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The electrical characteristics of the RAM SO-DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7. The specification is available from the Electronics Industry Association's web site, at
http://www.jedec.org/download/freestd/pub95/
The specification defines several attributes of the DIMM, including storage capacity and configuration, connector pin assignments, and electrical loading. The specification supports SO-DIMMs with either one or two banks of memory.
The JEDEC specification for the SO-DIMM defines a Serial Presence Detect (SPD) feature that contains the attributes of the module. SO-DIMMs for use in the iMac are required to have the SPD feature. Information about the required values to be stored in the presence detect EEPROM is in section 4.1.2.5 and Figure 4.5.6-C (144 Pin SDRAM SO-DIMM, PD INFORMATION) of the JEDEC standard 21-C specification, release 7.
Because the SO-DIMM connector has only two clock lines, and each clock line is limited to only 4 loads, an SO-DIMM with more than 8 SDRAM devices must have buffers on the clock lines. The buffers must be zero-delay type, such as phase-lock loop (PLL), which regenerates the clock signals. For example, the computer can support a 128-MB SO-DIMM using 16 devices and a PLL clock buffer.
The SDRAM devices used in the RAM expansion modules must be self-refresh type devices for operation from a 3.3-V power supply. The speed of the SDRAM devices must be 100 MHz or greater, corresponding to a cycle time of 10 ns or less.
The devices are programmed to operate with a CAS latency of 3. At that CAS latency, the access time from the clock transition must be 7 ns or less. The burst length must be at least 4 and the minimum clock delay for back-to-back random column access cycles must be a latency of 1 clock cycle.
Table 4-1 shows the sizes of the RAM expansion modules that can be used in the iMac computer and the different sizes of SDRAM devices that make up those modules.
Table 4-1 Sizes of RAM expansion modules and devices
The iMac accepts either one or two SO-DIMMs. Any of the supported SO-DIMM sizes can be installed in either slot, subject to the limits on the dimensions of the SO-DIMMS. The memory controller configures the combined memory of the SO-DIMMs into a contiguous array of memory addresses.
The iMac does not support memory interleaving, so installing two SO-DIMMs of the same size does not result in any performance gain.
Signals A[0] - A[13] on each RAM SO-DIMM make up a 14-bit multiplexed address bus that can support several different types of SDRAM devices. Table 4-2 lists the types of devices that can be used in the iMac computer by size, configuration, and sizes of row and column addresses.
The iMac supports only the types of SDRAM devices specified in Table 4-2 . Other types of DRAM devices should not be used with this computer.
Table 4-2 Types of DRAM devices