Rabbit 2000™ Microprocessor - UTN

5. Rabbit Memory OrganizationThe Rabbit architecture is derived from the original Z80 microprocessor. The original Z80 instructionset used 16-bit addresses to address a 64K memory space. All code and data had to fit in this64K space. The Rabbit adopts a scheme similar to that used by the Z180 to expand the availablememory space. The 64K space is divided into zones and a memory mapping unit or MMU mapseach zone to a block in a larger memory; the larger memory is 1 megabyte in the case of the Z180or the Rabbit 2000. The zones are effectively windows to the larger memory. The view from thewindow can be adjusted so that the window looks at different blocks in the larger memory.Figure 10 on page 24 shows the memory mapping schematically.5.1 Physical MemoryThe Rabbit has a 1-megabyte physical memory space. In special circumstances more than 1-megabyteof memory can be installed and accessed using auxiliary memory mapping schemes. TypicalRabbit systems have two types of physical memory: flash memory and static RAM memory. Flashmemory follows a write-once-in-a-while and read-frequently model. Depending on the particulartype of flash used, the flash memory will wear out after it has been written approximately 10,000to 100,000 times.5.1.1 Flash MemoryFlash memory in a Rabbit-based system may be small-sector type or large-sector type. Small-sectormemory typically has sectors of 128 to 1024 bytes. Individual sectors may be separately erasedand written. In large-sector memory the sectors are often 16K or 64K or more. Small-sector memoryprovides better support for program development and debugging, and large-sector memory isless expensive and has faster access time. The best solution will usually be to lay out a design toaccept several different types of flash memory, including the flexible small-sector memories andthe fast large-sector memories.At the present time development support for programs tested in flash memory is confined to flashmemories with small sectors. If larger sectors are used, the code must be debugged in RAM andthen loaded to flash. Large-sector flash is desirable for the better access time and power consumptionspecifications that are available. Dynamic C is being modified to handle large sector flash atthe time of this writing.5.1.2 SRAMStatic RAM memory may or may not be battery-backed. If it is battery-backed it retains its datawhen power is off. Static RAM chips typically used for Rabbit systems are 32K, 64K, 128K,256K, or 512K. When the memory is battery-backed, power is supplied at 2 V to 3 V from abackup battery. The shutdown circuitry must keep the chip select line high while preserving memorycontents with battery power.Designer’s Handbook 23