FC6 kernels & 4GB of memory

[Darryl]

I am setting up a new machine that has 4GB of ECC memory (Smolt UUID 
c0eefda6-6c8f-4663-a9b5-1e1adbcc0fc7) on an Asus M2V mobo (8GB max 
capacity, latest BIOS) with Athlon64 X2 5400+ but can't seem to get FC6 
to utilize all the available memory. BIOS memory hole remapping is 
enabled and the BIOS shows 4GB. memtest86 on the FC6 media writes 
to/reads from all 4095MB found for hours & hours without error. I've 
tried 32bit, 32bit PAE and 64bit FC6 kernels but they all seem to 
largely ignore the last 768MB of memory except the "regular" 32bit one 
which finds it but tells me to use a PAE kernel. Sometimes I would have 
to include mem=4095m on the kernel command line or the machine would not 
boot. Since memtest86 can get at all 4095MB it feels like a 
kernel/kernel boot option problem but I haven't been able to nail it. 
Any ideas?

dmesg snippets from each kernel:

32bit kernel 2.6.20-1.2948.fc6
  BIOS-e820: 0000000000000000 - 000000000009e800 (usable)
  BIOS-e820: 000000000009e800 - 00000000000a0000 (reserved)
  BIOS-e820: 00000000000e4000 - 0000000000100000 (reserved)
  BIOS-e820: 0000000000100000 - 00000000cffb0000 (usable)
  BIOS-e820: 00000000cffb0000 - 00000000cffbe000 (ACPI data)
  BIOS-e820: 00000000cffbe000 - 00000000cffe0000 (ACPI NVS)
  BIOS-e820: 00000000cffe0000 - 00000000d0000000 (reserved)
  BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
  BIOS-e820: 00000000fecc0000 - 00000000fecc1000 (reserved)
  BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)
  BIOS-e820: 0000000100000000 - 0000000130000000 (usable)
Warning only 4GB will be used.
Use a PAE enabled kernel.
3200MB HIGHMEM available.
896MB LOWMEM available.
Kernel command line: ro root=LABEL=/32
Memory: 3358536k/4194304k available (2156k kernel code, 47764k reserved, 
1128k data, 232k init, 2490048k highmem)

32bit 2.6.20-1.2948.fc6PAE
  BIOS-e820: 0000000000000000 - 000000000009e800 (usable)
  BIOS-e820: 000000000009e800 - 00000000000a0000 (reserved)
  BIOS-e820: 00000000000e4000 - 0000000000100000 (reserved)
  BIOS-e820: 0000000000100000 - 00000000cffb0000 (usable)
  BIOS-e820: 00000000cffb0000 - 00000000cffbe000 (ACPI data)
  BIOS-e820: 00000000cffbe000 - 00000000cffe0000 (ACPI NVS)
  BIOS-e820: 00000000cffe0000 - 00000000d0000000 (reserved)
  BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
  BIOS-e820: 00000000fecc0000 - 00000000fecc1000 (reserved)
  BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)
  BIOS-e820: 0000000100000000 - 0000000130000000 (usable)
user-defined physical RAM map:
  user: 0000000000000000 - 000000000009e800 (usable)
  user: 000000000009e800 - 00000000000a0000 (reserved)
  user: 00000000000e4000 - 0000000000100000 (reserved)
  user: 0000000000100000 - 00000000cffb0000 (usable)
  user: 00000000cffb0000 - 00000000cffbe000 (ACPI data)
  user: 00000000cffbe000 - 00000000cffe0000 (ACPI NVS)
  user: 00000000cffe0000 - 00000000d0000000 (reserved)
  user: 00000000fec00000 - 00000000fec01000 (reserved)
  user: 00000000fecc0000 - 00000000fecc1000 (reserved)
  user: 00000000ff780000 - 0000000100000000 (reserved)
2431MB HIGHMEM available.
896MB LOWMEM available.
Kernel command line: ro root=LABEL=/32 mem=4095m single
Memory: 3366128k/3407552k available (2197k kernel code, 40172k reserved, 
1159k data, 236k init, 2490048k highmem)

64bit 2.6.20-1.2948.fc6:
BIOS-provided physical RAM map:
  BIOS-e820: 0000000000000000 - 000000000009e800 (usable)
  BIOS-e820: 000000000009e800 - 00000000000a0000 (reserved)
  BIOS-e820: 00000000000e4000 - 0000000000100000 (reserved)
  BIOS-e820: 0000000000100000 - 00000000cffb0000 (usable)
  BIOS-e820: 00000000cffb0000 - 00000000cffbe000 (ACPI data)
  BIOS-e820: 00000000cffbe000 - 00000000cffe0000 (ACPI NVS)
  BIOS-e820: 00000000cffe0000 - 00000000d0000000 (reserved)
  BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
  BIOS-e820: 00000000fecc0000 - 00000000fecc1000 (reserved)
  BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)
  BIOS-e820: 0000000100000000 - 0000000130000000 (usable)
Entering add_active_range(0, 0, 158) 0 entries of 3200 used
Entering add_active_range(0, 256, 851888) 1 entries of 3200 used
Kernel command line: ro root=LABEL=/ single mem=4095m
Memory: 3340464k/3407552k available (2454k kernel code, 66696k reserved, 
1458k data, 316k init)