CompuServe Messages

Mike, Memory Management Unit. It remaps adresses to 'elsewhere' (usually) under CPU control. It also allows swapping in external memory on some systems. Its primary use is for allowing memory sections to be 'protected' and for swapping tasks in and out. There are other uses, such as multiple low memory setups, defragmenting memory, etc. It also allows for read one thing, write another (A la' the CBM 64 memory 'under' the ROM) and can ghost ROM/RAM in a similar manner. I first saw it in a primitve form with the PHANTOM memory under early Z80 systems, where you could have an additional 64K of memory swapped in and out as needed. The CPU still saw 64K, but one bank might hold something entirely different. The CPU 'sees' things where they ain't, as the address lines get switched around. The CPU might see the memory at, say, $80000 as being at $40000. Makes for some neat tricks. Doug