Atari 400 Hardware Manual

OS
Mode 0 Display
List
(40 chars x 24 l ines )
Address
(hex) Data (hex)
7C20 70
)
70 24 blank lines
70
42
)
reload memory scan
counter with 7C40,
40 IR mode 2
7C
2
2
2
23 more IR mode 2
instructions
2
2
2
41
)
~~
back to
7C20 and
20
wait for end of vertical
blank.
7C
7C40
)
960 bytes of display
data
(character names)
Cycle Counting: As explained previously, the ATARI 800 6502 micropro-
cessor runs at a rate of 114 machine cycles per TV line (1.79 MHZ). There
are 262 lines per TV frame and 60 frames per second on the NTSC (US) system.
(The PAL (European) system is different. See the section on NTSC vs. PAL.)
Each machine cycle is equivalent in length to 2 color clocks. There
are 228 color clocks on a TV line. The highest resolution graphics modes
have a pixel size of 1/2 color clock by 1 TV line. Horizontal blank takes
40 machine cycles. This is when the beam returns to the left edge of the
screen in preparation for displaying the next TV line. A wait for Sync
(WSYNC) instruction stops the 6502. The processor is restarted exactly 7
machine cycles before the beginning of the next TV line. The program can
thus change graphics or colors during horizontal blank in preparation for
the next line.
The ANTIC chip steals cycles from the 6502 in order to do memory
refresh and fetch graphics data when needed. The general rule to remember
is that each byte fetched from memory requires one machine cycle. If a
display list memory map instruction extends over several lines then the data
is only fetched on the first line. Memory refresh takes 9 cycles out of
every line, unless pre-empted by a high-resolution graphics mode. Memory
refresh continues during vertical blank.
Missile DMA takes one cycle per line in the one-line resolution mode
and one cycle every other line in the two line resolution mode. Missile
DMA can be enabled without doing player DMA. However, if player DMA is
enabled then missile DMA will also be done (see DMACTL, GRACTL bits).
Player DMA requires 4 cycles every one or two lines, depending on the
resolution used.
11.20

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)