The major option with memories is packaging. Some come encapsulated in plastic, some in a ceramic shell and so on. There are many different types of package options available and, obvi-ously, the package must match the sockets on the board. Of the many different types, four are commonly used: the dual in line package, zig–zag package, SIMM and SIP. The most common package encountered with the MAC is the SIMM, although all the others are used, especially with third party products.
Dual in line package
This package style, as its name implies, consists of two lines of legs either side of a plastic or ceramic body. It is the most commonly used package for the BIOS EPROMs, DRAM and SRAM. It is available in a variety of sizes with 24, 26 and 28 pin packages used for EPROMs and SRAMs and 18 and 20 pin packages for 1 Mbit × 1 and 256 kbit × 4 DRAMs. However, it has virtually been replaced by the use of SIMM modules and is now only used for DRAM on the original MAC 128K and 512K models and for DRAM and for EPROM on models up to the MAC IIx.
This is a plastic package used primarily for DRAM. Instead of coming out of the sides of the package, the leads protrude from the pattern and are arranged in a zig-zag — hence the name. This type of package can be more difficult to obtain, compared with the dual in line devices, and can therefore be a little more expensive. This format is often used on third party boards.
SIMM and DIMM
SIMM is not strictly a package but a subassembly. It is a small board with finger connection on the bottom and sufficient memory chips on board to make up the required configuration, such as 256 Kbit × 8 or × 9, 1 Mbit × 8 or × 9, 4 Mbit, and so on. SIMMs have rapidly gained favour and many new designs use these boards instead of individual memory chips. They require special sockets, which can be a little fragile and need to be handled correctly. There are currently two types used for PCs: the older 30 pin SIMM which uses an 8 or 9 bit (8 bits plus a parity bit) data bus and a more recent 72 pin SIMM which has a 32 or 36 bit wide data bus. The 36 bit version is 32 bits data plus four parity bits. Apple has used both types and a third which has 64 pins but like the IBM PC world standardised on the 72 pin variety which suited the 32 bit processors at the time. With the advent of wider bus CPUs, yet another variation appeared called the DIMM. This typically has 168 bits but looks like a larger version of the SIMM. With the wider buses came an increase in memory speeds and a change in the supply voltages. One method of getting faster speeds and reduced power consumption is to reduce the supply voltage. Instead of the signal levels going from 0 to 5 volts, today's CPUs and correspond-ing memories use a 3.3 volt supply (or even lower). As a result, DIMMs are now described by the speed, memory type and voltage supply e.g. 3.3 volt 133 MHz SDRAM DIMM.
The older 30 pin SIMMs are normally used in pairs for a 16 bit processor bus (80386SX, MC68000) and in fours for 32 bit buses (80386DX, 80486, MC68030, MC68040) while the 72 pin SIMMs are normally added singly although some higher performance boards need a pair of 72 pin SIMMs to support bank switching.
This is the same idea as SIMM, except that the finger connections are replaced by a single row of pins. SIP has been overtaken by SIMM in terms of popularity and is now rarely seen.