from site http://www.drix.be/numbers.htm

Memory chip numbers

Dram Numbers

For devices of 1Mbit and over, generally each manufacturer uses his own scheme to mark memory chips.However, some simple tricks can be used in most cases to determine the exact type of chip.

Usually the number starts with one or two characters and a few digits which are specific to the manufacturer. For an FPM, nibble, SC or EDO chip the rest of the sequence is usually buit up more or less the same : first an indication of the total memory and refresh address range is given, then the number of bits per chip, then the mode of operation terminating with the memory speed.

Format : ----AA B CC -J DD--

----: Manufacturer codes, see below
AA : Total number of megabits. 1=1Mb, 4=4Mb, 16=16Mb/4k refresh, 17=16Mb/2k refresh.
B : Data width. (1,4,8)
CC : I/O mode :

00 = Fast page mode (normal memory)
01 = Nibble mode
02 = Static Column mode
03 - 10 = Extended Data Out mode (EDO)

-J or -xJ : x is some letter irrelevant to this discussion, J is a package type
DD : Access time, should be 30 to 80ns. Sometimes the zero is omitted. In that case the number should be multiplied by 10 so that the speed falls into the 30 to 80ns range. (NOT FOR SDRAMS !)

Sometimes it may be easier to read the package number from the LAST digits and to read to the LEFT. Start with the access time for the rightmost digits, then the package type (either J or CJ) to the left, then the I/O mode, etc.

Examples :

1) HYB51 4 4 00 BJ -60 (8x)

The HYB51 denotes it is a Siemens device
The first 4 denotes it is a 4 megabit chip with 2k refresh.
The second 4 denotes the chip has 4 bits data width.
The 00 denotes it is a fast page device.
The character B denotes the production line or revision.
The J denotes SOJ housing (standard)
The -60 indicates the speed of 60ns.

The chip is thus 1Mx4 60ns FPM.
If there are 8 such chips on a 72pin simm module, then we have 1Mx4x8=1Mx32.Therefore, this is a 4Mbyte Fast page module with no parity.

2) HYB51 17 4 00 BJ -70 (2x)

The HYB51 denotes it is a Siemens device
The 17 denotes it is a 16 megabit chip with 2k refresh.
The second 4 denotes the chip has 4 bits data width.
The 00 denotes it is a fast page device.
The character B denotes the production line or revision.
The J denotes SOJ housing (standard)
The -70 indicates the speed of 70ns.

The chip is thus 4Mx4 70ns FPM.
If there are 2 such chips on a 30pin simm module, then we have 4Mx4x2=4Mx8.Therefore, this is a 4Mbyte Fast page 30 pin module with no parity.

The following are table of manufacturer codes.

Manufacturer codes:

Micron		MT4CTI		TMS4Fujitsu		MB81Goldstar	GM71CHitachi		HM51, HM61Hyundai		HY51Mitsubishi	M5MNEC		uPD4OKI		MSM5Panasonic	MN41Samsung	KM4nC, n=1 or 4 or 16 for data path widthToshiba	TC51

For older RAM chips of under a few megabytes, you'll see numbers like the following:

xxxxxxx4C256 or xxxxxxxC4256: 256K by 4
xxxxxx4160 or xxxxx4260: 256K by 16
xxxx4800 or xxxx8512: 512K by 8
xxxx8160: 1M by 16xxxx6160: 1M by 16, 4K refresh