Atari 400 Hardware Manual

When
Horizontal Blank DMA Timing
DMA is enabled, cycles
are stolen at the times shown below.
End of
rPrevious
Horizontal Blank
.. I
Line
20 machine
cycles
(40 color clocks)
5
411111412
8
~
WSYNC
1-9 refresh cycles.
char. and graphics
data DMA (depends on
graphic mode)
Interrupt
~-------Address DMA (3-byte display list
instruction)
~------------Player
~-----------------Display list instruction fetch DMA
~---------------------Missile DMA
Cycle Counting Example: This example uses the 40 character by 24 line
display list given on page II.24. This display list is 32 bytes long so
display list DMA takes 32 machine cycles. It takes 960 cycles to DMA the
characters and 8*960 to DMA the character data. The refresh DMA takes 9
cycles for each of 262 lines, except for the 24 lines where the characters
are read, where only 1 refresh cycle occurs.
DMA description
display list
characters
character data
refresh
total
Machine
40x24
960x8
262x9-24x8
cycles
32
= 960
=7680
=2166
10838
Thus the total DMA per frame is 10838 machine cycles. One frame
is 262 lines with 114 machine cycles per line for a total of 29868 machine
cycles per frame. Thus 36% of each frame is required for DMA in OS graphics
mode 0.
NTSC vs. PAL Systems: There are two versions of the ATARI 800: the NTSC
(United States T.V. standard) and PAL (one of the European T.V. standards).
The PAL system has been designed so that most programs will run without
being modified. However, some differences may be noticeable. There is a
hardware register (PAL) which a program can read to determine which type of
system it is running on and adjust accordingly.
II.22

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)