Editing Atari 520ST (section)

== Memory and Storage ==
RAM organization and ROM layout
* Standard RAM: 512 KB on system board, typically 16× 256k×1 DRAMs (varies by vendor), arranged as two 8‑bit banks to form the 16‑bit data path. Numerous later replacement boards allow 1–4 MB by adding new DRAM modules or SIMMs (wiring to MMU MAD/CAS/RAS lines as required).<ref>{{cite web|url=https://forums.atariage.com/topic/337575-520st-ram-upgrade-question/page/2/|title=520ST RAM upgrade discussion, C070243 boards|publisher=AtariAge Forums|date=2022-09-14|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://www.pcbway.com/project/shareproject/STRAM_4MB_Atari_520ST_M_Expansion_692e0348.html|title=STRAM 4MB Atari 520ST(M) Expansion|publisher=PCBWay|date=2023-06-07|access-date=2025-08-13}}</ref>
* ROM/TOS: Early machines shipped with tiny 2‑chip boot ROMs that loaded TOS from disk; later 192 KB TOS 1.00 in 6× 32 KB DIPs; subsequent 192 KB TOS 1.02/1.04 appear in 2‑chip sets; part numbers for 6‑chip US TOS 1.00 include CO26160..CO26165.<ref>{{cite web|url=https://mikrosk.github.io/doitarchive/doit_st/0f0601.htm|title=TOS ROM sizes and families|publisher=DOIT_ST archive|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://www.best-electronics-ca.com/custom-i.htm|title=Best Electronics – Atari TOS ROM part numbers|publisher=Best Electronics|access-date=2025-08-13}}</ref>
* Floppy subsystem: WD1772 FDC at 8 MHz (250 kbit/s). Standard external single‑sided 3.5" SF354 (360 KB). The WD1772‑02‑02 variant often tolerates 16 MHz for 1.44 MB HD mods with appropriate gating and TOS support.<ref>{{cite web|url=https://mikrosk.github.io/doitarchive/doit_st/0b03.htm|title=HD Disk ST (WD1772 02‑02 at 16 MHz)|publisher=DOIT_ST archive|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://exxosforum.co.uk/atari/last/floppy/index.htm|title=LaST 1.44 MB mod notes|publisher=exxosforum|access-date=2025-08-13}}</ref>
* Hard disks: External ACSI devices via DB‑19; up to eight chained targets with simple 8‑bit protocol similar to early SCSI (not electrically SCSI). Used by Atari SH204/205 and MegaFile drives and the SLM laser printer.<ref>{{cite web|url=https://info-coach.fr/atari/hardware/interfaces.php|title=Atari DMA/ACSI interface|publisher=info-coach.fr|access-date=2025-08-13}}</ref>