70s/80s, leading to trade-offs
How muchVRAM does an Atari have?

Memory Map
0000-002C – TIA (write)
0030-003D – TIA (read)
0280-0297 – RIOT (I/O,Timer)

Memory Map
0 bytes !!!!
????-???? – VRAM
right, now you
got a problem...

Racing the Beam
Since we can't just write pixels to
someVRAM frame buffer, our code
will need to work a bit closer to
the TV hardware, with a little help
from a very unique chip...

TIA
(Television Interface Adaptor)

How a TV set works
Image: cc-by wikipedia user Grm_wnr

Scanlines
public domain illustration by Ian Harvey
60 frames
per second
(NTSC
standard)

TIA is scanline-oriented
As the beam draws each scanline, the
game program must set TIA registers
to configure the objects drawn on it
Most of this objects have only one
color, making multiple colors on the
same scanline theoretically impossible...

constraints ⇒ creativity
vs.

Screen Objects
● Playfield (PF)
● Players (P0, P1)
● Missiles/Ball (M0, M1, BL)
Scanlines will be rendered based on
how we configure TIA's screen objects:

Playfield (PF)
20-bit pattern with a foreground and
a background color, rendered
over the left side of the scanline
The right side will either repeat or
refect the same pattern (the later
generating a symmetrical image)

Playfield configuration
PF0 = 0000 ←← order
PF1 = 00000000 order →→
PF2 = 00000000 ←← order
REFLECT = 0
resulting scanline

PF0 = 0001 ←← order
PF1 = 00000000 order →→
PF2 = 00000000 ←← order
REFLECT = 0
resulting scanline
_ _

PF0 = 0011 ←← order
PF1 = 00000000 order →→
PF2 = 00000000 ←← order
REFLECT = 0
resulting scanline
__ __

PF0 = 0111 ←← order
PF1 = 00000000 order →→
PF2 = 00000000 ←← order
REFLECT = 0
resulting scanline
___ ___

PF0 = 1111 ←← order
PF1 = 11110000 order →→
PF2 = 00000000 ←← order
REFLECT = 0
resulting scanline
________ _______

PF0 = 1111 ←← order
PF1 = 11111110 order →→
PF2 = 00010101 ←← order
REFLECT = 0
resulting scanline
___________ _ _ _ ___________ _ _ _

PF0 = 1111 ←← order
PF1 = 11111110 order →→
PF2 = 00010101 ←← order
REFLECT = 1
resulting scanline
___________ _ _ _ _ _ _ ___________

___________ _ _ _ _ _ _ ___________

Players (P0, P1)
Each one is an independent 8 bit pattern
(GRP0/GRP1) with a foreground color
(COLUP0 / COLUP1) that can be
positioned at any column of the scanline
e.g.: 10100001 → ████████

Players
Each player can be horizontally
stretched, multiplied or inverted by
setting NUSIZn / REFPn (n=0/1)
(number/size and reflect player)

NUSIZ0 (or NUSIZ1)
000
001
010
011
100
101
110
111
NUSIZn

With REFP0 (or REFP1) on
000
001
010
011
100
101
110
111
NUSIZn

Missiles/Ball (M0/M1/BL)
Can be positioned just like players, but
no bit pattern, just a pixel (although it
can be horizontally stretched 2/4/8x)
M0/M1 use P0/P1colors, while
BL uses the PF foreground color

Master Plan
For each scanline, configure the
options for each object before the
beam reaches its intended position
The time slot is very short, forcing
programmers to pick and choose what
to change, reusing as much as they can

How short?
6502 ≈ 1,19Mhz (1.194.720 cycles/sec)
NTSC: 60 frames per second
1.194.720/60 ≅ 19.912 cycles per frame

How short?
CPU: 19.912 cyles per frame
NTSC: 262 scanlines per frame
19.912 / 262 = 76 cycles per scanline

How short?
CPU: 19.912 cyles per frame
NTSC: 262 scanlines per frame
19.912 / 262 = 76 cycles per scanline
and what can we do with 76 cycles?
(by the way:WTF is a “cycle”?)

