THE SCOPE OF THIS DOCUMENT is limited to a description
of the ST computer system hardware characteristics and
idiosyncrasies. This document does not provide a detailed
description of ST component parts, peripheral devices, dev-
ice protocols, or software. References are provided for
detailed inquiry.
_1. _S_y_s_t_e_m _A_r_c_h_i_t_e_c_t_u_r_e
---------- ----------
|Graphics | |Music |
|Subsystem | |Subsystem |
---------- ----------
| |
| ---------- |
--------| Main |--------
| System |
----------
|
| ----------
The ST --------|Device |
|Subsystems|
----------
The hardware architecture of the Atari Corporation ST
(Sixteen/Thirty-two) computer system consists of a main sys-
tem, a graphics subsystem, a music subsystem, and several
device subsystems (most of the device subsystems require ST
resident intelligence). The ST is based on the MC68000 16
bit data/24 bit address microprocessor unit capable of
directly accessing up to 16 Mbytes of ROM and RAM memory.
Hardware features of the ST computer system include:
Main System
o 16 bit data/24 bit address 8 MHz microprocessor unit
o 192 Kbyte ROM, cartridge expandable to 320 Kbyte
o 512 Kbyte RAM or 1 Mbyte RAM
o direct memory access support
Graphics Subsystem
o 32 Kbyte BitMap video display memory (from above)
o 320 x 200 pixel, 16 color palette from 512 selections
o 640 x 200 pixel, 4 color palette from 512 selections
o 640 x 400 pixel, monochrome
Music Subsystem
o programmable sound synthesizer
o musical instrument network communication
Atari Corp Confidential
7 January 1986 2
Device Subsystems
o intelligent keyboard
o two button mouse
o RGB color and monochrome monitor interfaces
o printer parallel interface
o RS232 serial interface
o MIDI musical instrument interface
o on board floppy disk controller and DMA interface
o hard disk drive DMA interface
The following is a simplified hardware system block
diagram of the Atari ST:
Atari Corp Confidential
7 January 1986 3
----- Atari ST System Block Diagram ---------------
The main system includes the microprocessor unit, main
memory (both ROM and RAM), and general purpose DMA con-
troller. The main system is limited to memory expansion
only and should not be considered an open system (except via
the high speed device interfaces).
_2._1. _M_i_c_r_o_p_r_o_c_e_s_s_o_r _U_n_i_t
The ST computer system is based on an 8 MHz MC68000 16
bit data/24 bit address microprocessor unit (with an inter-
nal 32 bit architecture). Some features of the MC68000 are:
eight 32 bit data registers, nine 32 bit address registers,
a 16 Mbyte direct addressing range, 14 addressing modes,
memory mapped I/O, five data types, and a 56 instruction
set. The MPU is directly supported by an MK68901 Multi
Function Peripheral providing general purpose interrupt con-
trol and timers, among other things.
_2._2. _M_e_m_o_r_y _C_o_n_f_i_g_u_r_a_t_i_o_n
The configuration of main memory consists of five 64
Kbyte sets of ROM (standard set0 to set2, expansion set3 and
set4) and one configurable bank (standard bank0) of 128
Kbyte, 512 Kbyte, or 2 Mbyte RAM. The configuration of main
memory ROM is ascertained through software identification.
The configuration of main memory RAM is achieved via the
programming of the Memory Configuration Register
(read/write, reset: all zeros). RAM configuration must be
asserted during the first steps of the power up sequence and
can be determined by using the following shadow test algo-
rithm:
Atari Corp Confidential
7 January 1986 5
START o write 0x000a (2 Mbyte, 2 Mbyte) to the Memory
Configuration Register.
BANK0 o write Pattern to 0x000000 - 0x0001ff.
o read Pattern from 0x000200 - 0x0003ff.
o if Match, then Bank0 contains 128 Kbyte; goto BANK1.
o read Pattern from 0x000400 - 0x0005ff.
o if Match, then Bank0 contains 512 Kbyte; goto BANK1.
o read Pattern from 0x000000 - 0x0001ff.
o if Match, then Bank0 contains 2 Mbyte; goto BANK1.
o panic: RAM error in Bank0.
BANK1 o write Pattern to 0x200000 - 0x2001ff.
o read Pattern from 0x200200 - 0x2003ff.
o if Match, then Bank1 contains 128 Kbyte; goto FIN.
o read Pattern from 0x200400 - 0x2005ff.
o if Match, then Bank1 contains 512 Kbyte; goto FIN.
o read Pattern from 0x200000 - 0x2001ff.
o if Match, then Bank1 contains 2 Mbyte; goto FIN.
o note: Bank1 nonexistent.
FIN o write Configuration to the Memory Configuration
Register.
o note Total Memory Size (Top of RAM) for future
reference.
RAM memory access cycles are interleaved between the
microprocessor unit and the video controller, thus allowing
video display memory to reside efficiently as part of main
memory (the MPU still maintains full memory bandwidth).
_2._3. _D_i_r_e_c_t _M_e_m_o_r_y _A_c_c_e_s_s
A direct main memory RAM access channel is shared to
provide support for both low speed (250 to 500 Kbits/sec)
and high speed (up to 12 Mbits/sec) 8 bit device controll-
ers. The base address for the DMA read or write operation
is loaded into the DMA Base Address and Counter Register
(read/write, reset: all zeros). Since only one counter
register and channel is provided, only one DMA operation can
be executed at a time.
