Am 08.05.2018 um 19:58 schrieb Jim Brain <brain@jbrain.com>: > On 5/8/2018 12:23 AM, And Fachat wrote: >> >> >> Am 8. Mai 2018 02:20:11 schrieb Jim Brain <brain@jbrain.com>: >> >>> >>> I am keenly interested in MMU design, since I am working on one for >>> another platform. >>> >> >> FYI, here http://www.6502.org/users/andre/hwinfo/ls610/index.html I built a replacement for the 74ls610 MMU using cache RAM, where most address lines are kept zero. > I referenced your design as I worked on some designs, so a belated thanks. >> >> I use the ls610 in my system to map 16 pages of 4k from a 1MB physical address space. But external logic is used to have the MMU (all IO) available in all mappings, and to have a defined state at reset. Also the upper 4 bits of the MMU are used as write protect, valid and NoExecute bits. Still 4 data bits of one of the cache RAMs are unused and could be used as additional address lines. > I decided to implement my MMU in a CPLD with a fast SRAM for the "mapping" registers, and extended it to 16 bits, using the top 3 bits as you do. >> >> Using a task register for more of the address bits would indeed allow quickly switching between mappings. But extra logic should probably be included for shared memory windows on top or for IO to avoid trampoline code etc. But IO could also be protected from some mappings for "unprivileged" programs. > The CPLD provides that capability easily. > > The current features are: > > 5 bit task register (32 main tasks) > 8 16 bit values per task (8kB mapping granularity, >2MB address space capable) > optional common RAM area > 7 bits of "secondary task" capability (multiple mappings per task, etc.). >> >> >> >> I would like to add another aspect you might consider: If your MMU design allows the Z80 processor to see memory mappings similar to the ones which are possible in CPC 6128 or MSX computers, there's a good chance for Jörn Mika's magnificent SymbOS getting ported to the C128! (Until now the C128 memory organisation has been the "show stopper".) Regards, ChrizReceived on 2018-05-08 22:00:03
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