6502 (Atari-wise)
Reads a program from the cartridge
(ROM) composed of operations that
manipulate and transfer bytes between
cartridge, RIOT (RAM, I/O, timers) and
TIA, keeping state on internal registers

Operations
Each operation that composes a 6502
program in memory is identified by a
1-byte opcode and can be followed
by up to 2 bytes of parameters
An instruction can take up to 6
cycles to be executed

Registers
A = Accumulator (8 bits)
X,Y= Indexes (8 bits)
S = Stack Pointer (8 bits)
P = Status (fags, 8 bits)
PC = Program Counter (16 bits)

Example program
“add 2 to a value stored at a
memory position; store the result
into another memory position”

Implementation
● Read the byte stored on memory
position 0200 into register A
● Add 2 to register A's value
● Write register A's value into
memory position 0201

6502 Machine Code
AD Opcode (Memory → A)
00 Last part of “0200”
02 First part of “0200”
69 Opcode (value + A → A)
02 Value to add
8D Opcode (A → Memory)
01 Last part of “0201”
02 First part of “0201”

6502 Assembly Language
Associates the 151 opcodes
with 56 mnnemonic instructions
and a notation for their
parameters (access mode)

Assembly 6502
AD LDA $0200
00
02
69 ADC #02
02
8D STA $0201
01
02

Assembler
Program that reads a text file
written in Assembly language and
assembles a binary file with the
corresponding machine code
foo.asm
LDA $0200
ADC #02
STA $0201
...
foo.bin
AD000269
028D0102
...
ASSEMBLER

Macro Assembler
ORG $0100 ; Start @ memory 0100
...
SomeLabel:
LDX #$10 ; No idea where this
DEX ; will be in memory,
BNE SomeLabel ; and don't need to!
...

DASM
● 6502 Macro Assembler
● Includes Atari headers
● Multiplataform
● Free and open-source (GPLv2)
http://dasm-dillon.sourceforge.net/

Notation (for today)
#... = decimal value
#$... = hex value
$... = hex address
$... , X = hex address + X
http://www.obelisk.demon.co.uk/6502/addressing.html

6502 Instruction Set
= most relevant for Atari 2600 programming

Data Transfer
LDA, LDX, LDY = Load
STA, STX, STY = Store
TAX,TAY,TXA,
TYA,TSX,TXS = Transfer
LDA #$10 0x10→A
STY $0200 Y→m(0x0200)
TXA X→A

Arithmetic
ADC, SBC = +,- (w/ carry)
INC, INX, INY = ++
DEC, DEX, DEY = --
ADC $0100 m(0x100)+A→A
INC $0200 m(0x200)+1→
m(0x200)
DEX X-1→X

Bit Operations
AND, ORA, EOR = &, |, ^ (A)
ASL, LSR = Arithmetic shift
ROL, ROR = “Rotating” shift
AND #$11 A&0x11→A
LSR A>>1→A (A/2→A)
ROR A>>1 (bit 7=carry)

Comparing / Branching
CMP, CPX, CPY = compare A/X/Y (-)
BCS, BCC = ⇗ if carry set / clear
BEQ, BNE = ⇗ if equal / not equal
BVS, BVC = ⇗ if overfow set / clear
BMI, BPL = ⇗ if minus / plus
CPY $1234 if y=m(0x1234),
BEQ $0200 0x0200→PC

Stack and Subroutines
JSR, RTS = call/return subroutine
PHA, PLA = push / pull A
PHP, PLP = push / pull status (P)
JSR $1234 PC(+3)→stack,
0x1234→PC
RTS stack→PC

Everything else...
NOP = No Operation
JMP = Direct Jump (GOTO)
SEC, CLC = Set/Clear Carry
SEV, CLV = Set/Clear oVerfow
SEI, CLI = Set/Clear Interrupt-off
SED, CLD = Set/Clear Decimal
RTI = Return from Interrupt
BRK = Break

Neo:“I know kung fu.”
Morpheus:“Show me.”
© 1999 Warner Bros

Hello,World!
Horizontal writing is hard
(too many pixels per scanline)

Hello,World!
Vertical writing
is the way →
(less pixels/scanline)
We can use a player
or the playfield

Hello,World!
Our display kernel will configure each
pair of visible scanlines with a byte
from a “hello world” bitmap (stored
on the card, just after the code)
Let's find which are visible, and how
we need to deal with TV timings

Source: Stella Programmers Guide, SteveWright, 1979
GAMELOGIC
(3+37+30).76=5320cycles
KERNEL

Program structure
Vertical Sync
Vertical Blank
Overscan
Playfield

main
loop
(infinite)
Kernel
X: count 0 to 191
(192 scanlines)
Program structure
Vertical Sync
Vertical Blank
Overscan
11 chars *
8 bytes *
2 scanlines
per byte =
176
scanlines

