Technical Q&A HW75

Explanation of SDRAM configuration Nomenclature

Q: How do I interpret the three-part "Device configuration" numbers for SDRAM in Table 4-1, "Sizes of RAM expansion devices and modules," on page 64 of the Power Mac G4 Computer Developer Note?

A: The individual SDRAM IC configurations are typically described by a three-part device configuration number as in the example below:

4M x 16 x 2

From left to right the configuration is interpreted as follows:

The left-most number describes the RAM's memory capacity in bytes (K for Kilobytes, M for Megabytes … etc.) In the example above, the SDRAM capacity is 4 Megabytes.

The "middle" or second number represents that width of the data path in bits. In the example above, the data path is 16 bits wide.

The right-most number indicates the number of "banks" in the SDRAM IC package. The number of "banks" refers to the internal architecture of the individual SDRAM ICs and should not be confused with the "banks" for an SDRAM DIMM. Physically and electrically, the pins that control "bank" access to the SDRAM IC can be thought of as the upper address lines. In the example, the number indicates that there are 2 banks.

Completely translating the example reveals that the SDRAM IC it describes has 2 banks of 4MB memory with a 16-bit data path, making it an 8MB (4M x 2) SDRAM with 16-bit wide data.