SRAM PC133 - double sided vs single sided - detected half capacity?

[DiTBho]

I have several SRAM PC133 sticks: some are single-sided, some are double-sided, but each stick is 256 MByte.

Those single sided are correctly detected as 256MByte ram
Those double sided are detected as 128MByte ram

I fail to understand: why?!? Any idea?

If it's a firmware problem (u-boot), I can try to fix it

[oPossum]

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Note how the chip selects /S0 to /S3 are used. The double sided module probably requires /S1 and /S3 (in addition to /S0 and /S2).

[David Hess]

Exactly, each side counts as a "rank" and the memory controller may only support 2 instead of 4 ranks, or 4 instead of 8 ranks.

[rsjsouza]

Your post reminded me of how many times I assembled computers in the 1990s that had this and many other pitfalls - the motherboard/northbridge could be very particular in the types and models of components being used. It is amazing how so much of this knowledge quickly fades away...

[DiTBho]

It is amazing how so much of this knowledge quickly fades away...

Yesterday I contacted a RAM supplier who has a shop on eBay to find out if they can supply PC133 ram modules; talking to him ... he said that in the 2000s they had employees who sure know about my problem, but they are all retired and the information is lost without them.

[ejeffrey]

One other trap: "sides == ranks" is usually but not always true.  For many generations of DRAM the default PCB layouts followed that rule, but it isn't universal.  One thing that happens is that generation X memory is released, and single and double sided DRAM has 1 and 2 ranks.  Then, higher density chips become available, and the same chips now fit on a single side but are still dual rank.  Checking the sides is a good first check, but if you want to be absolutely sure you need a data sheet or to actually try and use it.

[DiTBho]

it's hard to find a datasheet of a ram stick, and it's hard to know *what* is mounted on ramsticks 

[DiTBho]

One other trap: "sides == ranks" is usually but not always true.  For many generations of DRAM the default PCB layouts followed that rule, but it isn't universal.  One thing that happens is that generation X memory is released, and single and double sided DRAM has 1 and 2 ranks.  Then, higher density chips become available, and the same chips now fit on a single side but are still dual rank.  Checking the sides is a good first check, but if you want to be absolutely sure you need a data sheet or to actually try and use it.

what about SDR ram? (before DDR) 

[ejeffrey]

Sure you can find data sheets for DRAM modules.  Here is an example I pulled at random from crucial: https://www.crucial.com/memory/ddr4/ct2k32g4s266m

You will notice that it says "dual rank", along with additional information about the performance and organization of the memory.  Finding information on obsolete no-name modules will obviously be harder.

I'm pretty sure SDR DRAM is the same way, not sure about older asynchronous DRAM standards.

[rsjsouza]

One other trap: "sides == ranks" is usually but not always true.  For many generations of DRAM the default PCB layouts followed that rule, but it isn't universal.  One thing that happens is that generation X memory is released, and single and double sided DRAM has 1 and 2 ranks.  Then, higher density chips become available, and the same chips now fit on a single side but are still dual rank.  Checking the sides is a good first check, but if you want to be absolutely sure you need a data sheet or to actually try and use it.

what about SDR ram? (before DDR) 

Just a correction: SDRAM or Synchronous Dynamic RAM.

It is a similar thing. Ranks are simply a row of memory devices tied to the same Chip Select (CS) signal. In the PC world, dual rank memory modules started to appear with the 72-pin SIMM DRAM modules (pre-SDRAM).

https://en.m.wikipedia.org/wiki/SIMM

Prior to that you would have single rank DRAMs either in 30-pin SIMM or SIPP or a sea of DRAM ICs in DIP package socketed to the motherboard.

[magic]

I found it easier to find datasheets of RAM chips used on the module. From those it's easy to deduce everything else, even the number of chips if only one side is shown on the photo.

Fun fact: old Intel i440BX/ZX chipsets work with most 256MB modules, but the single-sided ones are usable only to half capacity.
Why? Because the controller doesn't have enough bits to address a single 256MB rank.

[DiTBho]

I found it easier to find datasheets of RAM chips used on the module

I need to purchase something, and I don't find any datasheet of anything anywhere.
I bought several different ram-sticks to inspect chips, and see myself how they work.

[DiTBho]

Fun fact: old Intel i440BX/ZX chipsets work with most 256MB modules, but the single-sided ones are usable only to half capacity.
Why? Because the controller doesn't have enough bits to address a single 256MB rank.

That's exactly the opposite of what I have experimentally observed:

• double sided -> recognized as half capacity
• single sided -> recognized as full capacity

double-sided/single-sided ---> doesn't imply double-rank/single-rank
it's more complex to say

so far I have tested 4 ram-sticks, unfortunately I cannot read what's written on chips.
And there is no label on them.

[DiTBho]

What is a Memory Rank?
The term rank was created by JEDEC, the memory industry’s standards group, to distinguish between the number of memory banks on a module as opposed to the number of memory banks on a component, or memory chip. The concept of memory rank applies to all memory module form factors, though in general it tends to matter primarily on server platforms, due to the larger amounts of memory they manage.

A memory rank is a block or area of data that is created using some, or all, of the memory chips on a module. A rank is a data block that is 64 bits wide. On systems that support Error Correction Code (ECC) an additional 8 bits are added, which makes the data block 72 bits wide. Depending on how a memory module is engineered, it may have one, two, or four blocks of 64-bit wide data areas (or 72-bit wide in the case of ECC modules.) This is referred to as single-rank, dual-rank, and quad-rank. Crucial denotes this on the module label as 1Rx4, or 2Rx4, 2Rx8, or similar.

The x4 and x8 refer to the number of banks on the memory component or chip. It is this number, not the number of individual memory chips on a PCB, that determines the rank of the finished module. In other words, if a module has chips on both sides of the PCB, which makes it dual-sided, it can still be single-ranked, dual-ranked, or quad-ranked, depending on how those chips are engineered.

Because a rank is 64 or 72 bits, an ECC module made from x4 chips will need eighteen chips for one single rank (18 x 4 = 72). An ECC module made from x8 chips needs only nine of them for a rank (9 x 8 = 72). A module made from eighteen x8 chips would be dual-ranked (18 x 8 = 144, 144/72 = 2). An ECC module that has twice as many x8 chips becomes quad-ranked (36 x 8 = 288, 288/72 = 4).

Having a dual- or quad-ranked module is like having two or four DRAM modules combined onto one module. For example, you can instantly go from four single rank 4GB RDIMM modules to a single quad-rank 16GB RDIMM module (assuming the system is compatible with 16GB RDIMMs).

The drawback with higher ranked modules is that servers sometimes have a limit on how many ranks they can address. For example, a server with four memory slots may be limited to a total of eight ranks. This means you can install four single-ranked modules or four dual-ranked modules but only two quad-ranked modules, as installing more would exceed the amount of ranks that can be addressed.

On matters of rank and rank limitations, we recommend that you consult the manufacturer’s documentation for guidelines and directions that apply to your specific system.

[DiTBho]

Talking about labels ... as far as I understand ... memory Rank should be distinguished by the information that is displayed on the information sticker on the memory:

• Single Rank memory has a "1R%x" on the label, for example { 1Rx8, 1Rx16 }: "1R" --> Single Rank
• Dual Rank memory has a "2R%x"  on the label, for example { 2Rx8, 2Rx16 }: "2R" --> Dual Rank
• Quad Rank memory has a "4R%x"  on the label, for example { 4Rx8, 4Rx16 }: "4R" --> Quad Rank

In my case ... labels are missing or blank about this information