MC68030 Datasheet, PDF (289/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

SECTION 9
MEMORY MANAGEMENT UNIT
The MC68030 includes a memory management unit (MMU) that supports a demand-paged
virtual memory environment. The memory management is "demand in that programs do not
specify required memory areas in advance, but request them by accessing logical
addresses. The physical memory is paged, meaning that it is divided into blocks of equal
size called page frames. The logical address space is divided into pages of the same size.
The operating system assigns pages to page frames as they are required to meet the needs
of programs.
The principal function of the MMU is the translation of logical addresses to physical
addresses using translation tables stored in memory. The MMU contains an address
translation cache (ATC) in which recently used logical-to-physical address translations are
stored. As the MMU receives each logical address from the CPU core, it searches the ATC
for the corresponding physical address. When the translation is not in the ATC, the
processor searches the translation tables in memory for the translation information. The
address calculations and bus cycles required for this search are performed by microcode
and dedicated logic in the MC68030. In addition, the MMU contains two transparent
translation registers (TT0 and TT1) that identify blocks of memory that can be accessed
without translation. The features of the MMU are:
• 32-Bit Logical Address Translated to 32-Bit Physical Address with 3-Bit Function Code
• Supports Two-Clock Cycle Processor Accesses to Physical Address Spaces
• Addresses Translated in Parallel with Accesses to Data and Instruction Caches
• On-Chip Fully Associative 22-Entry ATC
• Translation Table Search Controlled by Microcode
• Eight Page Sizes: 256, 512, 1K, 2K, 4K, 8K, 16K and 32K Bytes
• Separate User and Supervisor Translation Table Trees Are Supported
• Two Independent Blocks Can Be Defined as Transparent (Untranslated)
• Multiple Levels of Translation Tables
MOTOROLA
MC68030 USER’S MANUAL
9-1