The actual DMA operation is performed through a 32 byte
FIFO programmed via the DMA Mode Control Register (word
access write only, reset: not affected) and DMA Sector Count
Register (word access write only, reset: all zeros). The
progress, success, or failure of a DMA operation is reported
through the DMA Status Register (word access read only,
reset: one) which is cleared by toggling Write/_Read in the
DMA Mode Control Register.
Atari Corp Confidential
7 January 1986 6
Bus accesses are granted to the DMA controller and
MC68000 MPU on an egalitarian first come, first served
basis. The access remains in effect until an operation is
complete or until control is otherwise relinquished.
The basic components of the graphics subsystem are
video display memory and the video controller. The defini-
tions of BitMap, BitBlk, BitBlt, coordinate system, and
color model are left to higher logical (software) levels of
system specification.
_3._1. _V_i_d_e_o _D_i_s_p_l_a_y _M_e_m_o_r_y
Video display memory is configured as n logical planes
interwoven by 16 bit words into contiguous memory to form
one 32 Kbyte (actually 0x7d00) physical plane starting at
any 256 byte half page boundary (in RAM only). The starting
address of display memory is placed in the Video Base
Address Register (read/write, reset: all zeros) which is
then loaded into the Video Address Counter Register (read
only, reset: all zeros) and incremented. The following is a
diagram of possible physical configurations of video display
memory:
Display memory resides as part of main memory and has
an identical bit, byte, and word arrangement with the physi-
cal screen origin located at top left (bit 15):
high low
-------- --------
word 0 | byte 0 | byte 1 |
-------- --------
fedcba98 76543210
_3._2. _V_i_d_e_o _C_o_n_f_i_g_u_r_a_t_i_o_n
The ST possesses three modes of video configuration:
320 x 200 resolution with 4 planes, 640 x 200 resolution
with 2 planes, and 640 x 400 resolution with 1 plane. The
modes are set through the Shift Mode Register (read/write,
reset: all zeros). A sixteen word color lookup palette is
provided with nine bits of color per entry. The sixteen
Color Palette Registers (read/write, reset: not affected)
contain three bits of red, green, and blue aligned on low
nibble boundaries. Eight intensity levels of red, eight
intensity levels of green, and eight intensity levels of
blue produce a total of 512 possible colors.
In 320 x 200 4 plane mode all sixteen palette colors
can be indexed, while in 640 x 200 2 plane mode only the
first four palette entries are applicable. In 640 x 400
monochrome mode the color palette is bypassed altogether and
is instead provided with an inverter for inverse video con-
trolled by bit 0 of palette color 0 (normal video is black
0, white 1). Color palette memory is arranged the same as
main memory. Palette color 0 is also used to assign a
border color while in a multi-plane mode. In monochrome
mode the border color is always black.
The general flow of the video controller is as follows:
BitMap planes are taken a word at a time from video display
memory and placed in the video shift register where one bit
from each plane is shifted out and collectively used as an
index (plane 0 is the least significant bit) to a color
lookup palette entry which is supplied to 3 bit digital to
analog converters to produce RGB output. The following is a
block diagram of the video controller:
Atari Corp Confidential
7 January 1986 9
----- Video Controller Block Diagram ---------------
The ST music subsystem is composed of a programmable
sound synthesizer and a musical instrument serial interface.
The interface provides high speed serial communication of
musical data to and from more sophisticated synthesizer dev-
ice subsystems.
_4._1. _S_o_u_n_d _S_y_n_t_h_e_s_i_z_e_r
The YM-2149 Programmable Sound Generator produces music
synthesis, sound effects, and audio feedback (eg alarms and
key clicks). With an applied clock input of 2 MHz, the PSG
is capable of providing a frequency response range between
30 Hz (audible) and 125 KHz (post-audible). The generator
places a minimal amount of processing burden on the main
system (which acts as the sequencer) and has the ability to
perform using three independent voice channels. The three
sound channel outputs are mixed, along with Audio In, and
sent to an external television or monitor speaker (the PSG
has built in digital to analog converters).
The sound generator's internal registers are accessed
via the PSG Register Select Register (write only, reset:
registers all zeros). The tone generator registers control
a basic square wave while the noise generator register con-
trols a frequency modulated square wave of pseudo random
pulse width. Tones and noise can be mixed over individual
channels by using the mixer control register. The amplitude
registers allow the specification of a fixed amplitude or of
a variable amplitude when used with the envelope generator.
The envelope generator registers permit the entry of a
skewed attack-decay-sustain-release envelope in the form of
a continue-attack-alternate-hold envelope.
The Musical Instrument Digital Interface (MIDI) allows
the integration of the ST with music synthesizers, sequenc-
ers, drum boxes, and other devices possessing MIDI inter-
faces. High speed (31.25 Kbaud) serial communication of
keyboard and program information is provided by two ports,
MIDI OUT and MIDI IN (MIDI OUT also supports the optional
MIDI THRU port).
