|
When
does 8ns not mean 8ns? In the new sales-speak of PC100 SDRAM!! While
many Web Vendors are claiming to be selling 8ns or less PC100 SDRAM
the reality is that no RAM of that speed yet exists outside of the
R&D labs of the major memory manufactures. Even the latest Samsung
-GH memory, which runs stable at 133mhz bus speed settings, is only
rated at 10ns! Many people are claiming to be running 6ns or 7ns
RAM, and I had quite an argument with a supplier in the UK who insisted
he had 6ns SDRAM in stock!! 6 ns SDRAM really doesn't exist yet
- trust me :)
However, since I originally wrote this article, faster SDRAM
is becoming available. Samsung -G8 appears to be genuine
8ns SDRAM, and is rated up to 125mhz. See this pdf
file for further information. Other manufacturers such as Micron
are also producing genuine 8ns parts. 7n RAM is also becoming available
- Samsung -G7 is rated up to 143Mhz! In the meantime, this RAM is
still scare and it is still a case of buyer beware - don't
be afraid to ask the vendor to quote ALL the RAM specs. Before you
part with your hard-earned. Try not to be persuaded that a figure
of -8 or something similar denotes 8ns RAM. In almost all cases,
the most important figure to be looking at is CAS.... And if your
vendor tries to persuade you a -7 tAC figure means that he is carrying
7ns parts - just say No!
If 133Mhz bus speed in not important to you, then many RAM
parts are rated up to 125mhz, making the new 124 MHz bus speed look
rather enticing! Celeron
owners note that due to multiplier locking, it is very unlikely
you will be able to run at 124 and 133 MHz speeds!
All
CPU's manufactured after August 1998 are now multiplier locked -
do not expect to run your CPU past 112Mhz FSB! This caveat applies
regardless of your RAM quality! Expect to use 124 and 133Mhz speed
speeds only if your CPU allows you to drop down a multiplier - i.e.
running a pre-august 1998 P2-333 @ either 3 x 133 or 3.5 x 124.
PC133 spec. is
available now - check out Micron's
site. LGS are also working on PC133 spec. SDRAM as well. 'True'
PC133 has a clock cycle time of 7.5ns AND tAC=5.4ns. PC100 may
be able to run at this clock speed, but PC100 has (for example)
tAC=6.0ns (less timing margin than true PC133). LGS PC133 will be
marked -75.
Enhanced also have PC133
SDRAM available, as do Corsair, who SDRAM uses Micron chips. Typical
ratings for PC133 SDRAM is 75 - do not confuse this with PC100 RAM
using a similar designation. PC133 SDRAM is designed to work in
Intel's upcoming Camino (i820) chip. VIA also have a PC133 chipset
available, and it looks as if they will beat Intel to market.
The purpose of this section is to get a list of as many brands
of SDRAM as possible, and show the maximum speeds they will reliably
run at.
| A
word on CAS latency: |
| When
choosing your SDRAM look carefully at Column Address Strobe
(or CL) rating. On non-premium SDRAM a CAS of 2 may (should)
allow you to run the 112 bus speed, and on the premium SDRAM
it should be good for 133mhz. Of course, your processor may
well have other ideas about the speed you are attempting... |
The reason why CAS exists is for the purpose of registered DIMMs.
The register allows a higher density DIMM to be built; a registered
DIMM can support up to 36 SDRAM. Unfortunately, the register adds
1 clock delay to the DIMM timing, as a result the timing of CAS2
SDRAM on registered DIMM is the same as CAS3 on a standard DIMM.
Likewise the timing of a CAS3 SDRAM on a registered DIMM is (supposedly)
the equivalent of a CAS4 standard DIMM.
For 10ns memory to run at 133Mhz you need CAS 2 parts.
Setting this RAM at a CAS of 3 in the BIOS will give you the required
latency (or breathing space) to run at 133Mhz stable. 8ns CAS
3 memory will run fine at 133Mhz also.
