HCS12 datasheet(360/452 Pages) FREESCALE

360 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

360

Freescale Semiconductor

pointer so that the instruction can resume correctly if it is interrupted. After

the REVW instruction finishes, X will point at the next address past the

$FFFF separator word that marks the end of the rule list.

The Y index register is set to the starting address of the list of rule weights.

Each rule weight is an 8-bit value. The weighted result is the truncated upper

eight bits of the 16-bit result, which is derived by multiplying the minimum

rule antecedent value ($00–$FF) by the weight plus one ($001–$100). This

method of weighting rules allows an 8-bit weighting factor to represent a

value between zero and one inclusive.

The 8-bit A accumulator is used to hold intermediate calculation results

during execution of the REVW instruction. During antecedent processing, A

starts out at $FF and is replaced by any smaller fuzzy input that is

referenced by a rule antecedent. If rule weights are enabled by the C

condition code bit equal one, the rule truth value is multiplied by the rule

weight just before consequent processing starts. During consequent

processing, A holds the truth value (possibly weighted) for the rule. This

truth value is stored to any fuzzy output that is referenced by a rule

consequent, unless that fuzzy output is already larger (MAX).

Before starting to execute REVW, A must be set to $FF (the largest 8-bit

value) because rule evaluation always starts with processing of the

antecedents of the first rule. For subsequent rules in the list, A is

automatically set to $FF when the instruction detects the $FFFE marker

word between the last consequent of the previous rule, and the first

antecedent of a new rule.

Both the C and V condition code bits must be set up prior to starting a REVW

instruction. Once the REVW instruction starts, the C bit remains constant

and the value in the V bit is automatically maintained as $FFFE separator

words are detected.

The final requirement to clear all fuzzy outputs to $00 is part of the MAX

algorithm. Each time a rule consequent references a fuzzy output, that fuzzy

output is compared to the truth value (weighted) for the current rule. If the

current truth value is larger, it is written over the previous value in the fuzzy

output. After all rules have been evaluated, the fuzzy output contains the

truth value for the most-true rule that referenced that fuzzy output.

After REVW finishes, A will hold the truth value (weighted) for the last rule

in the rule list. The V condition code bit should be one because the last

element before the $FFFF end marker should have been a rule consequent.

If V is zero after executing REVW, it indicates the rule list was structured

incorrectly.

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