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RAM Expansion

This chapter tells how to gain access to the expansion slots in the iMac computer and describes the RAM expansion modules.

RAM Expansion Slots

The iMac has two RAM expansion slots: one for an SO-DIMM (referred to as J22 in the System Profiler) and one for a DIMM, which is preloaded by the factory (referred to as J20 in the System Profiler). The iMac has 184-pin, 8-byte, DDR333 (PC2700) SO-DIMM (small outline, dual inline memory module) that uses DDR SDRAM devices The expansion slot capacity is an additional 512 MB, supporting a maximum of 1 GB.

For instructions on installing RAM, refer to the iMac User Guide that shipped with your computer or see the following Apple website:

http://www.info.apple.com/usen/cip

The RAM Expansion Modules

The RAM expansion slot accommodates a standard DDR333 PC2700 compliant SO-DIMM (small outline, dual inline memory module) that uses DDR SDRAM devices.

Mechanical Design of RAM SO-DIMM

The mechanical characteristics of the RAM expansion module are given in the JEDEC specification JESD95 for the 184-pin, 8-byte DRAM SO-DIMM. To find out how to obtain the specification, see RAM Expansion Modules.

The specification defines SO-DIMMs with nominal heights of 1.0, 1.25, 1.5, or 2.0 inches. The iMac can accommodate standard SO-DIMMs with a height of 1.0 or 1.25 inches.

The JEDEC specification defines the maximum depth or thickness of an SO-DIMM as 4 mm. That specification is also a maximum: modules that exceed the specified thickness can cause reliability problems.

Electrical Design of DDR RAM SO-DIMM

The electrical characteristics of the RAM SO-DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7 (JESD-21C). To find out how to obtain the specification, see RAM Expansion Modules.

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 (184-pin DDR SDRAM SO–DIMM, PD INFORMATION) of the JEDEC standard 21-C specification, release 7.

Configuration of DDR SDRAM Devices

The DDR SDRAM devices used in the RAM expansion modules must be self-refresh type devices for operation from a 2.5 V power supply. The speed of the DDR SDRAM devices must be 167 MHz or greater. The preloaded DIMM is 184-pin DDR333 (PC2700).

The devices are programmed to operate with a CAS latency of 2.5; the access time from the clock transition must be 7.5 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 information about the different sizes of DIMMs used in the iMac computer. The first three columns show the memory size, configuration, and number of banks in the DIMMs. The other three columns show the number, density, and configuration of the DDR SDRAM devices making up the memory modules.

**Table 4-1** Sizes of RAM expansion DIMMs and devices
Size of DIMM	Configuration of DIMM	Number of banks	Number of devices	Device density	Device configuration
128 MB	16 M x 64	1	4	256 Mbit	16 M x 16
256 MB	32 M x 64	2	8	256 Mbit	32 M x 8
512 MB	64 M x 64	2	16	256 Mbit	32 M x 8

Configuration of RAM SO-DIMMs

Table 4-2 shows information about the different sizes of SO-DIMMs used in the iMac computer. The first three columns show the memory size, configuration, and number of banks in the SO-DIMMs. The other three columns show the number, density, and configuration of the DDR SDRAM devices making up the memory modules.

**Table 4-2** Sizes of RAM expansion SO-DIMMs and devices
Size of SO-DIMM	Configuration of SO-DIMM	Number of banks	Number of devices	Device density	Device configuration
128 MB	16 M x 64	1	8	128 Mbit	16 M x 8
128 MB	16 M x 64	2	8	128 Mbit	8 M x 16
256 MB	32 M x 64	2	16	128 Mbit	16 M x 8
256 MB	32 M x 64	2	8	256 Mbit	16 M x 16
256 MB	32 M x 64	1	8	256 Mbit	32 M x 8
256 MB	32 M x 64	2	8	256 Mbit	16 M x 16
512 MB	64 M x 64	2	16	256 Mbit	32 M x 8

Address Multiplexing

Signals A[0] – A[12] and BA[0] – BA[1] on each RAM SO-DIMM make up a 15-bit multiplexed address bus that can support several different types of DDR SDRAM devices. Table 4-3 lists the types of devices that can be used in the iMac by size, configuration, and sizes of row, column, and bank addresses.

**Table 4-3** Types of DDR SDRAM devices
Device size	Device configuration (bytes x bits x banks)	Row address bits	Column address bits
128 Mbits	4 M x 8 x 4	12	10
128 Mbits	2 M x 16 x 4	12	9
256 Mbits	8 M x 8 x 4	13	10
256 Mbits	4 M x 16 x 4	13	9
512 Mbits	16 M x 8x 4	13	10

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