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III
4.1.1 New Instructions
Two instructions have been added to the M68000 instruction set for use in
controller applications. These are the low-power stop (LPSTOP) and the table
lookup and interpolation (TBL) commands.
4.1.1.1 low-Power Stop (lPSTOP)
In applications where power consumption is a consideration, the CPU32 can
force the device into a low-power standby mode when immediate processing is
not required.
The low-power mode is entered by executing the LPSTOP
instruction. The processor remains in this mode until a user-specified or higher
level interrupt, or a reset, occurs.
4.1.1.2 Table lookup and Interpolation (TBl)
To maximize throughput for real-time applications, reference data is often
precalculated and stored in memory for quick access. The storage of sufficient
data points can require an inordinate amount of memory. The TBL instruction
uses linear interpolation to recover intermediate values from a sample of data
points, and thus conserves memory.
When the TBL instruction is executed, the CPU32 looks up two table entries
bounding the desired result and performs a linear interpolation between them.
Byte, word, and long-word operand sizes are supported. The result can be
rounded according to a round-to-nearest algorithm, or returned unrounded
along with the fractional portion of the calculated result (byte and word results
only). This extra "precision" can be used to reduce cumulative error in complex
calculations.
See 4.6 Table lookup and Interpolation Instructions for
examples.
4.1.2 Unimplemented Instructions
The ability to trap on unimplemented instructions allows user-supplied code to
emulate unimplemented capabilities or to define special-purpose functions.
However, Motorola reserves the right to use all currently unimplemented
instruction operation codes for future M68000 enhancements.
See 6.2.8
Illegal or Unimplemented Instructions for more details.
MOTOROLA
4-2
INSTRUCTION SET
CPU32 REFERENCE MANUAL
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