Program structure
Vertical Sync
Vertical Blank
Kernel
Overscan

Let's begin!
PROCESSOR 6502
INCLUDE "vcs.h"
ORG $F000 ; Cart begins here
Vertical Sync
Vertical Blank
Kernel
Overscan

Frame (main loop) start
StartFrame:
lda #%00000010 ; Signal VSYNC start by
sta VSYNC ; setting bit 1
REPEAT 3 ; lasts 3 scanlines
sta WSYNC ; (WSYNC = wait until
REPEND ; scanline is finished)
lda #0 ; Signal VSYNC end (and
sta VSYNC ; VBLANK start)
Vertical Sync
Vertical Blank
Kernel
Overscan

Vertical Blank
PreparePlayfield:
lda #$00
sta ENABL ; Disable ball
sta ENAM0 ; Disable missiles
sta ENAM1
sta GRP0 ; Disable players
sta GRP1 ; (with a 0s-only shape)
Vertical Sync
Vertical Blank
Kernel
Overscan

Vertical Blank
sta COLUBK ; Background (0=preto)
sta PF0 ; PF0 and PF2 stay off
sta PF2
lda #$FF ; Playfield color
sta COLUPF ; (yellow-ish)
lda #$00 ; Clear CTRLPF bit 0 to
sta CTRLPF ; repeat playfield
ldx #0 ; X: scanline counter
Vertical Sync
Vertical Blank
Kernel
Overscan

FinishVertical Blank
REPEAT 37 ; VBLANK lasts 37 scanlines
sta WSYNC ; (useful for game logic)
REPEND ;
lda #0 ; Signals VBLANK end (will
sta VBLANK ; “turn on the beam”)
Vertical Sync
Vertical Blank
Kernel
Overscan

Scanline (inner loop)
Scanline:
cpx #174 ; Phrase over?
bcs ScanlineEnd; if so, skip
txa ; Y=X÷2 (logic shift →
lsr ; divides A by 2)
tay ;
lda Phrase,y ; label,Y = mem[label+Y]
sta PF1 ; PF1 = playfield (bits
; 4 to 11) Vertical Sync
Vertical Blank
Kernel
Overscan

Scanline (close inner loop)
ScanlineEnd:
sta WSYNC ; Finish current scanline
inx ; X=line counter
cpx #191 ; last visible scanline?
bne Scanline ; unless so, repeat!
Vertical Sync
Vertical Blank
Kernel
Overscan

Overscan (close main loop)
Overscan:
lda #%00000010 ; “turn off” beam again
sta VBLANK ; 30 scanlines of
REPEAT 30 ; overscan...
sta WSYNC
REPEND
jmp StartFrame ; ...and start it over,
; forever and ever!
Vertical Sync
Vertical Blank
Kernel
Overscan

Hello...
Phrase:
.BYTE %00000000 ; H
.BYTE %01000010
.BYTE %01111110
.BYTE %01000010
.BYTE %01000010
.BYTE %01000010
.BYTE %00000000
.BYTE %00000000 ; E
.BYTE %01111110
...

...world
...
.BYTE %00000000 ; D
.BYTE %01111000
.BYTE %01000100
.BYTE %01000010
.BYTE %01000010
.BYTE %01000100
.BYTE %01111000
.BYTE %00000000 ; PF1 last value (!)

6502 configuration
ORG $FFFA ; Located at the end
; of ROM (cart)
.WORD FrameStart ; NMI address
.WORD FrameStart ; BOOT address
.WORD FrameStart ; BRK address
END

Assemble!
dasm source.asm -ocart.bin -f3
http://stella.sourceforge.net/

Playfield-based score
The PF color can be replaced with
player colors (P0 = left side; P1 =
right side) by turning CTRLPF's
score mode bit on

(how could this improve our Hello World?)

