Texas Instruments TMS 99105 is the third generation of 16-bit
microprocessors. The family includes all instructions from the two
previous generations - TMS9900
and TMS9995, and fully object-code compatible with them. New
instructions, included in the TMS 99105, are 32-bit arithmetic and
logic, bit test, signed multiply and divide, and stack-related
instructions. In addition to these new instructions, it is possible to
re-define unused processor opcodes as new instructions. This can be
achieved either with the help of an attached processor (i.e.
co-processor), or by using a Macrostore feature. The Macrostore
allows system designers to map unused microprocessor opcodes to custom
functions located in a memory address space, which is separate from
main memory. This allows the CPU to access up to 128 KB memory (64 KB
of main memory + 64 KB of Macrostore memory). It is also possible to
split main memory into two memory segments - 64 KB instruction segment
and 64 KB data segment. In this case maximum addressable memory size
is increased to 192 KB.
The TMS99105 family uses memory-to-memory architecture. The main
advantage of this architecture is that the set of CPU registers
(called "Workspace") can be located anywhere in memory. This
architecture makes saving and restoring of the contents of all
CPU registers as simple as switching the base address of the
workspace. The disadvantage of this architecture is that the processor
speed is highly dependent on memory speed. The 99105 CPUs have
external clock frequency 24 MHz, which is divided by four internally.
It takes three or more machine cycles for the CPU to execute any
instruction when no wait states are required to access the memory.
This translates to maximum execution speed 2,000,000 instructions per
second or less. Execution speed drops significantly when the processor
is used with slow memory - instructions may execute up to two times
slower when memory access requires 1 wait-state, or up to 3 times
slower when 2 wait-states are required.
The TMS99105 is almost identical to TMS99110
16-bit processor, except that the TMS99110 has 1 KB on-chip Macrostore
memory with pre-programmed Floating Point instructions. Texas Instruments also
planned to release TMS99120 microprocessor which was supposed to
include commonly used utilities for PASCAL language as Macrostore
instructions.
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