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MC68340 USER’S MANUAL

MOTOROLA

5.3.1.1.2 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 5.3.4 Using the TBL Instructions for examples.

5.3.1.2 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 5.5.2.8

Illegal or Unimplemented Instructions for more details.

5.3.2 Instruction Format and Notation

All instructions consist of at least one word. Some instructions can have as many as

seven words, as shown in Figure 5-6. The first word of the instruction, called the operation

word, specifies instruction length and the operation to be performed. The remaining

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.

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 5-6. Instruction Word General Format

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