Atari Mega ST

Atari Mega ST

Atari Mega ST with detachable keyboard and SM124 monochrome monitor
Specifications
Manufacturer	Atari Corporation
Type	Professional desktop computer
Released	June 1, 1987
Discontinued	1991 (superseded by Mega STE)
Intro price	US$1,999 (Mega ST2); US$2,499 (Mega ST4)
CPU	Motorola MC68000 @ 8.00 MHz
Memory	1 MB (Mega ST1), 2 MB (Mega ST2), or 4 MB (Mega ST4); soldered DRAM
Storage	Internal 3.5-inch DSDD 720 KB floppy drive; external ACSI hard disks (MegaFile 20/30/60 series)
Display	—
Sound	Yamaha YM2149F (PSG, 3-channel mono)
OS / Firmware	Atari TOS 1.02 (shipped); upgradeable to TOS 1.04
Predecessor	Atari 1040STFM
Successor	Atari Mega STE

The Atari Mega ST is a professional desktop computer manufactured by Atari Corporation, released in 1987 as the business-oriented variant of the Atari ST family. Announced at the January 1987 Consumer Electronics Show (CES) in Las Vegas, the Mega ST was first available in Germany in June 1987, followed by the United Kingdom in August 1987, and the United States in October 1987.

Unlike the all-in-one 520ST and 1040ST designs, the Mega ST introduced a two-piece "pizza box" form factor comprising a separate low-profile system unit and a detachable keyboard. This design was intended to position the ST line as a viable competitor in the professional and small business markets, particularly for desktop publishing (DTP) and music production.

The Mega ST was produced in three configurations: the Mega ST1 (1 MB RAM, released spring 1989), Mega ST2 (2 MB), and Mega ST4 (4 MB). The Mega ST2 and Mega ST4 were the initial launch models.

Technical specifications

Processor and architecture

The Mega ST uses the same Motorola 68000 CPU clocked at 8 MHz as the earlier ST models. The 68000 features a 32-bit internal data path with a 16-bit external data bus and a 24-bit address bus, providing a maximum addressable memory space of 16 MB. Instruction execution is handled in a microcoded pipeline with prefetch, delivering performance of approximately 1 MIPS at 8 MHz.

BLiTTER chip

The Mega ST was the first ST model to ship with the Atari BLiTTER (BLock Image TransTER) chip as standard equipment. Designated C025912-38, the BLiTTER is a dedicated graphics co-processor that accelerates block transfer operations, line drawing, and area fills. It operates by performing memory-to-memory copy operations with logic functions (AND, OR, XOR, NOT) between source, destination, and a halftone pattern.

Key BLiTTER operations include:

• Block moves — Used for window redraws, icon rendering, and font display in GEM. Accelerates desktop operations by a factor of approximately 2–5x.
• Line drawing — Hardware-accelerated Bresenham line algorithm.
• Area fills — Pattern-based fills with 16 halftone patterns.

The BLiTTER accesses RAM via DMA, sharing the bus with the CPU. While the BLiTTER is active, the CPU is halted. The BLiTTER is memory-mapped at address $FFFF8A00 and controlled through a set of 26 registers.

Software must explicitly enable and detect the BLiTTER via the _blit system variable at address $0044C. Not all software utilises the BLiTTER; compatibility with non-BLiTTER ST models was typically maintained by checking for its presence at runtime.

Memory

RAM on the Mega ST is implemented with socketed or soldered DRAM chips, depending on the board revision. The Mega ST2 uses 16 × 1 Mbit DRAM chips (organized as 256K × 4), while the Mega ST4 uses 32 such chips. The Mega ST1 uses 8 × 1 Mbit DRAM chips. Memory access is handled by the MMU (Memory Management Unit, Atari C025913) and the GLUE chip (Atari C025914), which together manage bus arbitration, refresh, and address decoding.

