update memory map doc and anticipate a gotcha

boot.subx

@@ -15,10 +15,10 @@
 # understand hardware interfaces well enough to explain to others.
 
 # Memory map of a Mu computer:
-#   code: currently 4 tracks loaded from the primary disk to [0x00007c00, 0x00048600)
+#   code: some tracks loaded from primary disk to [0x00007c00, 0x00080000)
 #   stack: grows down from 0x02000000 to 0x01000000
-#   heap: [0x02000000, 0x08000000)
-#     see 120allocate.subx; Qemu initializes with 128MB RAM by default
+#   heap: [0x02000000, 0x80000000)
+#     see 120allocate.subx; Qemu initializes with 128MB RAM by default; simulating 2GB RAM is known to work
 # Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of
 # this. And don't forget to keep *stack-debug.subx in sync.
 

translate_subx

@@ -41,6 +41,12 @@ then
   exit 1
 fi
 
+if [ `stat --printf="%s" a.bin` -ge 492544 ]  # 15 tracks * 63 sectors per track * 512 bytes per sector
+then
+  echo "a.bin will overwrite BIOS/Video memory; you'll need to adjust boot.subx to load code to some other non-contiguous area of memory"
+  exit 1
+fi
+
 # Latter half of disk is for debug info.
 dd if=labels of=code.img seek=10080 conv=notrunc  # keep this sync'd with abort.subx
 if [ `stat --printf="%s" labels` -ge 524288 ]  # 4 reads * 256 sectors * 512 bytes per sector

translate_subx_emulated

@@ -45,6 +45,12 @@ then
   exit 1
 fi
 
+if [ `stat --printf="%s" a.bin` -ge 492544 ]  # 15 tracks * 63 sectors per track * 512 bytes per sector
+then
+  echo "a.bin will overwrite BIOS/Video memory; you'll need to adjust boot.subx to load code to some other non-contiguous area of memory"
+  exit 1
+fi
+
 # Latter half of disk is for debug info.
 dd if=labels of=code.img seek=10080 conv=notrunc  # keep this sync'd with abort.subx
 if [ `stat --printf="%s" labels` -ge 524288 ]  # 4 reads * 256 sectors * 512 bytes per sector