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The low-profile vertically mounted 4 MB SIMMs are easier to install in the NeXTcube than the horizontally mounted 4 MB SIMMs because of the small height clearance above the SIMM slots. It is possible to install the horizontally mounted 4 MB SIMMs, but you will be required to slide the CPU board and the center tower in simultaneously.
Parity (9-bit) SIMMs can be used in both 68030 and 68040 NeXT machines, but should not be mixed with non-parity SIMMs. Only 68040 boards with ROM levels of 2.2 (v63) and higher can use the parity memory to detect parity errors.
It is OK to mix parity and non-parity memory, but the system will not boot unattended. Cubes with early boot ROMs will not work with 4 Mb parity ram, unless at least 3 banks are used. The system gives an exception error on power up. The fix is to get a new boot rom from Next.
You can pay $30, or you may be able to squawk and get one for free. I have found Next to be pretty responsive, once I find the right person.
The correct version is v66 which was the last or final rev for this series of 040 boards. This version also fixed the problem in the second paragraph.
NeXT didn't officially bless the use of 8 MB SIMMs, but they seem to fit and work.
Faster SIMMS (70/80 ns) don't make the memory system work any faster than the 100 ns units.
For maximum performance use 70 ns SIMMs: SIMMs rated at 80 or 100 ns will be detected upon powerup and the memory system clock slowed to 100 ns. Faster RAM than 70 ns won't give you a speed increase anymore. In fact it could slow things down again, because some hardware drives 60 ns RAM as 100 ns RAM.
NeXT manufacturing introduced the new 25 MHz NeXTstation CPU board into production in late June '92. To verify which SIMM type your machine uses, check the system's memory configuration. You can do this by using the ROM monitor�s print memory configuration command m. Start with your machine powered down. Press the Power key to power on. As soon as the message �Testing system...� disappears, press command-command-tilde ( on the numeric keyboard). Under these circumstances, this will access the ROM monitor. In the ROM monitor, type m and press return. Turbo-designed boards�including new 25 MHz NeXTstations and all Turbo systems�will return messages reporting the memory configuration contained in four sockets (sockets 0 -3); old 25 MHz boards will return messages for more than four sockets (usually 8). You can tell a Turbo-designed board, and the accompanying 72 pin, 70 nanosecond SIMMs, by the fact it only reports information for only four sockets.
The memory system has programmable memory timing such that the number of processor clocks needed to access a given amount of data can be tailored to the speed of the memory installed. 70 ns memory is just enough faster than 80 ns memory to allow the cpu to access the data with fewer clock cycles. This improves memory system performance.
"70 ns" memory is faster than "80 ns" memory in many parameters other than just RAS access time. The faster CAS access time in particular allows the memory system to respond quicker to burst (16 bytes) bus transfers.