Atari 400 Hardware Manual

The OS uses 192 TV lines for its display and devotes the remaining
24 lines to overscan. It uses the standard display width of 160 color
clocks. The hardware will allow displays of any length, but it is recom-
mended that the standards be follo~ed. The exception might be a border
or other information which is merely decorative and not essential to use
of the program.
OS Shadowing: Since many of the hardware registers are write-only
and cannot be read the OS has a number of "shadow registers" in RAM.
Every TV frame during vertical blank the OS takes the values in some of
its shadow registers, and writes them out to the corresponding hardware
register. The OS does attract color shifting on all of the color registers
if ATRACT (on OS register) is negative. This is to prevent damage to the
TV screen phosphors which can occur if the brightness is turned up too high
and the same high-luminance display is left on for a long time. The OS also
reads the joysticks and other controllers during vertical blank and stores
the results in shadow registers, so that user programs do not have to include
code to unpack the data. There are a few interrupt-related registers which
the OS changes or reads during interrupt processing. Programs usually access
the OS shadow registers instead of accessing the hardware directly. However,
the OS shadowing can be disabled by changing the vertical blank and interrupt
vectors (see OS manual).
WSYNC: In addition to a Vertical Blank Interrupt, which allows the
Microprocessor to synchronize to the vertical TV display, this system also
provides a Wait for Horizontal Sync (WSYNC) command that allows the
microprocessor to synchronize itself to the TV horizontal line rate. This
sync takes effect when the processor writes to an I/O location called
WSYNC, whenever it desires horizontal synchronization. Writing to this
address sets a latch which pulls to zero a pin on the microprocessor
called READY. When READY goes to zero the microprocessor stops and waits.
The latch is automatically reset (returning READY true) at the beginning
of the next horizontal blank interval, releasing the microprocessor to
resume program execution.
Object DMA (Direct Memory Access): The primary function of the Antic
chip is to fetch data from memory (independent of the microprocessor) for
display on the TV screen. It does this with a technique called "Direct
Memory Access" or DMA. It requests the use of the memory address and data
bus by sending a signal called HALT to the microprocessor, causing the
processor to become "TRI-STATE" (open circuit) all during the next computer
cycle. The ANTIC chip then takes over the address bus and reads any data
it wishes from memory. Another name for this type of DMA is "cycle stealing".
Once initiated, this DMA is completely and automatically controlled by the
Antic chip without need for futher microprocessor intervention.
There are two types of DMA: Playfield and Player-Missile (see Figure
II.2). The playfield DMA control circuit on the Antic chip resembles a
small dumb microprocessor. By halting the main microprocessor it can
fetch its own instructions from memory (the display list) addressed by its
program counter(display list pointer). Each instruction defines the type
(alpha character or memory map), and the resolution (size of bits on the
screen), and the location of the data in memory which is to be displayed
on the next group of lines.
II. 2

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)