Two of the
most common CAS 2 parts to be found are:
Don't be afraid to look at the
manufacturers web-sites to check out the specs. for yourselves.
Look for numbers like this:
-7K device meets
PC100,222
specifications.(tCK=10ns,tAC=6ns,CL=2,tRCD=2,tRP=2)
-7J device meets
PC100,322
specifications.(tCK=10ns,tAC=6ns,CL=3,tRCD=2,tRP=2)
Note -
LGS now make a part (GM72V66841CT-7J) that runs up to 133 Mhz.
Look for either
a CL
or
CAS
rating. Ignore
the tAC setting - a figure of tAC = 6ns does not denote
6ns SDRAM. Ask Hitachi!!
See the FAQ
for
help in identifying your SDRAM.
| CAS
2 vs. CAS 3 - How much speed do I gain? |
| Setting aside
the fact that CAS 2 SDRAM may allow you to run at the
133 Mhz bus speed, how much do you gain between the two settings? |
I ran the Unreal timedemo
at 4 x 83:
| Unreal |
1st
Run |
2nd
Run |
| |
|
|
| CAS 2 |
12.48 |
12.48 |
| |
12.43 |
12.40 |
| |
12.28 |
12.33 |
| |
|
|
| CAS 3 |
12.33 |
12.31 |
| |
12.26 |
12.28 |
| |
12.20 |
12.18 |
This test was run in software rendering. As the results show,
the speed gain is negligible and certainly not noticeable in real
world situations. So should you get CAS 2 memory? If you have
a P2-400, then you may be able to run at either 3 x 133
or 3.5x 133 if you are really lucky. 3 x 133 may be achievable
on a P2-333. Remember that your Level 2 cache is the main limiter
in overclocking at such high bus speeds! See Overclocking
tips for more information on Level 2 cache limitations.
Celeron owners please not that due to the locking on your CPU's
running the 124/133 Mhz is not realistic!
| Mixing
DIMMs - Any problems I should know about? |
Provided all the DIMM's are PC100 compliant there shouldn't be
any problems. I had 128 megs of LGS -7j and 2x 32 sticks of Megatrends
10ns PC66 running at 100 Mhz. without any problems. However, it
is important to make sure that all your memory can run
at the bus speed you are attempting. My Megatrends RAM will not
run at 112 Mhz, so mixing in that case is not appropriate.
| Registered
DIMM's |
| There
any some reports of sticks of "non-registered" DIMM's
failing to be recognised properly. |
This usually manifests its by the BIOS only identifying half
of the memory on the board. I.e. a 128 meg DIMM is recognised
as only 64 megs. My advice for now is to only purchase SDRAM that
is found on this survey and avoid too good to be true purchases,
which normally turns out to be generic. There are reports that
these DIMM's show the correct amount on other motherboards, so
this is something that may be solved by a BIOS update.
In addition, 256 megabytes DIMM's are not currently supported
on the BX6:
>to: technical@abit.com.tw
>Subject: 256Mb memory modules
>It is my understanding the BX6 cannot currently accept a 256Mb
PC100
>module. Is that something that can be changed through a BIOS
revision,
>and does Abit plan to do so sometime in the future?
>
>Thank you very much.
Dear Sir:
this might be possible,but currently we
do not have the plan and solution for it yet.
best regards,
abit,.fae
| Meet
the SDRAM Vendors.... |
| The following
is an excerpt of a reply I received from an SDRAM Vendor, and
will make interesting reading - if you can understand the techno-babble
:) |
On
the PC66 memory the industry standard was a sort of an amalgam
of the tCK and tAC timings. So a DIMM with 15ns tCK and 9ns tAC
would be classed as 10ns and 66MHz. Under the PC100 spec, some
people quote this old figure, some just the tCK. So a 7ns (old
measure) would be 10ns tCK and 6ns tAC. But the new measure is
10ns, but because the tAC is better than 7ns, it is PC100, and
won't work on the old boards. So, the new PC100 8ns standard might
be 8ns tCK and 6ns tAC or 12ns tCK and 6ns tAC!! Hyundai
PC66 10ns (old measure) product appears faster than the 7J, but
is not guaranteed stable. They say because it is 2-2-2 timing,
which is the same as the LG 7K. However, we are still looking
for a PC100 7nstCK, 6nstAC, 2-2-2 timing product. The 8ns we have
seen was only 3-3-3 timing and thus was slower at 100 than the
cheaper stuff (which was 2-2-2).
