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RAM Expansion

This chapter describes the DDR SDRAM expansion features of the iMac G5 computer.

RAM Expansion

The main logic board of the iMac G5 computer has two DDR SDRAM expansion slots for unbuffered DDR400 (PC3200) dual in-line memory modules (DIMMs) for a maximum memory of 2 GB.

Each memory slot can contain 256 MB, 512 MB, or 1 GB of double data rate synchronous dynamic RAM (DDR SDRAM). The iMac G5 contains 512 MB of factory installed DIMM. The iMac G5 supports CAS latencies of 2, 2.5, 3, 4, and 5.

Additional DIMMs can be installed. The combined memory of all of the DIMMs installed is configured as a contiguous array of memory. The throughput of the 400 MHz memory bus is dependent on the DIMMs installed. If only one DIMM is installed, the memory bus is 64-bit. If two non-identical DIMMs are installed, there are two 64–bit memory buses. If two identical DIMMs are installed, the memory bus is 128-bit. Identical DIMM pairs have the same size and composition and provide the fastest and most efficient throughput.

DIMM Specifications

The RAM expansion slots accept 184-pin DDR SDRAM DIMMs that are unbuffered, 8-byte, nonparity, and DDR400-compliant (PC3200). The iMac G5 only supports DIMMs up to 1.25” in height.

Mechanical Specifications

The mechanical design of the DDR SDRAM DIMM is defined by the JEDEC specification JESD21-C, MODULES4_20_4, Release 11b. To find this specification on the World Wide Web, refer to RAM Expansion Modules.

Electrical Specifications

The electrical design of the SDRAM DIMM is defined by the JEDEC specification JESD21-C, MODULES4_20_4, Release 11b. To find this specification on the World Wide Web, refer to RAM Expansion Modules.

The Serial Presence Detect (SPD) EEPROM specified in the JEDEC standard is required and must be set to properly define the DIMM configuration. The EEPROM is powered on 2.5 V. Details about the required values for each byte on the SPD EEPROM can be found on pages 68–70 of the JEDEC specification.

DIMM Configurations

The largest DIMM supported is 1 GB. The maximum number of devices per DIMM is 16.

Table 4-1 shows information about the different sizes of DDR SDRAM devices used in the memory modules. The memory controller supports 256 Mbit, 512 Mbit, and 1Gbit DDR SDRAM devices. The device configurations include three specifications: address range, word size, and number of banks. For example, a 1 M by 16 by 4 device addresses 1 M, stores 16 bits at a time, and has 4 banks.

The first column in Table 4-1 shows the memory size of the largest DIMM with that device size that the computer can accommodate. The third column specifies the number of devices needed to make up the 8-byte width of the data bus. The fourth column in the table shows the size of each bank of devices, which is based on the number of internal banks in each device and the number of devices per bank.

**Table 4-1** Sizes of DDR SDRAM expansion DIMMS and devices
DIMM size	Device size	Configuration	Devices per rank	Rank size	# of ranks
128 MB	256 Mb	4 M x 16 x 4	4	128 MB	1
256 MB	256 Mb	4 M x 16 x 4	4	128 MB	2
256 MB	256 Mb	8 M x 8 x 4	8	256 MB	1
256 MB	512 Mb	8 M x 16 x 4	4	256 MB	1
512 MB	256 Mb	8 M x 8 x 4	8	256 MB	2
512 MB	512 Mb	8 M x 16x 4	4	256 MB	2
512 MB	512 Mb	16 M x 8 x 4	8	512 MB	1
512 MB	1 Gb	16 M x 16 x 4	4	512 MB	1
1 GB	512 Mb	16 M x 8 x 4	8	512 MB	2
1 GB	1 Gb	16 M x 16x 4	4	512 MB	2
1 GB	1 Gb	32 M x 8 x 4	8	1 GB	1

RAM Addressing

Signals A[0–12] on each SDRAM DIMM make up a 13-bit multiplexed address bus that can support several different sizes of SDRAM devices. Table 4-2 shows the address multiplexing modes used with the devices.

**Table 4-2** Address multiplexing modes for SDRAM devices
Device size	Device configuration	Size of row address	Size of column address
256 Mb	4 M x 16x 4	13	9
256 Mb	8 M x 8 x 4	13	10
512 Mb	8 M x 16 x 4	13	10
512 Mb	16 M x 8 x 4	13	11
1 Gb	16 M x 16x 4	14	10
1 Gb	32 M x 8x 4	14	11

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