Editing Atari Mega ST (section)

== 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 ('''BL'''ock '''I'''mage '''T'''rans'''TER''') 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.<ref name="atarifaq">{{cite web |url=https://www.fenarinarsa.com/?p=2242 |title=Atari ST FAQ |publisher=Fenarinarsa |access-date=2026-03-27}}</ref>

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 &times; 1 Mbit DRAM chips (organized as 256K &times; 4), while the Mega ST4 uses 32 such chips. The Mega ST1 uses 8 &times; 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:

{| class="wikitable"
|-
! 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:

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Mode !! Resolution !! Colours !! Monitor
|-
| Low resolution || 320 &times; 200 || 16 of 512 palette || SC1224 (colour)
|-
| Medium resolution || 640 &times; 200 || 4 of 512 palette || SC1224 (colour)
|-
| High resolution || 640 &times; 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 &times; 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 ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! 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 &times; 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.