The MIDI bus permits up to 16 channels in one of three
network addressing modes: Omni (all units addressed simul-
taneously, power up default), Poly (each unit addressed
separately), and Mono (each unit voice addressed
separately). Information is communicated via five types of
data format (data bytes, most significant bit: status 1,
data 0) which are prioritized from highest to lowest as:
System Reset (default conditions, should not be sent on
power up to avoid deadlock), System Exclusive (manufacturer
unique data: Sequential Circuits, Kawai, Roland, Korg,
Yamaha), System Real Time (synchronization), System Common
(broadcast), and Channel (note selections, program data,
etc).
The ST supports seven device subsystems: an intelli-
gent keyboard, video interface, parallel interface, RS232
interface, MIDI interface, floppy disk interface, and hard
disk interface. Included with each device interface
description is a port pin assignment chart with the ST and
programmable signals justified left (pins that are not con-
nected are not shown). The connector type on the ST is
shown above each pin list with an S designating a female
socket and a P designating a male plug.
_5._1. _I_n_t_e_l_l_i_g_e_n_t _K_e_y_b_o_a_r_d
The Atari Intelligent Keyboard (ikbd) transmits encoded
make/break key scan codes (with two key rollover),
mouse/trackball data, joystick data, and time of day. The
ikbd receives commands as well, with bidirectional communi-
cation controlled on the ST side by an MC6850 Asynchronous
Communications Interface Adapter supplied with transmit and
receive clock inputs of 500 KHz. The data transfer rate is
a constant 7812.5 bits/sec which can be generated by setting
the ACIA Counter Divide Select to divide by 64. All ikbd
functions such as key scanning, mouse tracking, command
parsing, etc. are performed by a 1 MHz HD6301V1 8 bit Micro-
computer Unit.
The ikbd is equipped with a combination mouse/joystick
port and a joystick only port. The Atari Two Button Mouse
is a mechanical, opto-mechanical, or optical mouse with the
following minimal performance characteristics: a resolution
of 100 counts/inch (4 counts/mm), a maximum velocity of 10
inches/second (250 mm/second), and a maximum pulse phase
error of 50 percent. The Atari Joystick is a 4 direction
Atari Corp Confidential
7 January 1986 13
switch-type joystick with one fire button. The ikbd can
report movement using one of three mouse/joystick modes:
mouse with joystick, disabled mouse with joystick, and joys-
tick with joystick.
----- Mouse/Joystick0 Port Pin Assignments ---------------
ST DB 9P
---- ----
IKBD MATRIX 1 |<--- XB Pulse / Up Switch --------|
IKBD MATRIX 2 |<--- XA Pulse / Down Switch ------|
IKBD MATRIX 3 |<--- YA Pulse / Left Switch ------|
IKBD MATRIX 4 |<--- YB Pulse / Right Switch -----|
IKBD MCU 6 |<--- Left Button / Fire Button ---|
7 |---- Power ---------------------->|
8 |---- Ground ----------------------|
IKBD MCU 9 |<--- Right Button / Joy1 Fire ----|
---- ----
----- Joystick1 Port Pin Assignments ---------------
ST DB 9P
---- ----
IKBD MATRIX 1 |<--- Up Switch -------------------|
IKBD MATRIX 2 |<--- Down Switch -----------------|
IKBD MATRIX 3 |<--- Left Switch -----------------|
IKBD MATRIX 4 |<--- Right Switch ----------------|
IKBD MCU 6 |<--- Fire Button -----------------|
7 |---- Power ---------------------->|
8 |---- Ground ----------------------|
---- ----
Signal Characteristics
pins 1-4,6 TTL levels.
pin 7 +5 VDC.
_5._2. _V_i_d_e_o _I_n_t_e_r_f_a_c_e
The ST video display interface supports low resolution
(320 x 200 or 640 x 200) television receivers and composite
monitors, low resolution (320 x 200 or 640 x 200) RGB moni-
tors, and medium resolution (640 x 400) monochrome monitors.
Both NTSC and PAL color encoding standards are available in
two separate configurations of the video interface hardware.
For United States television receivers, the modulated RF
display signal is provided on two adjacent broadcast chan-
nels: channel 2 (55.25 MHz) and channel 3 (61.25 MHz). In
order to protect against monitor damage while in medium
resolution monochrome mode, the Monochrome Monitor Detect
signal is provided as an interrupt on the MK68901 Multi
Function Peripheral and can be tested to determine the pres-
ence of a monochrome monitor (active low, any change will
generate the interrupt if enabled).
External/Internal syncs and 50/60 Hz field rates are
selected via the Sync Mode Register (read/write, reset: all
zeros). The YM-2149 Programmable Sound Generator I/O Port A
provides a General Purpose Output for use in the control of
functions such as the remote selection of monitor external
or internal sync. Two autovector interrupts are generated
to allow software synchronization with horizontal and verti-
cal blanking intervals. The Horizontal Blanking (down)
Counter MFP Timer B has an active high input signal and pro-
duces an interrupt when the counter times out (Event Count
Mode). The horizontal blanking counter actually uses
display enable, the first of which occurs at the end of the
Atari Corp Confidential
7 January 1986 15
first display line. The video display shift mode should be
selected during the vertical blanking interval.
