I've been doing some more in-depth reading in
the MC68060 manual, and learned a few interesting little tidbits. First, there is no such thing as an MC68060RC75 - the only 75MHz versions of this processor are the LC and EC models. So much for my previous post, eh? With this news, I'm once again up in the air about which CPU to go with. Some pros and cons:
Pros:
68060: Complete version with MMU, FPU, everything you could possibly need.
68LC060: Fastest version made runs at 75MHz, but has successfully been run at 80 MHz with no issues.
68EC060: Fastest version made runs at 75MHz, but rumored to be even more easily overclocked without causing a thermal nightmare, making 100MHz within reach. Commonly available at very reasonable prices.
Cons:
68060: Fastest version made runs at only 66MHz instead of 75.
68LC060: Harder to find and insanely expensive, around $650 per chip. No MMU may be an issue for protected memory.
68EC060: Again, no MMU.
Even though there is no MMU in the LC and EC, Appendix B of the manual states:
"Although the MC68EC060 has no paged MMU, the four TTRs (ITT0, ITT1, DTT0, and DTT1) and the default transparent translation (defined by certain bits in the TCR) operate normally and can still be used to assign cache modes and supervisor and write protection for given address ranges. All addresses can be mapped by the four TTRs and the default transparent translation."
Contrast that with the info found in section 4 regarding the MMU:
"The data transparent translation registers (DTTR0 and DTTR1) and instruction transparent translation registers (ITTR0 and ITTR1) are 32-bit registers that define blocks of logical address space that are untranslated by the MMU (the logical address is the physical address)."
I could care less about paging, but protected memory is still
really high on my wish list. However, I don't see how these TTR registers can be used for memory protection when it seems all they do is control whether addresses are translated or not. What am I missing here?
I'm still doing some reading, so I may answer my own question eventually, but if there are any experienced Motorola guys following along, maybe one could point me in the right direction?