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ER
microcontroller, represent system integration and performance that were previously
unattainable. The system level benefits of this integration make the Am186ER
microcontroller a compelling product.
Memory integration forms
The memory integration has taken several forms: caches, scratchpads, and system level
memory. Each form of the memory integration has its advantages and disadvantages.
A cache generally improves performance in a system if the external memory is not able
to respond with no wait states. The cache keeps a subset of the large memory
information, usually by storing the most recently used. Because of locality of reference,
the most recently used is also the most likely to be used next. The typical example is a
system using DRAM (which will generally have multiple wait states to respond to
accesses) and a cache. Some systems (e.g., Am486廬) would even have multiple levels
of caches with one level on-chip and the second level external between the processor
and the main system memory. The cache is good for enhancing system performance, but
it does not replace system components. On the Am186ER microcontroller, AMD chose
not to integrate the RAM in the form of a cache because we were already able to use 70-
ns commodity memory for no wait state operation at 40 MHz. No performance
enhancement would be achieved and, as importantly, no system cost reduction would be
made. A cache augments external RAM, it does not replace it.
Several microcontrollers have integrated a small piece of memory as a scratchpad. Like
a cache, a scratchpad generally doesn鈥檛 replace other system components. The one
exception to this is non-volatile RAM. Many systems, but not all, use some small
section of non-volatile memory to store parameters. The rise of the use of Flash in
systems has integrated this feature with the instruction store. A portion of the Flash is
used as the scratchpad non-volatile RAM. A use of scratchpad RAM is similar to a
cache. Instead of the no-wait-state memory being a variation of most recently used (and
thus most likely to be used again next), the scratchpad contains data or program code
that is known to be speed critical. While this provides a performance increase, it again
does not replace system components.
The combination of AMD and the AMD logo, Am186, Am188, and E86 are trademarks of Advanced Micro Devices, Inc.
AMD, the AMD logo, and Am486 are registered trademarks of Advanced Micro Devices, Inc.
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