The memory map follows the standard ST layout:

Address range	Function
$000000–$3FFFFF	ST RAM (up to 4 MB)
$E00000–$E7FFFF	TOS ROM (192 KB)
$FA0000–$FBFFFF	Cartridge port ROM space (128 KB)
$FF8000–$FFFFFF	I/O hardware registers

Graphics

The video subsystem is identical to the standard ST line, using the Shifter chip (C025914) to generate three display modes:

Mode	Resolution	Colours	Monitor
Low resolution	320 × 200	16 of 512 palette	SC1224 (colour)
Medium resolution	640 × 200	4 of 512 palette	SC1224 (colour)
High resolution	640 × 400	Monochrome (black/white)	SM124 (mono)

Display mode selection is hardware-locked by the monitor type connected — the high-resolution monochrome mode requires the SM124 or SM125 monitor, which uses a separate sync frequency (71.2 Hz). Colour modes output 15 kHz horizontal sync, compatible with the SC1224 or a television via the RF modulator.

Sound

Audio is provided by the Yamaha YM2149F Programmable Sound Generator (PSG), producing three channels of square wave output with frequency range of approximately 30 Hz to 125 kHz, plus a single noise channel. Envelope control provides hardware volume modulation. Output is mono through the monitor speaker or the 13-pin DIN video connector.

Internal expansion — MegaBus

The Mega ST introduced the MegaBus, an internal expansion connector providing direct access to the 68000's address bus, data bus, and control signals. This 120-pin connector is located on the motherboard and provides:

• Full 68000 address and data bus access
• R/W, AS, DS, DTACK, and other control lines
• +5V and +12V power rails
• Interrupt lines (IPL0–IPL2)

The MegaBus was used by third-party peripherals including:

• Moniterm Viking — 19-inch high-resolution (1280 × 960) monochrome display adapter, widely used in DTP.
• Matrix VideoBoard — Additional video output card.
• 68881/68882 FPU cards — Floating-point co-processor daughtercards via the co-processor slot.
• Various network adapters and memory expansion boards.

Note: The MegaBus is electrically distinct from the VMEbus used in the later Atari TT030 and Mega STE, though both serve as internal expansion systems.

Real-time clock

The Mega ST was the first ST model to include a battery-backed real-time clock (RTC) using the Ricoh RP5C15 chip. This allows the system to maintain accurate date and time across power cycles. The RTC is accessed via I/O registers at $FFFC20–$FFFC3F. The backup battery is a CR2032 lithium coin cell mounted in a holder on the motherboard.

Ports and connectivity

Port	Connector	Description
MIDI In	5-pin DIN	Standard MIDI input at 31.25 kbaud
MIDI Out	5-pin DIN	Standard MIDI output at 31.25 kbaud
RS-232 serial	DB-25 male	Standard serial port via MFP 68901, up to 19,200 baud
Centronics parallel	DB-25 female	Standard parallel printer output via PSG port B
ACSI DMA	DB-19 male	Atari Computer Systems Interface; hard disk and laser printer port
External floppy	DB-14	Additional floppy disk drive connector
Video	13-pin DIN	RGB analogue + mono detect + audio output
Joystick × 2	DE-9 male	Standard Atari-compatible joystick/mouse ports
Cartridge	128 KB ROM cartridge slot on left side of case
Power	External Mitsumi SR98/SR118 PSU

Keyboard

The Mega ST features a detachable keyboard connected to the system unit via a coiled cable and RJ-11-style modular connector. The keyboard uses a dedicated Motorola HD6301V1 microcontroller for key scanning and IKBD (Intelligent Keyboard) protocol communication. The IKBD protocol also handles mouse and joystick input, as the mouse and joystick ports are located on the rear of the keyboard unit. The keyboard provides 94 keys with full numeric keypad, cursor keys, and 10 function keys.

Motherboard revisions

The primary Mega ST motherboard is designated C100167-001. Known revisions include:

Board designation	Notes
C100167-001 Rev.B	Standard Mega ST2/ST4 motherboard
CA2000019	Later production variant, electrically equivalent to C100167-001 Rev.B

The board uses a combination of custom Atari ASICs (GLUE, MMU, Shifter, DMA, BLiTTER) in PLCC packages alongside discrete logic and the 68000 in a DIP-64 package.

Operating system

The Mega ST shipped with TOS 1.02 (commonly called "Mega TOS"), stored in two or six ROM chips depending on the board revision. TOS 1.02 was the first version to include BLiTTER support and introduced minor bug fixes over TOS 1.00.

