From 9f45c30789b6133d4b5fb4ef07f0818a3ea4de48 Mon Sep 17 00:00:00 2001
From: Richard Qin <richard.qin.x@gmail.com>
Date: Mon, 9 Nov 2026 23:49:35 +0800
Subject: [mubios] Derive the E820 map from the CMOS memory size

The E820 map ended the usable region at a hardcoded 0x08000000, so a guest
with more RAM had the extra memory reported as reserved and ran out of memory
during initramfs unpacking.

Add cmos_read8() and size the extended memory entry from CMOS registers 0x34
and 0x35, falling back to the 16 MiB units in 0x30 and 0x31 and then to the
previous 128 MiB end when neither is programmed.
---
 mubios/services/int15.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

(limited to 'mubios/services')

diff --git a/mubios/services/int15.c b/mubios/services/int15.c
index eb66f5c..b9edb76 100644
--- a/mubios/services/int15.c
+++ b/mubios/services/int15.c
@@ -37,9 +37,42 @@ static const struct e820_entry e820_map[]
     = { { 0x00000000u, 0x00000000u, 0x0009C000u, 0x00000000u, 1u },
         { 0x0009C000u, 0x00000000u, 0x00004000u, 0x00000000u, 2u },
         { 0x000A0000u, 0x00000000u, 0x00060000u, 0x00000000u, 2u },
-        { 0x00100000u, 0x00000000u, 0x07F00000u, 0x00000000u, 1u } };
+        { 0x00100000u, 0x00000000u, 0x00000000u, 0x00000000u, 1u } };
 
 #define E820_ENTRY_COUNT ((u32)(sizeof (e820_map) / sizeof (e820_map[0])))
+#define E820_MEMORY_BASE 0x00100000u
+#define E820_FALLBACK_END 0x08000000u
+#define CMOS_EXTENDED_MEMORY_LOW_16M 0x30u
+#define CMOS_EXTENDED_MEMORY_HIGH_16M 0x31u
+#define CMOS_EXTENDED_MEMORY_LOW 0x34u
+#define CMOS_EXTENDED_MEMORY_HIGH 0x35u
+
+static u32
+get_memory_end (void)
+{
+  u16 units;
+  u16 extended_kib;
+  u32 memory_end;
+
+  units = (u16)cmos_read8 (CMOS_EXTENDED_MEMORY_LOW)
+          | (u16)((u16)cmos_read8 (CMOS_EXTENDED_MEMORY_HIGH) << 8);
+  if (units != 0u)
+    {
+      memory_end = 0x01000000u + ((u32)units << 16);
+    }
+  else
+    {
+      extended_kib = (u16)cmos_read8 (CMOS_EXTENDED_MEMORY_LOW_16M)
+                     | (u16)((u16)cmos_read8 (CMOS_EXTENDED_MEMORY_HIGH_16M)
+                             << 8);
+      if (extended_kib != 0u)
+        memory_end = E820_MEMORY_BASE + ((u32)extended_kib << 10);
+      else
+        memory_end = E820_FALLBACK_END;
+    }
+
+  return memory_end;
+}
 
 static void
 int15_fail (struct bios_regs *regs)
@@ -51,9 +84,11 @@ int15_fail (struct bios_regs *regs)
 void
 int15_service (struct bios_regs *regs)
 {
+  struct e820_entry entry;
   struct e820_entry *output;
   u32 index;
   u32 address;
+  u32 memory_end;
 
   if (regs->eax != E820_FUNCTION || regs->edx != E820_SIGNATURE
       || regs->ecx < E820_ENTRY_SIZE)
@@ -73,7 +108,14 @@ int15_service (struct bios_regs *regs)
   address = ((u32)regs->es << 4) + (u16)regs->edi;
   output = (struct e820_entry *)address;
 
-  *output = e820_map[index];
+  entry = e820_map[index];
+  if (index == 3u)
+    {
+      memory_end = get_memory_end ();
+      entry.length_low = memory_end - E820_MEMORY_BASE;
+    }
+
+  *output = entry;
 
   regs->eax = E820_SIGNATURE;
   regs->ecx = E820_ENTRY_SIZE;
-- 
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