----- Video Port Pin Assignments ---------------
Television
ST RCA Pin Jack
---- ----
core |---- RF Modulated Video --------->|
shield |---- Ground ----------------------|
---- ----
Monitor
ST Circular DIN 13S
---- ----
1 |---- Audio Out ------------------>|
2 |---- Composite Video ------------>|
PSG I/O A 3 |---- General Purpose Output ----->|
MFP 4 |<--- Monochrome Detect -----------|
5 |<--- Audio In --------------------|
6 |---- Green ---------------------->|
7 |---- Red ------------------------>|
8 |---- Peritel Power -------------->|
9 |---- Horizontal Sync ------------>|
10 |---- Blue ----------------------->|
11 |---- Monochrome ----------------->|
12 |---- Vertical Sync -------------->|
13 |---- Ground ----------------------|
---- ----
Signal Characteristics
pin 1 1 VDC peak to peak, 10 Kohm.
pin 2 1 VDC peak to peak, 75 ohm.
pin 3 TTL levels, assertion not assigned.
pin 4 TTL levels, active low,
1 Kohm pullup resistor to +5 VDC.
pin 5 1 VDC peak to peak, 10 Kohm.
pins 6-7 1 VDC peak to peak, 75 ohm.
pin 8 +12 VDC pullup.
pin 9 5 VDC active low, 3.3 Kohm.
pin 10 1 VDC peak to peak, 75 ohm.
pin 11 1 VDC peak to peak, 75 ohm.
pin 12 5 VDC active low, 3.3 Kohm.
Atari Corp Confidential
7 January 1986 16
_5._3. _P_a_r_a_l_l_e_l _I_n_t_e_r_f_a_c_e
The ST parallel interface supports Centronics STROBE
from the YM-2149 PSG for data synchronization and Centronics
BUSY to the MK68901 MFP (ACKNLG is not supported) for
handshaking. Eight bits of read/write data are handled
through I/O Port B on the PSG at a typical data transfer
rate of 4000 bytes/second.
----- Parallel Port Pin Assignments ---------------
ST DB 25S
---- ----
PSG I/O A 1 |---- Centronics STROBE ---------->|
PSG I/O B 2 |<--- Data 0 --------------------->|
PSG I/O B 3 |<--- Data 1 --------------------->|
PSG I/O B 4 |<--- Data 2 --------------------->|
PSG I/O B 5 |<--- Data 3 --------------------->|
PSG I/O B 6 |<--- Data 4 --------------------->|
PSG I/O B 7 |<--- Data 5 --------------------->|
PSG I/O B 8 |<--- Data 6 --------------------->|
PSG I/O B 9 |<--- Data 7 --------------------->|
MFP 11 |<--- Centronics BUSY -------------|
18-25 |---- Ground ----------------------|
---- ----
Signal Characteristics
pin 1 TTL levels, active low.
pins 2-9 TTL levels.
pin 11 TTL levels, active high,
1 Kohm pullup resistor to +5 VDC.
_5._4. _R_S_2_3_2 _I_n_t_e_r_f_a_c_e
The ST RS232 interface provides voltage level synchro-
nous or asynchronous serial communication. Five EIA RS232C
handshake control signals are supported: Request To Send
and Data Terminal Ready are transmitted through the YM-2149
PSG I/O Port A while Clear To Send, Data Carrier Detect, and
Ring Indicator are received through the MK68901 MFP. The
MFP USART transmit and receive clock inputs are controlled
by the Baud Rate Generator MFP Timer D which is supplied
with 2.4576 MHz and can support asynchronous data transfer
rates from 50 to 19200 baud. One byte transmit and receive
data buffers are managed by the MFP USART, which provides
monitoring of buffer conditions and communication errors.
Atari Corp Confidential
7 January 1986 17
----- RS232 Port Pin Assignments ---------------
ST DB 25P
---- ----
1 |---- Protective Ground -----------|
MFP 2 |---- Transmitted Data ----------->|
MFP 3 |<--- Received Data ---------------|
PSG I/O A 4 |---- Request To Send ------------>|
MFP 5 |<--- Clear To Send ---------------|
7 |---- Signal Ground ---------------|
MFP 8 |<--- Data Carrier Detect ---------|
PSG I/O A 20 |---- Data Terminal Ready -------->|
MFP 22 |<--- Ring Indicator --------------|
---- ----
The ST MIDI interface provides current loop asynchro-
nous serial communication controlled by an MC6850 ACIA sup-
plied with transmit and receive clock inputs of 500 KHz.
The data transfer rate is a constant 31.25 Kbaud which can
be generated by setting the ACIA Counter Divide Select to
divide by 16. The MIDI specification calls for serial data
to consist of eight data bits preceded by a start bit and
followed by one stop bit.
Atari Corp Confidential
7 January 1986 18
----- MIDI Port Pin Assignments ---------------
MIDI OUT/THRU
ST Circular DIN 5S
---- ----
MIDI IN 1 |---- THRU Transmit Data --------->|
2 |---- Shield Ground ---------------|
3 |<--- THRU Loop Return ------------|
MIDI ACIA 4 |---- OUT Transmit Data ---------->|
5 |<--- OUT Loop Return -------------|
---- ----
MIDI IN
ST Circular DIN 5S
---- ----
MIDI ACIA 4 |<--- IN Receive Data -------------|
5 |---- IN Loop Return ------------->|
---- ----
Signal Characteristics
current loop 5 ma, zero is current on.
