Introduction
1-14
MC68030 USER’S MANUAL
MOTOROLA
1.7 THE MEMORY MANAGEMENT UNIT
The MMU supports virtual memory systems by translating logical addresses to physical ad-
dresses using translation tables stored in memory. The MMU stores address mappings in
an address translation cache (ATC) that contains the most recently used translations. When
the ATC contains the address for a bus cycle requested by the CPU, a translation table
search is not performed. Features of the MMU include:
• Multiple Level Translation Tables with Short- and Long-Format Descriptors for Efficient
Table Space Usage
• Table Searches Automatically Performed in Microcode
• 22-Entry Fully Associative ATC
• Address Translations and Internal Instruction and Data Cache Accesses Performed in
Parallel
• Eight Page Sizes Available Ranging from 256 to 32K Bytes
• Two Optional Transparent Blocks
• User and Supervisor Root Pointer Registers
• Write Protection and Supervisor Protection Attributes
• Translations Enabled/Disabled by Software
• Translations Can Be Disabled with External MMUDIS
Signal
• Used and Modified Bits Automatically Maintained in Tables and ATC
• Cache Inhibit Output (CIOUT
) Signal Can Be Asserted on a Page-by-Page Basis
• 32-Bit Internal Logical Address with Capability To Ignore as many as 15 Upper Address
Bits
• 3-Bit Function Code Supports Separate Address Spaces
• 32-Bit Physical Address
The memory management function performed by the MMU is called demand paged memory
management. Since a task specifies the areas of memory it requires as it executes, memory
allocation is supported on a demand basis. If a requested access to memory is not currently
mapped by the system, then the access causes a demand for the operating system to load
or allocate the required memory image. The technique used by the MC68030 is paged
memory management because physical memory is managed in blocks of a specified
number of bytes, called page frames. The logical address space is divided into fixed-size
pages that contain the same number of bytes as the page frames. Memory management
assigns a physical base address to a logical page. The system software then transfers data
between secondary storage and memory one or more pages at a time.
To Next Page
Loading...
Need help?
General
