diff --git a/src/apple2.mem.c b/src/apple2.mem.c
index 5c651bb..92a0716 100644
--- a/src/apple2.mem.c
+++ b/src/apple2.mem.c
@@ -16,6 +16,12 @@ SEGMENT_READER(apple2_mem_read_bank)
 
     mach = (apple2 *)_mach;
     
+    // In the case of bank-switchable memory, BANK_ALTZP is the ultimate
+    // arbitrator; if it's on, we have to use aux, and if not, we have
+    // to use main. Whatever the segment was that was passed in will
+    // turn out to be immaterial.
+    segment = (mach->bank_switch & BANK_ALTZP) ? mach->aux : mach->main;
+
     if (~mach->bank_switch & BANK_RAM) {
         // We need to account for the difference in address location
         // before we can successfully get any data from ROM.
@@ -52,6 +58,9 @@ SEGMENT_WRITER(apple2_mem_write_bank)
         return;
     }
 
+    // See my spiel in the read bank mapper; the same applies here.
+    segment = (mach->bank_switch & BANK_ALTZP) ? mach->aux : mach->main;
+
     // You will note, if we've gotten here, that it's possible to write
     // to the bank-switch addresses even if the ROM flag is 1. It's
     // true! Except that writes never go to ROM. That is to say, it's