_5._6. _D_i_s_k _D_r_i_v_e _I_n_t_e_r_f_a_c_e
The ST floppy disk drive interface is provided through
the DMA controller to an on board WD1772 Floppy Disk Con-
troller. A total of two daisy chained floppy disk drives
(drive 0 or 1) can be supported. Commands are sent to the
FDC by first writing to the DMA Mode Control Register to
select the FDC internal command register and then writing
the desired one byte command to the Disk Controller Regis-
ter. The entire floppy disk DMA read or write sequence is
as follows:
o select floppy drive 0 or 1 (PSG I/O Port A).
o select floppy side 0 or 1 (PSG I/O Port A).
o load DMA Base Address and Counter Register.
o toggle Write/_Read to clear status (DMA Mode Control Register).
o select DMA read or write (DMA Mode Control Register).
o select DMA Sector Count Register (DMA Mode Control Register).
o load DMA Sector Count Register (DMA operation trigger).
o select FDC internal command register (DMA Mode Control Register).
o issue FDC read or write command (Disk Controller Register).
o DMA active until sector count is zero (DMA Status Register,
do not poll during DMA active).
o issue FDC force interrupt command on multi-sector transfers
Atari Corp Confidential
7 January 1986 19
except at track boundaries (Disk Controller Register).
o check DMA error status (DMA Status Register, nondestructive).
The detection of floppy disk removal is not supported
in hardware.
The ST hard disk drive interface is also provided
through the DMA controller, however the Atari Hard Disk Con-
troller is off board and is sent commands using an ANSI
X3T9.2 SCSI-like (Small Computer Systems Interface) command
descriptor block protocol. The Atari Hard Disk Interface
(AHDI) supports a minimal subset of SCSI commands (Class 0
OpCodes), which are dispatched using the following fixed six
byte Atari Computer System Interface (ACSI) command packet
format:
Commands are issued to the Atari HDC in a manner simi-
lar to that of the FDC, with the major difference being the
handshaking of a multi-byte command descriptor block. The
entire hard disk DMA read or write sequence is as follows:
o load DMA Base Address and Counter Register.
o toggle Write/_Read to clear status (DMA Mode Control Register).
o select DMA read or write (DMA Mode Control Register).
o select DMA Sector Count Register (DMA Mode Control Register).
o load DMA Sector Count Register (DMA operation trigger).
o select HDC internal command register (DMA Mode Control Register).
o issue controller select byte while clearing A0.
o set A0 for remaining command bytes.
o after last command byte select controller (DMA Mode Control
Register).
o DMA active until sector count is zero (DMA Status Register,
do not poll during DMA active).
o check DMA error status (DMA Status Register, nondestructive).
o check HDC status byte and if necessary perform ECC correction
following a Verify Track or Read Sector command.
The format of both floppy and hard disks contain 512
byte data sectors.
Atari Corp Confidential
7 January 1986 21
----- Floppy Disk Port Pin Assignments ---------------
ST Circular DIN 14S
---- ----
1 |<--- Read Data -------------------|
PSG I/O A 2 |---- Side0 Select --------------->|
3 |---- Logic Ground ----------------|
4 |<--- Index Pulse -----------------|
PSG I/O A 5 |---- Drive0 Select -------------->|
PSG I/O A 6 |---- Drive1 Select -------------->|
7 |---- Logic Ground ----------------|
8 |---- Motor On ------------------->|
9 |---- Direction In --------------->|
10 |---- Step ----------------------->|
11 |---- Write Data ----------------->|
12 |---- Write Gate ----------------->|
13 |<--- Track 00 --------------------|
14 |<--- Write Protect ---------------|
---- ----
NOTE: shield ground must not be connected on the ST side,
the cable must have Read Data and Write Data paired
with Logic Grounds.
Signal Characteristics
pin 1 TTL levels, active low,
1 Kohm pullup resistor to +5 VDC.
pin 2 TTL levels, active high,
high at system reset.
pin 4 TTL levels, active low,
1 Kohm pullup resistor to +5 VDC.
pins 5-6 TTL levels, active low,
high at system reset.
pins 8-12 TTL levels, active low (inverted).
pins 13-14 TTL levels, active low,
1 Kohm pullup resistors to +5 VDC.
Atari Corp Confidential
7 January 1986 22
----- Hard Disk Port Pin Assignments ---------------
ST DB 19S
---- ----
1 |<--- Data 0 --------------------->|
2 |<--- Data 1 --------------------->|
3 |<--- Data 2 --------------------->|
4 |<--- Data 3 --------------------->|
5 |<--- Data 4 --------------------->|
6 |<--- Data 5 --------------------->|
7 |<--- Data 6 --------------------->|
8 |<--- Data 7 --------------------->|
9 |---- Chip Select ---------------->|
MFP 10 |<--- Interrupt Request -----------|
11 |---- Ground ----------------------|
12 |---- Reset ---------------------->|
13 |---- Ground ----------------------|
14 |---- Acknowledge ---------------->|
15 |---- Ground ----------------------|
16 |---- A1 ------------------------->|
17 |---- Ground ----------------------|
18 |---- Read/Write ----------------->|
19 |<--- Data Request ----------------|
---- ----
Signal Characteristics
pins 1-8 TTL levels.
