RAM modules for Apple notebooks: the test
Non-certified RAM issues have always affected Apple computers. We will show that those problems could arise due to both the specific design of the RAM controller used by Apple and also by the poor quality of the memory chips often used for "no name" RAM modules .
The chronic RAM module compatibility issue is coupled to the usual habit of Apple to deliver desktop and laptop computers with a ridiculously low amount of memory installed by default . Originally, this was a way for Apple to allow Apple resellers to generate a larger profit margin by selling Cupertino-designed computers (hardware margins have also been really tiny). However, the launch of the physical Apple Store and the open war between Apple and its resellers have amplified the problem that there is not enough RAM installed by default, which is the main cause of slow OS responsiveness, GUI fluidity, and in general, the overall performance of Apple-branded computers. This is especially true since OSX and its Unix-based kernel are , by definition, RAM hungry , leading users to look for alternatives to the Apple-certified modules .
One of the reasons, for which we have decided to start this large-scale test, is the difficulty in finding compatible RAM modules for some Apple notebooks. After investigating , we have found some explanations for the extreme sensitivity to RAM quality found in the PowerBook 15" 1.25 GHz, for example. In order to save on the battery life span, Apple has been using a special feature allowing the decrease of the memory bus frequency based on CPU usage and environment temperature (it was mainly turned on when the computer was in "sleep mode") on some notebook models released from August 2003 to April 2004 . Apple named this system: "Bus slewing". Decreasing the frequency of a RAM controller is nothing really special, except that it takes at least 200 cycles before being able to send a new request to the RAM controller. The design used by Apple does not allow the RAM controller to wait for this "long" awaking time, and it has been generating many instability and compatibility issues with most RAM modules. In fact, it is the reactivity and the ability of the memory chips to respond to less than 200 cycles awake procedure that define the compatibility of the RAM modules for notebooks equipped with such a bus slewing system. More importantly , the design used by Apple was not following JEDEC specifications, which certainly explains why most non -Apple certified RAM modules failed to work properly in those models. Apple modified the system with the April 2004 revision. One can find information on the "Bus Slewing" design, as well as which models have been equipped with it, in Apple Tech Notes . The following is a Tech Note describing the system for an iBook G4 of September 2003 :
To lower power consumption and heat generation, the iBook G4 incorporates an automatic power management technique called bus slewing. Bus slewing is designed to run at high processor speed, bus speeds, and voltage when the demand on the processor is high, and to run at low processor speed, bus speeds, and voltage when the demand on the processor is low. Switching between different processor/bus speeds and voltages is achieved by a transition that operates seamlessly to the user and should not impact system or application performance. The iBook G4 allows the user to control bus slewing mode. The options for specifying either high, reduced, or automatic processor and bus speeds are located at System Preferences>Energy Saver>Show Details>Options>Processor Performance; then select Automatic, Highest, or Reduced. Slewing is enabled with the automatic setting, which is the system default. If the iBook G4 detects a system temperature that is too high, due to high ambient temperatures or other factors, it will automatically force the system to slew to reduced mode regardless of the selected setting. When the iBook G4 is in slewing mode, the processor, processor bus, and memory bus dynamically adjust their speeds based on the current needs of the system. In the 1.0 GHz configuration, the processor speed will switch between 1 GHz and 765 MHz. In the 933 MHz configuration, the processor speed will switch between 933 MHz and 714 MHz. In the 800 MHz configuration, the processor speed will switch between 800 MHz and 612 MHz. In all configurations, the processor bus and memory bus will switch between 133 MHz and 102 MHz.
So, it is a power management technique used in many models, but that has been creating problems only for 166MHz-based notebooks (using DDR333 SODIMM PC2700 RAM modules). Apple modified it in April 2004 and added an interesting information to the power consumption section , indirectly confirming that this design could be the basis of many instability issues: the memory controller frequency is NOT altered during sleep mode anymore ( see the bottom of the page).
Based on this information, in order to give Mac users a larger choice of RAM modules to install in their favorite machines, we have initiated a large scale RAM modules test. We decided to test a defined number of RAM modules in as many different Apple notebooks as possible : iBooks and PowerBooks .
Before continuing, we want to mention that we contacted 3 of the main RAM modules manufacturers, and only one, SwissBit, sent us RAM modules to be tested .
