Editing Atari Mega STE (section)

== Technical specifications ==

=== Processor and cache ===

The Mega STE uses the Motorola 68000 CPU clocked at '''16 MHz''' — double the clock speed of all prior ST models. A '''software-switchable speed''' feature allows the CPU to be set to either 16 MHz or 8 MHz via the '''GENERAL.CPX''' control panel accessory. The 8 MHz mode exists for compatibility with older software that relies on timing loops calibrated for the original ST's clock speed.<ref name="wp"/>

The system defaults to '''8 MHz with cache disabled''' at every cold boot. To operate at 16 MHz, the user must either load the CPX control panel or place an AUTO folder utility (such as MSTE_CC) that sets the speed and cache at boot time.<ref name="github">{{cite web |url=https://github.com/phjanderson/MSTE_CC |title=MSTE_CC: Atari Mega STE CPU speed and cache control utilities |publisher=GitHub |access-date=2026-03-27}}</ref>

The '''16 KB CPU cache''' is implemented using discrete SRAM chips (typically 2 &times; 8 KB, designated U3 and U4 on the motherboard) and cache control PAL logic. The cache stores recently accessed instructions and data, providing a significant performance boost at 16 MHz — approximately 80% faster than a standard 8 MHz ST in CPU-bound operations. The cache is automatically invalidated when the CPU relinquishes bus control (e.g., during DMA transfers or BLiTTER operations) to maintain coherency.

The CPU speed and cache are controlled via hardware registers at address $FFFF8E21:
* Bit 0: CPU speed (0 = 8 MHz, 1 = 16 MHz)
* Bit 1: Cache enable (0 = disabled, 1 = enabled)

An optional '''Motorola 68881 or 68882 FPU''' (floating-point unit) can be installed in a PLCC socket on the motherboard, providing hardware floating-point acceleration for CAD, scientific, and 3D applications.

=== Memory ===

The Mega STE uses '''30-pin SIMM''' (Single Inline Memory Module) slots for RAM, a significant improvement over the soldered DRAM of the Mega ST. Four SIMM slots accept 256 KB or 1 MB SIMMs in matched pairs, providing configurations of 1 MB, 2 MB, or 4 MB.

{| class="wikitable"
|-
! Configuration !! SIMMs required
|-
| 1 MB || 4 &times; 256 KB
|-
| 2 MB || 2 &times; 1 MB (+ 2 empty slots, or 2 &times; 256 KB in remaining slots)
|-
| 4 MB || 4 &times; 1 MB
|}

'''Parity SIMMs should not be used.''' Multiple documented cases have shown that parity SIMMs cause instability, particularly under load or when the power supply is degraded. If parity SIMMs are installed, the parity chip should be removed from each module.<ref name="mandatory">{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1585 |title=Mandatory Fixes |publisher=exxos Forum |access-date=2026-03-27}}</ref>

Memory access is managed by the GST MCU (GLUE-Shifter-MMU combined chip), which handles address decoding, DRAM refresh, and bus arbitration with the CPU, BLiTTER, and DMA controller.

=== Graphics ===

The video subsystem is derived from the STE, using the enhanced Shifter chip with an expanded palette of '''4,096 colours''' (12-bit RGB: 4 bits per channel), compared to the 512-colour palette (9-bit, 3 bits per channel) of the original ST.

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Mode !! Resolution !! Colours !! Monitor
|-
| Low resolution || 320 &times; 200 || 16 of 4,096 palette || SC1435 (colour)
|-
| Medium resolution || 640 &times; 200 || 4 of 4,096 palette || SC1435 (colour)
|-
| High resolution || 640 &times; 400 || Monochrome (black/white) || SM146 (mono)
|}

The STE enhancements include:
* '''Hardware fine scrolling''' — Pixel-level horizontal scrolling via hardware registers, without the performance cost of software scrolling.
* '''Extended palette''' — 4,096 colours with STE-format palette registers at $FFFF8240–$FFFF825E.

The BLiTTER co-processor is present, functioning identically to the unit in the Mega ST and 1040STE.

=== Sound ===

The Mega STE retains the dual audio system of the STE:

* '''Yamaha YM2149F PSG''' — Three-channel mono square wave synthesis, identical to all prior ST models.
* '''DMA stereo audio''' — An 8-bit PCM DMA sound system capable of playback at 6.258 kHz, 12.517 kHz, 25.033 kHz, or 50.066 kHz sample rates. Audio output is mixed through the '''National Semiconductor LMC1992''' programmable audio mixer, which provides software-controlled volume, bass, and treble adjustment.

