Editing Atari 130XE Troubleshooting Guide (section)

== Extended Memory (Bank-Switching) Faults ==

The 130XE's 128 KB memory system relies on the EMMU (CO25953) and PIA (CO14795) for bank-switching:

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
! Symptom !! Probable Cause !! Action
|-
| Base 64K OK, extended 64K test fails entirely || EMMU (CO25953) fault || Replace EMMU chip
|-
| Some extended banks fail, others work || Individual DRAM chips in extended bank failed || Identify and replace specific DRAM(s)
|-
| PIA port B writes have no effect on banking || PIA (CO14795) fault || Replace PIA
|-
| Random data corruption in extended RAM || Intermittent DRAM or EMMU timing issue || Test DRAM; check for cold solder joints on EMMU
|}

The bank-switching register is at address $D301 (PIA port B). Bits 2–3 select the bank (0–3), bit 4 controls CPU access, and bit 5 controls ANTIC access to extended memory.