Playfield-based score
Q: How can different patterns be
shown at each side of the playfield?
A: Change the pattern when the
beam is halfway through the scanline
(“race the beam”)

beam
...and you have a different shape
at the other half!

http://pitfallharry.tripod.com/MapRoom/PitfallMap.html

Pitfall!
Screen configuration (logs, vines,
trees, stairs, crocs) was squeezed
into a single byte, but 255 screens
(bytes) are still a huge ROM table
(for Atari standards)
http://pitfallharry.tripod.com/MapRoom/PitfallMap.html

Pitfall!
David Crane implemented a sequence
generator (LFSR), which, for a given
value, would give the previous/next
ones, replacing the 255-byte table
with a 50-byte piece of code
http://en.wikipedia.org/wiki/Linear_feedback_shift_register

River Raid
Carol Shaw had used a 16-bit generator,
resulting in thousands of non-repeating
river sectors (tweaking the interpreter
to make the first few ones easier)
When a player loses a life, rendering
restarts from the last generated number,
that is, back on the last bridge

Horizontal positioning
The horizontal position of a player /
missile / ball is not a writable register
Games must sync to with the beam and
write to the appropriate strobe register
when it is on the desired location

You can calculate...
1 CPU cycle = 3 “color clocks” (pixels)
Horizontal Blank = 22.6 cycles
horiz. position ≈ (cycles – 22.6) * 3
...but it is an estimate, because TIA only
reads its registers every 5 CPU cycles
Smooth ↔ movement is also hard

Alternatives
A 4-bit register allows moving a player,
missile or ball relative to its previous
position (adding -7 to +8 pixels)
The missile can also be reset to the
middle of its player, making it easy to
“fire” it over and over

VERTICAL MOVEMENT
Just start on a different scanline at each frame
HORIZONTAL MOVEMENT
strobe registers HMP0/1 e HMM0/1

Multi-digit score
The trick is the same of the playfield-
based score (change the registers while
the beam is drawing the scanline), but
timing is much more of an issue here
Let's say the score is 456789...

Multi-digit score
Begin the scanline with the bits for 4
on GRP0 and those for 5 on GRP1
Configure NUSIZ0 and NUSIZ1 for
a triple repetition:
4 4 4 5 5 5
Player 0 Player 1

Multi-digit score
Set player 1 position just to the right
of player 0, overlapping the triplets
454545
Player 0
Player 1

Multi-digit score
Change the patterns (GRP0/GRP1)
syncing with the beam, like this:
BEAM
454545

Multi-digit score
When the beam is about to finish
player 1's 1st
copy, change player 0 to
6 and player 1 to 7:
BEAM
454545

Multi-digit score
Repeat the trick after the 2nd
player 2
copy, this time changing player 0 to 8
and player 1 to 9
BEAM
456767

Multi-digit score
Repeat it for each scanline.
Easy! #not
BEAM
456789

Multi-digit score
There are other hurdles: we can't load
replaced digits from memory, and we
only have 3 memory-writable registers
to store 6 bit patterns...
...and that is why it is fun!

A new look to the old school
Knowing what the Atari 2600 was
designed to do, we can appreciate
games that push it beyond its limits
by identifying “impossible” things

It is just the beginning!
The Atari homebrew scene is alive and
kicking, and these are the basics you
need to create your own games/demos
There are several uncovered topics
(sound, timers, collision, I/O...), but with
time and dedication, you can do it!

To learn more
Hello, World: https://gist.github.com/chesterbr/5864935
Sorteio 2600 http://github.com/chesterbr/sorteio2600
Racing The Beam (book): http://bit.ly/dSqhjS
David Crane's talk: http://youtu.be/MBT1OK6VAIU
David Crane's iOS tutorials: http://bit.ly/9pwYHs and http://bit.ly/qWBciZ
Stella Programmer's Guide: http://emu-docs.org/?page=Atari%202600
Classic games disassembled: http://classicdev.org/wiki/2600/Source_Code
Atari 2600 specs: http://nocash.emubase.de/2k6specs.htm
6502 reference: http://bit.ly/hxG5c6
In-browser emulator: http://jogosdeatari.com.br/
Andrew Dave's tutorial: http://bit.ly/ptQDdA (the whole site is great)
Harmony (SD-reader cart): http://harmony.atariage.com/
BAtari (BASIC compiler): http://bataribasic.com
TIA sound examples: http://bit.ly/tnbPrp
Bankswitching (more ROM/RAM): http://bit.ly/tqhLZk

Questions?
Thank you!
@chesterbr
http://chester.me

Credits and License
This presentation is available under the
licença Creative Commons “by-nc” 3.0 l,
noticing the exceptions below
Images from third parties were included (with due credits) under
fair use assumption and/or under their respective licenses.
These are excluded from the license above.
Atari™,Adventure™, Donkey Kong™, Pitfall™, Super Mario™ and
likewise characters/games/systems are mentioned uniquely for
illustrative purposes, also under fair use assumption.They are property
of their rights holders, and are also excluded from the license above.

Atari 2600 VCS Programming

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