Path: utzoo!attcan!uunet!tektronix!tekig5!tekig4!mstevens From: mstevens@tekig4.TEK.COM (Michael Stevens) Newsgroups: comp.sys.ibm.pc Subject: RAM speed per clock rate Message-ID: <3075@tekig4.TEK.COM> Date: 5 Aug 88 17:05:20 GMT Organization: Tektronix Inc., Beaverton, Or. Lines: 39 There have been numerous requests for info on how fast a RAM chip is needed for a clock rate of X. Generally, the equation is (assuming 0 wait state operation is desired): 1 / (Clock Speed in Hertz) = RAM speed in nanoSeconds Examples: -------- 4.77Mhz clock: 1 / 4,770,000 = 210 nS or faster parts. 6Mhz clock: 1 / 6,000,000 = 167 nS or faster parts. 8Mhz clock: 1 / 8,000,000 = 125 nS or faster. 10Mhz: 1 / 10,000,000 = 100 nS or faster. 12Mhz: 1 / 12,000,000 = 83 nS or faster. 16Mhz: 1 / 16,000,000 = 62 nS or faster. etc. RAM is generally available in 150nS, 120nS, 100ns, and 80nS in "reasonable" prices. If your system runs with 1 wait state, it means that 2 clock cycles are used to access the RAM (among other things), so a 12Mhz 1 Wait state system can use 150ns RAMS, but to run a 12Mhz system at 0 wait states you should use 80ns RAMS. This is why many people have problems when they increase the crystal frequency and/or try to go from 1 to 0 wait states without considering their RAM speeds. Note that the CPU or other system parts can cause the same problems (they are also rated for speed - you cannot automaticallly assume adding a faster clock will give you a "faster" system that works reliably). Note that you may be able to get away with slower RAMs in a particular configuration at room temperatures. But for running your dual hard disk and RAM-expanded PC in 95 to 100 degree F temperatures, you will probably have timing problems show up if you try to "cheat" and use slower RAMS than you clock speeds indicate. Considerations which may affect RAM performance include temperature, I/O impedences, system noise, etc. Fully loaded systems (many slots used, multiple disk drives) will generally run hotter than lightly loaded (basic configuration) systems. Also remember that each RAM vendor will give different speed margins (a 100nS part may run at 87nS or 97 nS or 99.99nS), and even different lots of parts from the same vendor can have these differences.