Later Mega ST units shipped with TOS 1.04 ("Rainbow TOS"), which added improved desktop functionality, faster boot times, and better hard disk support. TOS 1.04 can be retrofitted to earlier Mega ST units by replacing the ROM chips.

TOS includes the GEM (Graphics Environment Manager) desktop, GEMDOS file system, the BIOS, and the XBIOS. The GEM desktop provides a graphical file manager with drag-and-drop file operations, resizable windows, and desk accessory support.

Desktop publishing

The Mega ST became closely associated with desktop publishing following its launch alongside the Atari SLM804 laser printer (US$1,500). The SLM804 was unusual in that it connected via the ACSI DMA port rather than a parallel interface, using the ST's RAM as its page buffer — the printer itself contained no processor or memory of its own. This required a minimum of 2 MB RAM in the host computer and made the Mega ST2 or ST4 the natural companion.

Key DTP software on the Mega ST included:

• Calamus — Professional page layout and typesetting application by DMC. Widely used in German-speaking countries.
• PageStream — Page layout by Soft-Logik.
• Publishing Partner — Earlier DTP package by SoftLogik.
• Fleet Street Publisher — UK-oriented DTP package.

The Mega ST's high-resolution monochrome mode (640 × 400) on the SM124 monitor was the standard display for DTP work, and the Moniterm Viking adapter provided 1280 × 960 resolution via the MegaBus for true full-page WYSIWYG layout.

Music production

With built-in MIDI In and MIDI Out ports — a feature present on all ST models — the Mega ST was widely adopted in professional music studios. The 68000's interrupt-driven MIDI timing proved exceptionally stable compared to competing platforms of the era, with jitter typically below 1 ms.

Major MIDI sequencing software included:

• Steinberg Pro-24 and Cubase — Industry-standard sequencers. Steinberg originally developed Cubase on the Atari ST before porting to other platforms.
• C-Lab Creator/Notator — Professional sequencer and score editor.
• Dr. T's KCS (Keyboard Controlled Sequencer)
• Hybrid Arts SMPTE Track

The detachable keyboard and pizza-box form factor made the Mega ST particularly well-suited to studio environments, as the system unit could be rack-mounted or placed beneath a MIDI controller keyboard.

Known issues

Common issues affecting the Mega ST after decades of use include:

• Power supply degradation — The Mitsumi SR98/SR118 PSU contains electrolytic capacitors that degrade over time, causing voltage ripple, random crashes, floppy read errors, video noise, and in severe cases, damage to the motherboard. PSU recapping is considered mandatory maintenance.
• Reset circuit malfunction — The reset circuit on ST/Mega ST boards can fail due to aged capacitors, causing the machine to require multiple power cycles or a manual reset button press to start. A modification involving the reset capacitor addresses this issue.
• DMA bus pull-up resistors — The DMA data bus lacks pull-up resistors in the original design, which can cause intermittent hard drive and floppy drive failures. Adding 10K pull-up resistors to the DMA data bus is a recommended preventive modification.
• 1772 floppy controller pull-ups — Replacement floppy drives lacking internal pull-up resistors (such as the MPF920) may exhibit unreliable operation. Pull-up resistors should be added to the 1772 FDC output lines.
• RTC battery leakage — The CR2032 battery can leak over time, causing corrosion on nearby PCB traces and components. Regular battery replacement is advised.
• Bus resistor values — Stock 10K bus resistors on the address and data bus can cause instability when cartridges or certain CPU/ROM combinations are used. Replacing with 2.2K resistors improves reliability.

See also

• Atari 520ST
• Atari 1040STFM
• Atari Mega STE
• Atari Mega ST General Maintenance
• Atari Mega ST Troubleshooting Guide
• Atari Mega ST Capacitor Replacement Guide

External links

• Atari Mega ST at OldComputers.net
• Atari Mega ST at Atari Museum
• Mega ST at Atari-Wiki
• The LaST Upgrade — Exxos Atari ST upgrades and fixes
• Atari Document Archive (dev-docs.org)