pin 9 TTL levels, active low.
pin 10 TTL levels, active low,
1 Kohm pullup resistor to +5 VDC.
pin 12 TTL levels, active low,
system reset.
pin 14 TTL levels, active low.
pins 16,18 TTL levels.
pin 19 TTL levels, active low,
1 Kohm pullup resistor to +5 VDC.
Atari Corp Confidential
7 January 1986 23
_6. _C_o_m_p_o_n_e_n_t_s
The standard configurations of the Atari ST main sys-
tem, graphics subsystem, music subsystem, and device subsys-
tems are made up of the following major hardware components:
Main o 8 MHz MC68000 Microprocessor Unit
o MK68901 Multi Function Peripheral
o 192 Kbyte ROM
o 512 Kbyte or 1 Mbyte RAM
o Memory Controller Chip
o Control Logic Chip
o DMA Controller Chip
Graphics o 32 Kbyte Display Memory (from main RAM)
o Video Shift Register Chip
Music o YM-2149 Programmable Sound Generator
Device o Atari Intelligent Keyboard (ikbd)
1 MHz HD6301V1 Microcomputer Unit
o Atari Two Button Mouse
o 2 MC6850 Asynchronous Communications
Interface Adapters
o WD1772 Floppy Disk Controller
Atari Corp Confidential
7 January 1986 24
_7. _E_x_p_a_n_s_i_o_n
The Atari ST can be expanded by cartridge ROM only
which can contain a maximum of 128 Kbyte. The disclosure of
cartridge presence or memory size is not supported in
hardware. The ROM cartridge slot has the following pin
assignments (only the lower 15 address lines are available):
----- ROM Port Pin Assignments ---------------
ST 40S
---- ----
1 |---- Power +5 VDC --------------->|
2 |---- Power +5 VDC --------------->|
3 |<--- Data 14 ---------------------|
4 |<--- Data 15 ---------------------|
5 |<--- Data 12 ---------------------|
6 |<--- Data 13 ---------------------|
7 |<--- Data 10 ---------------------|
8 |<--- Data 11 ---------------------|
9 |<--- Data 8 ----------------------|
10 |<--- Data 9 ----------------------|
11 |<--- Data 6 ----------------------|
12 |<--- Data 7 ----------------------|
13 |<--- Data 4 ----------------------|
14 |<--- Data 5 ----------------------|
15 |<--- Data 2 ----------------------|
16 |<--- Data 3 ----------------------|
17 |<--- Data 0 ----------------------|
18 |<--- Data 1 ----------------------|
19 |---- Address 13 ----------------->|
20 |---- Address 15 ----------------->|
21 |---- Address 8 ------------------>|
22 |---- Address 14 ----------------->|
23 |---- Address 7 ------------------>|
24 |---- Address 9 ------------------>|
25 |---- Address 6 ------------------>|
26 |---- Address 10 ----------------->|
27 |---- Address 5 ------------------>|
28 |---- Address 12 ----------------->|
29 |---- Address 11 ----------------->|
30 |---- Address 4 ------------------>|
31 |---- ROM3 Select ---------------->|
32 |---- Address 3 ------------------>|
33 |---- ROM4 Select ---------------->|
34 |---- Address 2 ------------------>|
35 |---- _Upper Data Strobe --------->|
36 |---- Address 1 ------------------>|
37 |---- _Lower Data Strobe --------->|
38-40 |---- Ground ----------------------|
---- ----
Atari Corp Confidential
7 January 1986 25
_8. _M_e_m_o_r_y _M_a_p
The first 2 Kbyte of ST memory is reserved for the
exception vector table and supervisor stack. This area
along with I/O space is protected for supervisor references
only. Accessing supervisor protected areas while in the
user state will result in a bus error. A 4 word portion of
ROM is shadowed at the start of RAM for the reset stack
pointer and program counter. Writing to this area or any
ROM location will also result in a bus error. The following
is a map of ST memory:
Atari Corp Confidential
7 January 1986 26
----- ST Memory Map ---------------
----------------
00 0000 | ROM | Reset: Supervisor Stack Pointer
00 0004 | ROM | Reset: Program Counter
00 0008 | RAM | 0 Kbyte RAM
----------------
.
----------------
08 0000 | RAM | 512 Kbyte RAM
----------------
.
----------------
10 0000 | RAM | 1 Mbyte RAM
----------------
.
.
.
----------------
fa 0000 | ROM | 320 Kbyte ROM
----------------
.
----------------
fc 0000 | ROM | Reset: Supervisor Stack Pointer
fc 0004 | ROM | Reset: Program Counter
fc 0008 | ROM | 192 Kbyte ROM
fe ffff | ROM | 0 Kbyte ROM
----------------
.
.
.
----------------
ff 8000 | I/O | Configuration Registers
ff 8200 | I/O | Display Registers
ff 8400 | I/O | Reserved
ff 8600 | I/O | DMA/Disk Registers
ff 8800 | I/O | Sound Registers
----------------
.
