Atari 400 Hardware Manual

l· INTRODUCTION
The ATARI (R) 800™ and ATARI 400™ Personal Computer Systems
contain a 6502 microprocessor, 4 I/O chips, operating system ROM, expandable
RAM, and several MSI chips for address decoding and data bus buffering.
This manual is primarily intended to describe the 4 I/O chips in sufficent
detail to allow experienced programmers to create assembly language programs,
such as video games. All four Input/Output chips are controlled by the
microprocessor by writing directly into their registers which are decoded
to exist in microprocessor memory space just as RAM does. These I/O chips
can also be interrogated by the microprocessor by reading similar registers.
Many registers are write only and cannot be read after they are
written. In some cases, reading from the same address gives the value
contained in a separate read only register. Some write only registers are
strobes. No data bits are needed in this case since the presence of the
address on the bus is what triggers the requested action. The usual
convention is to use the STA (Store Accumulator) instruction for such
registers. For example, STA WSYNC performs the wait for Sync function.
STX (Store X) or STY (Store Y) would work just as well. In BASIC, a POKE
could be used (the data could be anything). Reading a register is accomp-
lished by using any of the load instructions (LDA, LDX etc.). In BASIC
a PEEK would be used. When the hardware register names are defined in an
equate list, the programmer can refer to the registers by name rather than
using the addresses directly.
It is really not necessary for the programmer to know which
I/O functions are performed by which of the 4 chips, however it
does help in learning these functions.
This manual should be used in conjunction with the Operating
System (OS) Manual, a 6502 programming manual, and the ATARI 400/800
Basic Reference Manual.
CHIP NAME
ANTIC
CTIA
FUNCTION
DMA(Direct Memory Access) control
NMI(Non-Maskable Interrupt) control
Vertical and Horizontal fine scrolling
Light pen position registers
Vertical line counter
WSYNC(wait for horizontal sync)
Priority control (display of overlapping objects)
Color-Lumimance control (colors and brightness assigned
to all objects including DMA objects from ANTIC )
PLAYER-MISSILE
objects (4 players and 4 missiles)
Graphics registers
Size control
Horizontal position control
Collision detection between all objects
Switches and triggers (miscellaneous I/O functions)
I.l

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)