IBM PC 300GL Types 6275, Information Manual

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imposed on the microprocessor. The cache controller manages the use of the cache so that
the number of cache hits far exceeds the number of cache misses.
Level-2 Cache
Your computer has level-2 (L2) cache memory integrated into the microprocessor. L2 cache
complements L1 cache to increase the probability of cache hits. If the microprocessor cannot
find what it needs in L1 cache, it searches L2 cache. If the data or instruction is not in
either cache, the cache controller locates it in main memory and copies it into both caches.
Cache Mode
The microprocessor frequently updates cache memory with changed data. Caches, in turn,
pass these changes to main memory.
When updating cache memory, your computer uses the write-back mode. In write-back
mode, the microprocessor updates the cache, then goes on to perform other functions while
the cache controller updates main memory.
Write-back mode provides better performance than write-through mode, which is a type of
cache architecture used in some other computers. In write-through mode, a microprocessor
updates main memory directly. Write-through mode is slower because the microprocessor
interacts directly with main memory, which is slower than cache memory.
Memory Organization
Operating systems are responsible for allocating memory space, assigning addresses, and
performing many other tasks associated with memory management.
DOS Memory Management
DOS organizes memory into the following types:
Ÿ
Conventional
Ÿ
Extended
Ÿ
Expanded
Conventional memory is the first 1 MB of memory-address space. Of this, the first 640 KB is
available for use by DOS and application programs. Memory from 640 KB through 1 MB is
reserved as a work space for hardware devices and the BIOS.
Extended memory starts above the first 1 MB, appended to conventional memory. Use of
this space is dependent on the operating system and application programs. Not all
programs can use extended memory.
Expanded memory is controlled through an expanded memory specification (EMS) device
driver, such as the one provided with DOS. The EMS device driver is commonly known as
a memory manager. The EMS device driver uses part of the reserved area of conventional
memory as a work space, and the memory above the first 1 MB as a storage area. The
Microprocessors and Memory 5