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4.1.1.2 Table Lookup and Interpolation (TBL)
To maximize throughput for real-time applications, reference data is often precalculat-
ed 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 mem-
ory.
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 Inter-
polation 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.
4.2 Instruction Format
All instructions consist of at least one word. Some instructions can have as many as
seven words, as shown in Figure 4-1. The first word of the instruction, called the op-
eration word, specifies instruction length and the operation to be performed. The re-
maining words, called extension words, further specify the instruction and operands.
These words may be immediate operands, extensions to the effective address mode
specified in the operation word, branch displacements, bit number, special register
specifications, trap operands, or argument counts.
15
MOTOROLA
4-2
OPERATION WORD
(ONE WORD, SPECIFIES OPERATION AND MODES)
SPECIAL OPERAND SPECIFIERS
(IF ANY, ONE OR TWO WORDS)
IMMEDIATE OPERAND OR SOURCE ADDRESS EXTENSION
(IF ANY, ONE TO THREE WORDS)
DESTINATION EFFECTIVE ADDRESS EXTENSION
(IF ANY, ONE TO THREE WORDS)
Figure 4-1 Instruction Word General Format
INSTRUCTION SET
0
CPU32
REFERENCE MANUAL
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