Why does the usable memory of my 32-bit XP system decrease from 3.25 GB to 3 GB when I add RAM?

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Peter Hunt

Commendable
Oct 27, 2016
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NOTE: This is not the question about the 3.25GB limit on 32-bit XP. But it might be related.

I had 4GB (2x2GB) in my Dell Precision T3400 desktop, and XP reported 3.25GB in the "My Computer" properties. Sweet.

I then added 2 more 2GB DIMMs to the remaining two RAM slots, and now XP only reports 3GB, not 3.25GB. It's the same RAM vendor, type, part number, everything. BIOS sees all 4 DIMMs fine. I boot Win7 x64 and it reports 8GB.

But XP seems to have "lost" 250MB of memory by adding RAM.

Can anyone please help me try to get it back? :)


System specs:

Operating System: MS Windows XP Professional 32-bit SP3
CPU: Intel Core 2 Duo E6850 @ 3.00GHz (Conroe)
RAM: 8.0GB Dual-Channel DDR2 @ 399MHz (5-5-5-18)
Motherboard: Dell Inc. 0TP412 (CPU)

Memory:

  • ■Type DDR2
    ■Size 8192 MBytes
    ■Channels # Dual
    ■DRAM Frequency 399.0 MHz
    ■CAS# Latency (CL) 5 clocks
    ■RAS# to CAS# Delay (tRCD) 5 clocks
    ■RAS# Precharge (tRP) 5 clocks
    ■Cycle Time (tRAS) 18 clocks
    ■Command Rate (CR) 2T

Each SPD:

  • ■ Type DDR2
    ■ Size 2048 MBytes
    ■ Manufacturer Crucial Technology
    ■ Max Bandwidth PC2-6400 (400 MHz)
    ■ Part Number CT25672AA80EA.M18F
    ■ Week/year 22 / 16
    ■ SPD Ext. EPP

    • ■ JEDEC #3

      • ■ Frequency 400.0 MHz
        ■ CAS# Latency 6.0
        ■ RAS# To CAS# 5
        ■ RAS# Precharge 5
        ■ tRAS 18
        ■ tRC 23
        ■ Voltage 1.800 V
      ■ JEDEC #2

      • ■ Frequency 400.0 MHz
        ■ CAS# Latency 5.0
        ■ RAS# To CAS# 5
        ■ RAS# Precharge 5
        ■ tRAS 18
        ■ tRC 23
        ■ Voltage 1.800 V
      ■ JEDEC #1

      • ■ Frequency 266.7 MHz
        ■ CAS# Latency 4.0
        ■ RAS# To CAS# 4
        ■ RAS# Precharge 4
        ■ tRAS 12
        ■ tRC 16
        ■ Voltage 1.800 V
 
Solution
BFG-9000
I agree, Gavotte RAMDisk 1.0.4096.5_200811130 works great and easily makes a RAM Drive the size of the unaccessible memory. After that you can put a 4GB pagefile on it.

Dataram v3.5 to 4.3 works too (4.4 no longer supports XP or FAT16/FAT32) and is free under 4GB.

Alternately eBoostr can use the memory as cache for the HDD (like ReadyBoost for XP) but it prevents the HDD from spinning down and hibernation. I used to use that but small SSDs have become so cheap I even use one just for XP.
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Barty1884

Barty1884

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Apr 16, 2015
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I can't say definitively, but I believe it has something to do with the structure for 'reserving' system memory for graphics (and a few other 'reserved' aspects).

Your system/OS reserves X% or xMB, based on the total *installed*, not a % of the 'useable' RAM.
That %/amount can vary depending on the quantity installed - but you should be able to lower the reserved amount/% in the BIOS - although being a Dell, you might have limited BIOS options.

0.75GB reserved of 4GB = 18.75% of the *installed* memory is being reserved, leaving 81.25% useable to the OS

1GB reserved of 8GB = 12.5% of the *installed* memory is being reserved (kind of a sliding scale, with ~1GB being the automatic 'cap' usually).
Although only 12.5% of the installed memory is being reserved, your OS can't handle that very well.
Since 32bit OS can only 'use' 4GB max, it's still reserving the same quantity of RAM vs your 'installed' amount.

In this case, 8GB installed (of which 1GB is reserved), but your OS can only use 4GB.... yet still reserves the same 1GB.
 
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Peter Hunt

Commendable
Oct 27, 2016
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1,510


Thanks for the helpful explanation. This does make sense, and the Dell BIOS doesn't seem to have any way to tweak these settings.

I have seen other posts asking about the 3.25 (not 3) GB limit on systems with 8GB installed, so maybe this is something funky about the Dell. I'll post a question on their forum just to see what they say.

 
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Peter Hunt

Commendable
Oct 27, 2016
4
0
1,510


Well, I'm hoping that I *can* use the 250MB of RAM that was "lost" by XP when I added DIMMs. That's why I want to know why it disappeared - I'd like to get it back.

If you're asking "why install the additional 4GB is you can never use it?", then I should explain that this is a dual boot system with Win7 x64 on the other disk. So it's useful having the extra RAM for that OS, but I still boot to XP often enough that I'd like to be able to use as much RAM as possible when I do.
 
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BFG-9000

BFG-9000

Splendid
Sep 17, 2016
3,424
439
27,090
I agree, Gavotte RAMDisk 1.0.4096.5_200811130 works great and easily makes a RAM Drive the size of the unaccessible memory. After that you can put a 4GB pagefile on it.

Dataram v3.5 to 4.3 works too (4.4 no longer supports XP or FAT16/FAT32) and is free under 4GB.

Alternately eBoostr can use the memory as cache for the HDD (like ReadyBoost for XP) but it prevents the HDD from spinning down and hibernation. I used to use that but small SSDs have become so cheap I even use one just for XP.
 
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