Stereo audio output is available via RCA phono jacks on the rear of the system unit.

The enhanced joystick ports (DE-15) also support '''paddle controllers''' and '''light guns''' via the enhanced joystick interface, sharing the same connector pinout as the 1040STE.

=== Storage ===

==== Floppy disk drive ====

The Mega STE includes a single internal 3.5-inch floppy drive. Early production units shipped with a '''720 KB double-density''' drive; later units include a '''1.44 MB high-density''' drive capable of reading both DD and HD media. The floppy controller is the WD1772 (clocked at 8 MHz regardless of CPU speed setting) with DMA transfer to main memory.

==== SCSI hard disk ====

The 2 MB and 4 MB models shipped with an internal SCSI hard disk — typically a '''Seagate ST-157N''' (48 MB) or similar SCSI drive.<ref name="seagate">{{cite web |url=http://www.verycomputer.com/10_587e9c5649cc406d_1.htm |title=Seagate ST-157N Hard Drive in Mega STe |publisher=verycomputer.com |date=1993-08-06 |access-date=2026-03-27}}</ref> The SCSI interface is provided by an '''ACSI/SCSI adapter daughterboard''' that bridges the Atari Computer System Interface (ACSI) DMA port to a standard SCSI bus. This daughterboard connects internally; the external ACSI DMA port on the rear panel remains available for additional ACSI peripherals.

The 1 MB base model did not include a hard disk, SCSI adapter, or high-resolution monitor.

==== Hard disk driver software ====

The internal SCSI disk requires driver software loaded at boot time. Atari shipped '''AHDI''' (Atari Hard Disk Interface) as the standard driver. Third-party alternatives include '''HDDriver''' (by Uwe Seimet) and '''CBHD''', both of which provide improved performance, larger partition support, and broader device compatibility.

=== VMEbus expansion ===

The Mega STE includes a single '''3U VMEbus''' expansion slot inside the case, accessible by removing the top cover.<ref name="vmespec">{{cite web |url=http://dev-docs.atariforge.org/files/VME_Spec_7-19-1991.pdf |title=Atari VME Expansion for TT030 and Mega STE Products |publisher=Atari Corporation |date=1991-07-19 |access-date=2026-03-27}}</ref> This slot conforms to a subset of the VMEbus standard and provides access to the 68000's address and data buses.

VME expansion cards used with the Mega STE include:
* '''ET4000-based VGA cards''' — Providing higher-resolution display output (e.g., 1024 &times; 768) via VME-to-VGA adapters.
* '''Ethernet adapters''' — PAMs NetUS and similar cards providing 10 Mbit/s Ethernet networking.
* '''Crazy Dots''' — Graphics accelerator cards by Matrix.

The VME slot is physically identical between the Mega STE and the TT030, and most VME cards are compatible with both systems, though software driver compatibility varies.

=== Networking — LocalTalk/RS-422 port ===

The Mega STE includes a dedicated '''Serial LAN''' port implementing '''RS-422''' electrical signalling on a mini-DIN 8 connector. This port is physically and electrically compatible with Apple '''LocalTalk''' hardware, enabling connection to LocalTalk networks.

However, '''no AppleTalk protocol software was ever produced''' by Atari for TOS.<ref name="wp"/> Third-party solutions existed but were limited in availability. The port can also be used as a high-speed serial connection independent of networking, providing speeds up to 1 Mbit/s.

=== Serial ports ===

The Mega STE provides '''three RS-232 serial ports''', all using DB-9 connectors (a change from the DB-25 connectors used on earlier ST models):

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Port !! Controller !! Maximum speed !! Notes
|-
| Serial 1 (Modem) || MFP 68901 || 19,200 baud || Standard ST serial port, directly compatible with existing ST serial software
|-
| Serial 2 (Serial LAN) || SCC 85C30 || 1 Mbit/s || RS-422; also the LocalTalk port
|-
| Serial 3 || SCC 85C30 || 230,400 baud || Additional RS-232 port via the Zilog SCC
|}

The Zilog '''85C30 SCC''' (Serial Communications Controller) provides the two additional serial ports, offering significantly higher throughput than the MFP-based Serial 1.

=== Keyboard ===

The detachable keyboard connects via a coiled cable and uses the same IKBD protocol as the Mega ST. The keyboard includes mouse and joystick ports on its rear panel. The keyboard is functionally identical to the TT030 keyboard and uses the same TT-style case design. Membrane-type keyswitches are used, with occasional failure of individual key traces being a known wear issue.