Editing Atari ST Troubleshooting Guide (section)

== RAM Fault Diagnosis ==

RAM faults are a common cause of white screens, crashes, and corrupted displays on both the 520ST and 1040ST.

=== RAM Architecture ===

* '''520ST/520STM/520STF/520STFM (512 KB):''' Typically 16× 41256 (256K×1) DRAMs arranged as two banks of 8, or 8× 44256 (256K×4) DRAMs on later boards. RAM is soldered on most revisions.
* '''1040ST/1040STF/1040STFM (1 MB):''' 32× 41256 or 16× 44256, depending on board revision. On the shared C070789-001 board, the 1040 simply has more RAM positions populated than the 520. RAM is soldered.

The MMU handles row/column address multiplexing (MAD0–MAD9), RAS (Row Address Strobe), and CAS (Column Address Strobe) signals to the DRAM array.

=== Diagnosing RAM Faults ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Likely Cause !! Diagnostic Steps
|-
| White screen at power-on || One or more RAM chips completely dead; multiple data lines stuck || Use diagnostic cartridge with serial output — it will report stuck data lines and the address of the failing location.<ref name="diagcart"/> If no diagnostic cartridge, desoldering RAM to test in isolation may be necessary.
|-
| Machine detects less RAM than installed (e.g., 512 KB on a 1040ST) || Failed RAM chip(s) in upper bank; MMU configuration register mismatch || Boot with diagnostic software and run memory test. Check the MMU bank configuration register at $FF8001.
|-
| Random crashes during use || Intermittent RAM failure; marginal cell that fails under thermal stress || Run extended RAM test software (YAART is recommended). If crashes worsen as the machine warms up, use freeze spray on RAM chips to isolate the failing device.
|-
| Corrupted graphics (random pixels, garbled text) || RAM fault in the video display area (ST-RAM is shared between CPU and video) || Specific corrupted patterns can indicate which data bit is failing. A single stuck bit produces a repeating pattern every N pixels. Use diagnostic software to identify the failing bit.
|-
| Bomb screen (row of bombs at top of display) || Software crash; the number of bombs indicates the 68000 exception type || While not always a RAM fault, persistent bomb screens at random addresses suggest RAM corruption. Run memory diagnostics.
|}

=== 68000 Exception Types (Bomb Count) ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Bombs !! Exception !! Common Hardware Cause
|-
| 2 || Bus Error || GLUE timeout — accessed unmapped address space. Often caused by bad ROM contact, failed GLUE, or bus contention.
|-
| 3 || Address Error || 68000 attempted word/long access at odd address. Usually a software bug, but can indicate RAM corruption of pointer values.
|-
| 4 || Illegal Instruction || CPU fetched invalid opcode. Corrupt ROM, failing RAM in code area, or bus noise.
|-
| 5 || Division by Zero || Almost always a software issue.
|-
| 8 || Privilege Violation || Software attempted supervisor-mode instruction in user mode.
|}