Another Vendor writes:
<snip>
For
instance, those 32 chip 64 meg SDRAM parts you speak of on your
page. Those in actuality are against JEDEC standards and in reality
could void the warranty on a motherboard. Parts like those are
one of the main reasons the PC100 specs were introduced. Way over
a year ago, when NEC started manufacturing SDRAM chips, the put
misleading markings on their DRAMS. The part numbers ended with
a -6. So everyone who new about SIMM memory instantly thought
it was 6 ns SDRAM. It was hilarious. Hitachi has part numbers
out right now that end with G60. Distributors and retailers alike
are selling this memory as 6ns parts. If
you take a cruise over to Hitachi's web page, you will see that
they are just 10ns PC100 DRAM chips. Engineers at Hitachi said
the 0 in the G60 stands for 10ns:)
Pretty
tricky huh? As far as 8ns parts go, that's pretty simple. Part
of the PC100 specs and Engineering requirements are that PC100
memory has been tested down to 8ns. But, these are not always
the fastest parts. Samsung has GL, GH and G8 parts. The first
two are 10ns and the last is 8ns. The faster of those parts are
the GH chips which run CAS 2 latency at 100mhz. There others are
CAS 3 at 100 MHz. Simple reverse engineering will tell you if
a part can run CAS 2 at a 100mhz, then it is capable of running
CAS 3 latency a lot higher, i.e. the 133mhz bus speeds. Hitachi's
memory, if my memory serves me correctly can only run CAS 2 at
66MHz. while Microns 8ns parts can only run CAS 2 at 66MHz. Quite
confusing without doing some research.
As
far as 6ns parts being out, it just isn't true That would be around
150 Mhz., and Samsung, the developer of SDRAM, who always leads
the market, in still in the engineering stages of 6ns memory.
So
if you happen upon someone trying to sell you 6ns parts, I recommend
you run, because if they are clueless on what they are selling,
it's not someone you want to count on.
| What
speed can I expect from my DIMM? |
| The first point
of call is to look at the RAM chip numbers. It
is also important to examine the circuitry on which the chips
are mounted as well. Although the mounted chips may be
capable of running at the rated speeds, poor PCB (Printed Circuit
Board) quality can introduce instability. |
Avoid DIMM's with more than 24 chips
on like the plague! They won't work on the BX6. This really shouldn't
be a problem for most people, as the PC100 doesn't allow for this.
However some very early DIMM's, usually with 64 megs. or 128 megs.
Onboard do carry more than 24 chips (usually 32) and will not
work -at any bus speed!!