----------------
ff fa00 | I/O | MC68xxx Registers
ff fc00 | I/O | MC68xx Registers
----------------
Atari Corp Confidential
7 January 1986 27
_9. _I/_O _M_a_p
The ST I/O space ranges from ff 0000 to ff ffff, with
MC68000 and MC6800 peripheral internal registers starting at
ff fa00 and ff fc00 respectively. Accessing reserved I/O
addresses will result in a bus error. Bit values for vari-
ous read and/or write registers are labeled as active
One/_Zero (always mask out unused field bits). The follow-
ing is a map of ST I/O space:
ff 8604 R/W |--------xxxxxxxx| Disk Controller (Word Access)
ff 8606 R |-------------xxx| DMA Status (Word Access)
|||
|| ---- _Error Status
| ----- _Sector Count Zero Status
------ _Data Request Inactive Status
ff 8606 W |-------xxxxxxxx-| DMA Mode Control (Word Access)
||||||||
||||||| ----- A0
|||||| ------ A1
||||| ------- HDC/_FDC Register Select
|||| -------- Sector Count Register Select
|||0 Reserved
|| ---------- Disable/_Enable DMA
| ----------- FDC/_HDC
------------ Write/_Read
ff 8609 R/W |xxxxxxxx| DMA Base and Counter High
ff 860b R/W |xxxxxxxx| DMA Base and Counter Mid
Atari Corp Confidential
7 January 1986 29
ff 860d R/W |xxxxxxxx| DMA Base and Counter Low
_____
Sound
ff 8800 R |xxxxxxxx| PSG Read Data
|||||||| I/O Port B
-------------- Parallel Interface Data
ff 8800 W |xxxxxxxx| PSG Register Select
||||
------- Register Number
0000 Channel A Fine Tune
0001 Channel A Coarse Tune
0010 Channel B Fine Tune
0011 Channel B Coarse Tune
0100 Channel C Fine Tune
0101 Channel C Coarse Tune
0110 Noise Generator Control
0111 Mixer Control - I/O Enable
1000 Channel A Amplitude
1001 Channel B Amplitude
1010 Channel C Amplitude
1011 Envelope Period Fine Tune
1100 Envelope Period Coarse Tune
1101 I/O Port A (Output Only)
1111 I/O Port B
ff 8802 W |xxxxxxxx| PSG Write Data
|||||||| I/O Port A
||||||| ------- Floppy Side0/_Side1 Select
|||||| -------- Floppy _Drive0 Select
||||| --------- Floppy _Drive1 Select
|||| ---------- RS232 Request To Send
||| ----------- RS232 Data Terminal Ready
|| ------------ Centronics _STROBE
| ------------- General Purpose Output
-------------- Reserved
|||||||| I/O Port B
-------------- Parallel Interface Data
_______
MC68xxx
ff fa01 |xxxxxxxx| MFP General Purpose I/O
ff fa03 |xxxxxxxx| MFP Active Edge
ff fa05 |xxxxxxxx| MFP Data Direction
ff fa07 |xxxxxxxx| MFP Interrupt Enable A
ff fa09 |xxxxxxxx| MFP Interrupt Enable B
ff fa0b |xxxxxxxx| MFP Interrupt Pending A
ff fa0d |xxxxxxxx| MFP Interrupt Pending B
ff fa0f |xxxxxxxx| MFP Interrupt In-Service A
ff fa11 |xxxxxxxx| MFP Interrupt In-Service B
ff fa13 |xxxxxxxx| MFP Interrupt Mask A
Atari Corp Confidential
7 January 1986 30
ff fa15 |xxxxxxxx| MFP Interrupt Mask B
ff fa17 |xxxxxxxx| MFP Vector
ff fa19 |xxxxxxxx| MFP Timer A Control
ff fa1b |xxxxxxxx| MFP Timer B Control
ff fa1d |xxxxxxxx| MFP Timers C and D Control
ff fa1f |xxxxxxxx| MFP Timer A Data
ff fa21 |xxxxxxxx| MFP Timer B Data
ff fa23 |xxxxxxxx| MFP Timer C Data
ff fa25 |xxxxxxxx| MFP Timer D Data
ff fa27 |xxxxxxxx| MFP Sync Character
ff fa29 |xxxxxxxx| MFP USART Control
ff fa2b |xxxxxxxx| MFP Receiver Status
ff fa2d |xxxxxxxx| MFP Transmitter Status
ff fa2f |xxxxxxxx| MFP USART Data
______
MC68xx
ff fc00 |xxxxxxxx| Keyboard ACIA Control
ff fc02 |xxxxxxxx| Keyboard ACIA Data
ff fc04 |xxxxxxxx| MIDI ACIA Control
ff fc06 |xxxxxxxx| MIDI ACIA Data
Atari Corp Confidential
7 January 1986 31
_1_0. _I_n_t_e_r_r_u_p_t _T_a_b_l_e
The following tables list the ST interrupt and signal
priority assignments:
NOTE: the MC6850 ACIA Interrupt Request status bit must be tested
to differentiate between keyboard and MIDI interrupts.
