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S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

5.18 Table Interpolation Instructions

The table interpolation instructions (TBL and ETBL) interpolate values from

tables stored in memory. Any function that can be represented as a series

of linear equations can be represented by a table of appropriate size.

Interpolation can be used for many purposes, including tabular fuzzy logic

membership functions. TBL uses 8-bit table entries and returns an 8-bit

result; ETBL uses 16-bit table entries and returns a 16-bit result. Use of

indexed addressing mode provides great flexibility in structuring tables.

Consider each of the successive values stored in a table to be y-values for

the endpoint of a line segment. The value in the B accumulator before

instruction execution begins represents the change in x from the beginning

of the line segment to the lookup point divided by total change in x from the

beginning to the end of the line segment. B is treated as an 8-bit binary

fraction with radix point left of the MSB, so each line segment is effectively

divided into 256 smaller segments. During instruction execution, the change

in y between the beginning and end of the segment (a signed byte for TBL

or a signed word for ETBL) is multiplied by the content of the B accumulator

to obtain an intermediate delta-y term. The result (stored in the A

accumulator by TBL, and in the D accumulator by ETBL) is the y-value of

the beginning point plus the signed intermediate delta-y value. Table 5-16

shows the table interpolation instructions.

Table 5-16. Table Interpolation Instructions

Mnemonic

Function

Operation

ETBL

16-bit table lookup

and interpolate

(no indirect addressing

modes allowed)

: M + 1) + [(B) × ((M + 2 : M + 3)

– (M

: M + 1))] ⇒ D

Initialize B, and index before ETBL.

<ea> points to the first table entry (M

: M + 1)

B is fractional part of lookup value

TBL

8-bit table lookup

and interpolate

(no indirect addressing

modes allowed)

(M)

+ [(B) × ((M + 1) – (M))] ⇒ A

Initialize B, and index before TBL.

<ea> points to the first 8-bit table entry (M)

B is fractional part of lookup value.

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