Editing Atari 520ST (section)

== Architecture and Processor ==
Exact CPU and core architecture
* CPU: Motorola MC68000 (DIP‑64 MC68000P8 on early boards; later boards sometimes PLCC‑68 MC68000FN8), nominal 8.00 MHz.<ref>{{cite web |url=https://www.alldatasheet.net/datasheet-pdf/pdf/135463/MOTOROLA/MC68000P8.html |title=MC68000P8 8 MHz Datasheet |publisher=Motorola |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.datasheets360.com/part/detail/mc68000fn8/-3094971039532773301/ |title=MC68000FN8 8 MHz summary |publisher=Datasheets360 |access-date=2025-08-13}}</ref>
* Architecture: 16/32‑bit 68000 core (32‑bit registers/ALU with 16‑bit external data bus; 24‑bit address bus for 16 MB address space).<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
* System bus: 24 address lines, 16 data lines; unified memory (video fetches contend with CPU). Video DMA is performed by the Shifter reading DRAM under GLUE/MMU timing; the separate Atari “DMA” IC arbitrates ACSI/floppy transfers.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Atari_ST/STe/MSTe/TT/F030_Hardware_Register_Listing |title=Atari ST hardware register listing and memory map |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
* Region timings: CPU runs at 8.00 MHz in all regions; display vertical refresh differs (50.05 Hz PAL, 60.04 Hz NTSC, mono ~71.5 Hz), derived from ~32.084988 MHz master clock on color modes.<ref>{{cite web |url=https://www.atari-forum.com/viewtopic.php?t=32842 |title=ST/STE video frequencies derivation |publisher=Atari‑Forum |date=2017-12-24 |access-date=2025-08-13}}</ref>

Major custom and support ICs (typical 520ST/520STM/early STF family)
* Shifter (video DMA/serializer): Atari C025914‑38/‑38A; also seen as CO70713/I10356 on some boards.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC part numbers list (community collation) |publisher=AtariAge Forums |date=2016-01-05 |access-date=2025-08-13}}</ref>
* GLUE (bus/timing/GLUe logic): Atari C025915‑38A (and SMD successors).<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC part numbers list |publisher=AtariAge Forums |access-date=2025-08-13}}</ref>
* MMU (DRAM controller/multiplexer): Atari C025912‑38; later C100109 on later boards.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC list |publisher=AtariAge Forums |access-date=2025-08-13}}</ref>
* DMA controller (ACSI/floppy DMA): Atari C025913‑38.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0904.htm |title=DOIT: DMA and ROM discussion with WD1772 photo |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
* Interrupt/timers/UART: Motorola MC68901 MFP (system timers, interrupt controller, 200 Hz tick, handshake lines).<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=Concise Atari ST Programmer’s Reference – MC68901 MFP |publisher=bighole.nl mirror |access-date=2025-08-13}}</ref>
* ACIAs: 2× Motorola MC6850 ACIA (keyboard/IKBD and MIDI), clocked at 500 kHz for MIDI 31.25 kbaud (÷16).<ref>{{cite web |url=https://www.muzines.co.uk/articles/atari-520st/9092 |title=Atari 520ST (MIDI hardware notes) |publisher=Electronics & Music Maker |date=1985-07 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_2.htm |title=Concise Atari ST Programmer’s Reference – MIDI/IKBD ACIAs |publisher=bighole.nl mirror |access-date=2025-08-13}}</ref>
* Floppy controller: Western Digital WD1772, commonly WD1772‑00‑02 or ‑02‑02; later chips tolerate 16 MHz for HD‑floppy mods.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0b03.htm |title=HD Disk ST (WD1772 02‑02, 16 MHz HD mods) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://exxosforum.co.uk/atari/last/floppy/index.htm |title=The LaST upgrade – Atari 1.44 MB floppy mod notes |publisher=exxosforum |access-date=2025-08-13}}</ref>
* PSG audio + parallel I/O: Yamaha YM2149F (AY‑3‑8910 compatible) provides 3 tone + noise channels and two 8‑bit I/O ports used by ST for printer and control lines.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Yamaha_YM2149_Manual |title=Yamaha YM2149 Manual |publisher=AtariForumWiki |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=Atari ST Internals – YM2149 registers and I/O |publisher=Ctrl‑Alt‑Rees Atari Museum mirror |access-date=2025-08-13}}</ref>