Atari Corp Confidential
7 January 1986 32
_1_1. _C_a_s_e _D_e_s_i_g_n
The ST is primarily designed as a keyboard computer
with external video display, disk drives, and power supply.
The top panel contains the keyboard, ventilation slots (bot-
tom also), and power on LED. The side panels contain the
expansion ROM slot and mouse/joystick ports. The back panel
contains the on/off switch, reset button, channel select
switch, power connector, and all remaining device connec-
tors: video, parallel, RS232, MIDI, and disk drive inter-
faces.
The device ports are labeled with International Elec-
trotechnical Commission-like symbols in combinations of pic-
tograms and phonetic transcriptions (captions). In the case
of MIDI, the specification explicitly requires that the MIDI
connectors be labeled MIDI OUT, MIDI IN, and MIDI THRU (if
applicable).
Atari Corp Confidential
7 January 1986 33
_1_2. _P_o_w_e_r _S_u_p_p_l_y
An external DC power supply provides power to the main
system board, keyboard, expansion ROM, and expansion RAM.
All power levels are regulated for over-voltage and over-
current protection. The following are minimal power supply
specifications:
AC Input 115 VAC 10% at 60 Hz (fused)
DC Output +5 VDC at 3 A 5%
+12 VDC at .03 A 10%
-12 VDC at .03 A 10%
The surface of the Atari Intelligent Keyboard is formed
from four distinct ergonomic units: a QWERTY typewriter
matrix, a function key array, a screen control cluster, and
a calculator numeric keypad. As a whole, the United States
version of the keyboard layout is DEC VT100-like with the
exception of the following items:
o repositioned keys -- [caps lock], cursor control keys,
programmable function keys.
o added keys -- [alternate], [help], [undo], [insert], [clear/home],
10 programmable function keys.
o improvements -- DEC VT200-style inverted T cursor control
keys, full calculator function numeric keypad.
Please note that the left [shift] key is intentionally
large to allow space for an extra ISO required key (see ISO
2530-1975). Atari ikbd International Layouts will be
designed for the following countries:
o United Kingdom
o United States
o Finland
o Norway / Denmark
o Sweden
o Japan
o Germany
o French Canada
o France
o Italy
o Spain
NOTE: the 'HELP' and 'UNDO' keys will be left untranslated.
Mostek MK68901 Multi Function Peripheral Data Sheet
Graphics Subsystem
Adele Goldberg and David Robson, 'Smalltalk-80: The
Language and Its Implementation', Addison-Wesley, Read-
ing Massachusetts, 1983, Chapter 18.
Music Subsystem
General Instrument AY-3-8910 Programmable Sound Genera-
tor Data Sheet
MIDI Musical Instrument Digital Interface Specification
1.0
Device Subsystems
Atari Intelligent Keyboard (ikbd) Protocol and Specifi-
cation
Motorola MC6850 Asynchronous Communications Interface
Adapter Data Sheet
Centronics Parallel Interface Specification
Electronic Industries Association RS232C Standard
Western Digital WD1770/1772 Floppy Disk Controller Data
Sheet
Specification of the Atari Computer System Interface
(ACSI)
Specification of the Atari Hard Disk Interface (AHDI)
Atari Corp Confidential
7 January 1986 36
_A_p_p_e_n_d_i_x _C -- _N_o_t_e_s
General
851125A An address error occurs when a word instruction is
used on a byte address.
Main System
841017B The DMA Base Address and Counter Register must be
loaded in low, mid, high order.
Graphics Subsystem
841017C None.
Music Subsystem
841017D The YM-2149 PSG I/O space and registers should be
set up as critical regions in software.
Device Subsystems
841017F Poll or service the Disk Drive Controller inter-
rupt on the MK68901 MFP General Purpose I/O Regis-
ter to detect the completion of a WD1772 FDC com-
mand. Do not poll the FDC Busy or DMA Sector
Count Zero status bits.
841017G Select the Sector Count Register before testing
the DMA Status Register Error bit.
841017H Do not set the 30 ms Settling Delay bit on WD1772
FDC type 2 and 3 command executions.
841017I A force interrupt should be issued after a few
seconds (ie time out) on all commands sent to the
WD1772 FDC.
841017J Wait until the WD1772 FDC Motor On status is low
before deselecting a floppy drive.
841017K A floppy disk drive configuration table should be
maintained in software to accommodate a diverse
selection of 3.5 inch floppy disk drives. Two
floppy disk drives currently under evaluation have
the following characteristics:
o 500 Kbyte unformatted, 80 cylinders, one head,
3 ms stepping rate.
o 1 Mbyte unformatted, 80 cylinders, two heads, 3
ms stepping rate.
Atari Corp Confidential
Table of Contents
1. System Architecture ............................... 1
2. Main System ....................................... 4
2.1. Microprocessor Unit ........................... 4
2.2. Memory Configuration .......................... 4
2.3. Direct Memory Access .......................... 5
3. Graphics Subsystem ................................ 7
3.1. Video Display Memory .......................... 7
3.2. Video Configuration ........................... 8
4. Music Subsystem ................................... 10
4.1. Sound Synthesizer ............................. 10
4.2. Musical Instrument Communication .............. 10
