Atari 400 Hardware Manual

There are six charcter map modes, IR modes 2 through 7. Modes 2,6
and 7 are supported by the OS and BASIC (GRAPHICS 0,1 and 2).
In IR modes 6 and 7, the upper two bits of each character name select
one of four playfield colors. For each data bit that contains a one, the
selected playfield color is displayed. For each zero data bit, the
background color is displayed. The four character colors plus the background
color gives a total of five different colors. the mode 6 characters are
eight lines high and the mode 7 characters are sixteen lines high (each
data byte is displayed for two lines).
In IR modes 4 and 5, each character is only four pixels wide instead
of eight (as in the other modes). Two bits per pixel of data are used to
select one of three playfield colors, or background. Seven~ bits are
used to select the character. If the most significant name bit is a zero
then data of 10 (binary) selects PF1. If the name bit 7 is one, then data
bits of 10 select PF2. This makes it possible to display two characters
with different colors, using the same data but different name bytes.
In IR modes 2 and 3, each pixel is half of a color clock in width.
This makes it possible to have forty eight-pixel-wide characters in a
standard width line. These modes are similar to memory mode F in that two
luminances can be displayed, but only one color is available at a time.
In IR mode 3, each character is 10 lines high. This makes it possible to
define lower case characters with descenders. The last fourth of the
character set (name bits 5 and 6 equal to one) is lowered. The hardware
takes the first two data bytes and moves them to the bottom of the character,
displaying two blank lines at the top of the character (see next page).
In IR modes 2 and 3, bit 7 of the character name is used for inverse
video or blanking. This is controlled by CHACTL (Character Control). If
bit 2 of CHACTL is a one then all of the characters will be displayed
upside down, regardless of mode. If CHACTL bit 1 is set, then each
character which has bit 7 of its name set will be displayed in inverse
video (the luminances will be reversed). If CHACTL bit 0 is set, then
each character which has bit 7 set will be blanked (only background wil be
displayed) . Characters can be blinked on and off by setting name bit 7 to
1 and toggling CHACTL bit 0. Inverse video and blank apply only to IR
modes 2 and 3. If both inverse video and blank are set then the character
will appear as an inverse video blank character (solid square).
Hardware Collision Detection: 60 bits of collision register are
provided to detect and store overlap (hits) between players, missiles and
playfield. These collisions can be read by the microprocessor from
addresses DOOO through DOOF. There are no bits for missile to missile
collisions.
16 bits for Missile to Playfield
16 bits for Player to Playfield
16 bits for Missile to Player
12 bits for Player to Player (PO to PO always reads as zero, etc.)
The 1/2 clock memory map mode (IR code 1111) and the 1/2 clock Character
mode (IR codes 0011 and 0010) are both playfield type 2 collisions and will
be stored irr bit 2 of the playfield collision registers.
II.15

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)