Memory expansion board with NeXTcube?

Started by nlk, June 27, 2023, 11:42:05 PM

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I was thinking whether a memory expansion board (suppose that we design an extended version of one shown in the Appendix A of the "NextBus Interface Chip Specification") for NeXTcube (both 25 MHz and 33 MHz version) could extend the system memory beyond the 64/128 MB motherboard limit.

Based on the NBIC Specification, since the NextBus bus has 32 bit address lines, I guess the 68040 CPU on the NeXTcube motherboard could access almost 4 GB memory (up to 3840 MB considering the reserved 256 MB at the end of the 4GB address space) on a memory expansion board residing at a different back-plane slot.

Assuming that we have such a memory expansion board installed, wondering if PROM and NeXTSTEP OS could detect more than 1GB memory? I was also curious if it could be somehow tested by the Previous emulator.




I would ask the question in the 'official' Previous emulator.

It's a good idea to see if it can be done there first.


It might be possible hardware-wise to expand memory by up to 128 or probably 256 MB per expansion slot. But it would require a custom kernel to detect and use the extra memory. AFAIK there is no programming interface to add such functionality to the kernel externally.


What is the max memory the kernel can detect. I'm pretty sure it can detect more than 128MB (black hardware limit) on intel machines. But do not recall what the maximum was. I always assumed it was around 2-4GB.


I seem to remember people reporting crashes above 512? I'm pretty sure I ran a sparc with 512 and had no problems. I have 384mb in my machine now with no issues.

Rob Blessin Black Hole

512MB was the limit for ram on Intel boxes, you can install more ram but NeXTSTEP / Openstep , Rhapsody(?) does not use it.
Rob Blessin President computerpowwow ebay  [email protected]
303-741-9998 Serving the NeXT Community  since 2/9/93


The upper limit varies on Intel hardware.. In my experience it depends on the video hardware/ driver. I can get Openstep 4.2 to run for several days with maxmem=980992 for non VESA graphics and maxmem=917504 for systems using the VESA driver. So in my experience somewhat less than 1 GB for Openstep 4.2 Intel.
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