**Most Motherboards require
REGISTERED DIMMs for 256meg DIMMs**
Note: Chip markings can vary
depending on size - the numbers marked in BOLD are the
key markings to look for
| PC66
SDRAM |
Works
up to.. 100mhz |
| |
|
| Siemens |
AT-10 |
| LGS |
T-10K |
| Samsung |
-G12
G2 |
| NEC |
D4516821G5 |
| Hyundai |
10B-TC10 |
| PC66
SDRAM |
Works
up to.. 103mhz |
| |
|
| Samsung |
-G10 |
| Hyundai |
TFG-10 |
| Fujitsu |
MB81117822A-100 |
| Siemens |
HYB39S16800T-8
|
| PC66
SDRAM |
Works
up to.. 112mhz |
| |
|
| LGS |
-10K |
| Hitachi |
HM5216805TT10H |
| AMM* |
TI-10 |
| Hyundai |
HY57V168010A
TC -10 |
| Siemens |
39S64160
AT-10 @ CAS2 |
*Works at 112 Mhz with CAS
set to 3
| PC66
SDRAM |
Works
up to.. 124mhz |
| |
|
| Micron |
48LC2M8A1
TG -10S |
| PC100
SDRAM |
Works
up to.. 100mhz |
| |
|
| Fujitsu |
8UV64B4C-10xT-S |
| PC100
SDRAM |
Works
up to.. 103mhz |
| |
|
| Mitsubishi |
M5M64S30ATP-8 |
| PC100
SDRAM |
Works
up to.. 112mhz |
| |
|
| Corsair
PC100 |
(CM-8-64-BX) |
| Toshiba |
-80H |
| LGS |
7K |
| LGS |
7J* |
| Mitsubishi |
M5M4V64S40ATP-8 |
| |
M5M4V64S30ATP-8 |
| Micron |
-8b |
| Samsung |
-GL |
| Hitachi |
HM5264165TTB60 |
| Siemens |
-AT10 |
| Hyundai |
HY57V658020TC-10 |
| TI |
TI-8
TMS664814DGE |
*CAS 3, but
works at 112mhz, but appears to suffer on poor constucted PCB's.
LGS now make a -7j SDRAM that runs up to
133Mhz.
| PC100
SDRAM |
Works
up to.. 124mhz |
| |
|
| Toshiba
8ns |
CAS
3 -80H |
| |
CAS
2 MC16V64E8S4G8T |
| |
TC59S6408BFT-10
|
| Mitsubishi
8ns |
M5M4V64S30ATP-8 |
| LGS |
LG8
(CT8) |
| Siemens |
AT-8 |
| TI |
-8 |
| |
-8A |
| Hyundai |
HY-8 |
| NEC
CAS2 |
A10-JF |
| PC100
SDRAM |
Works
up to.. 133mhz |
| |
| Samsung
CAS2 |
KM48S8030BT-GH |
| Samsung
CAS3 |
-G8 |
| Samsung |
KM48S8030CT-G7* |
| NEC
CAS2 |
D4564841G5 |
| Micron
CAS3 |
48LC8M8A2
TG -8C |
| LGS
(tAA = 16ns) |
GM72V66841CT-7J** |
For -GH set CAS to 2 for 124 Mhz
and below. CAS to 3 for 133 Mhz.
*Stable at 143Mhz
**Note
LGS also make other SDRAM with a -7j marking that only runs
at 112Mhz. This can be idenitfied with a tAA rating of 24ns.
| PC100
SDRAM |
Works
up to.. 139mhz |
| |
| Samsung
CAS3 |
-G8 |
139Mhz tested on AOpenAX6BC
Rev 2.10
| PC100
SDRAM |
Works
up to.. 140mhz |
| |
| Siemens
|
HYB39S64800-AT
|
140Mhz
tested on Asus
P2B-F
| PC100
SDRAM |
Works
up to.. 143mhz |
| |
| Samsung
CAS3 |
-G8 |
143Mhz
tested on MSI
6163
| PC133
SDRAM |
Works
up to.. 153mhz |
| |
| Enhanced
PC133 |
SM12808DT-7.5
|
153Mhz
tested on MSI
6163
Note on Enhanced PC133
SDRAM: This is the best
SDRAM I have tested to date (1st June 1999) ! It works flawlessly
at 133Mhz as expected with all latencies set to minimum.
In testing on the Five
Board Overclockers Round-Up this memory consistently
outperformed the reference Samsung -G8 SDRAM used for product
reviews.
Please help me fill in the gaps on these tables! What brand
of SDRAM are you running, what chip marking and maximum speeds please!!!
Mail me at bxpage.survey@virgin.net
Copyright 1998 Andy Drake
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