Motorola MC68340 manual Following value has been calculated for independent variable

Freescale Semiconductor, Inc...

Freescale Semiconductor, Inc.

The following value has been calculated for independent variable X:

31	16	15	0
NOT USED		0	0 0 0 0 0 0 0 1 0 1 1 1 1 0 1

Since X is an 8-bit value, the upper four bits are used as a table offset and the lower four bits are used as an interpolation fraction. The following results are obtained from the subroutine:

Table Entry Offset ⇒ Dx [4:7] = $B = 11

Interpolation Fraction ⇒ Dx [0:3] = $D = 13

Thus, Y is calculated as follows:

Y = 80 + (13 (64 – 80)) / 16 = 67

If the 8-bit value for X were used directly by the table instruction, interpolation would be incorrectly performed between entries 0 and 1. Data must be shifted to the left four places before use:

LSL.W #4, Dx

The new range for X is 0 ≤ X ≤ 4096; however, since a left shift fills the least significant digits of the word with zeros, the interpolation fraction can only have one of 16 values.

After the shift operation, Dx contains the following value:

31	16	15	0
NOT USED		0 0 0 0 1 0 1 1 1	1 0 1 0 0 0 0

Execution of the table instruction using the new value in Dx yields:

Table Entry Offset ⇒ Dx [8:15] = $0B = 11

Interpolation Fraction ⇒ Dx [0:7] = $D0 = 208

Thus, Y is calculated as follows:

Y = 80 + (208 (64 – 80)) / 256 = 67

5.3.4.4TABLE EXAMPLE 4: MAINTAINING PRECISION. In this example, three TBL operations are performed and the results are summed. The calculation is done once with the result of each TBL rounded before addition and once with only the final result rounded.

Assume that the result of the three interpolations are as follows (a ".'' indicates the binary radix point).

TBL # 1	0010	0000 . 0111	0000
TBL# 2	0011	1111 . 0111	0000
TBL # 3	0000	0001 . 0111	0000

5- 34MC68340 USER’S MANUALMOTOROLA

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